Measurement of prompt photon production in sNN√=8.16 TeV p+Pb collisions with ATLAS

Physics Letters B 796 (2019) 230–252 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Measurement of prompt photon production in √sNN =8.16 TeV p +Pb collisions with ATLAS .The ATLAS Collaboration a r t i c l e i n f o a b s t r a c t Article history: Received 7 March 2019 Received in revised form 19 June 2019 Accepted 15 July 2019 Available online 17 July 2019 Editor: M. Doser The inclusive production rates of isolated, prompt photons in p +Pb collisions at √sNN =8.16 TeV are studied with the ATLAS detector at the Large Hadron Collider using a dataset with an integrated luminosity of 165 nb−1recorded in 2016. The cross-section and nuclear modification factor RpPb are measured as a function of photon transverse energy from 20 GeV to 550 GeV and in three nucleon– nucleon centre-of-mass pseudorapidity regions, (−2.83, −2.02), (−1.84, 0.91), and (1.09, 1.90). The cross-section and RpPb values are compared with the results of a next-to-leading-order perturbative QCD calculation, with and without nuclear parton distribution function modifications, and with expectations based on a model of the energy loss of partons prior to the hard scattering. The data disfavour a large amount of energy loss and provide new constraints on the parton densities in nuclei. ©2019 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 1. Introduction Measurements of particle and jet production rates at large transverse energy are a fundamental method of characterising hard-scattering processes in all collision systems. In collisions involving large nuclei, production rates are modified from those measured in proton +proton (pp) collisions due to a combination of initial- and final-state effects. The former arise from the dynamics of partons in the nuclei prior to the hard-scattering process, while the latter are attributed to the strong interaction of the emerging partons with the hot nuclear medium formed in nucleus–nucleus collisions. Modification due to the nuclear environment is quantified by the nuclear modification factor, RAA, defined as the ratio of the cross-section measured in A +A to that in pp collisions, scaled by the expected geometric difference between the systems. Measurements of prompt photon production rates offer a way to isolate the initial-state effects because the final-state photons do not interact strongly. These initial-state effects include the degree to which parton densities are modified in a nuclear environment [1–3], as well as potential modification due to an energy loss arising through interactions of the partons traversing the nucleus prior to the hard scattering [4,5]. Constraints on such initial-state effects are particularly important for characterising the observed modifications of strongly interacting final states, such as jet and hadron production [6,7], since they are sensitive to effects E-mail address: atlas .publications @cern .ch. from both initial- and final-state. Due to the significantly simpler underlying-event conditions in proton–nucleus collisions, measurements of photon rates can be performed with better control over systematic uncertainties than in nucleus–nucleus collisions, allowing a more precise constraint on these initial-state effects. Prompt photon production has been extensively measured in pp collisions at a variety of collision energies [8–12]at the Large Hadron Collider (LHC). It was also measured in lead–lead (Pb+Pb) collisions at a nucleon–nucleon centre-of-mass energy √sNN = 2.76 TeV [13,14]at the LHC, and in gold–gold collisions at √sNN = 200 GeV at the Relativistic Heavy Ion Collider (RHIC) [15], where the data from both colliders indicate that photon production rates are unaffected by the passage of the photons through the hot nuclear medium. At RHIC, photon production rates were measured in deuteron–gold collisions at √sNN =200 GeV [16,17] and were found to be in good agreement with perturbative QCD (pQCD) calculations. Additionally, jet production [18,19] and electroweak boson production [20–22]were measured in 28 nb−1of proton– lead (p +Pb) collision data at √sNN =5.02 TeV recorded at the LHC; the former is a strongly interacting final state, while the latter is not. All measurements provided some constraints on initial-state effects. The data used in this measurement were collected with the ATLAS detector during the p +Pb collision running period in 2016, and correspond to an integrated luminosity of 165 nb−1, approximately six times larger than the measurements made in the previous 5.02 TeV data. The proton and lead beams had an energy of 6.5TeV and 2.51 TeV per nucleon respectively, resulting in a nucleon–nucleon centre-of-mass collision energy of 8.16 TeV and https://doi.org/10.1016/j.physletb.2019.07.031 0370-2693/©2019 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 231 a rapidity boost of this frame by ±0.465 units relative to the ATLAS laboratory frame, depending on the direction of the Pb beam.1 By convention, the results are reported as a function of photon pseudorapidity in the nucleon–nucleon collision frame, η∗, with positive η∗corresponding to the proton beam direction, and negative η∗corresponding to the Pb beam direction. At leading order, the process p +Pb →γ+Xhas contributions from direct processes, in which the photon is produced in the hard interaction, and from fragmentation processes, in which it is produced in the parton shower. Beyond leading order the direct and fragmentation components are not separable and only their sum is a physical observable. To reduce contamination from the dominant background of photons mainly from light-meson decays in jets, the measurements presented here require the photons to be isolated from nearby particles. This requirement also acts to reduce the relative contribution of fragmentation photons in the measurement, and thus, the same fiducial requirement must be imposed on theoretical models when comparing with the data. Specifically, as in previous ATLAS measurements [9,10], the sum of energy transverse to the beam axis within a cone of R ≡(η)2+(φ)2=0.4 around the photon, Eiso T, is required to be smaller than 4.8 +4.2 × 10−3Eγ T[GeV], where Eγ Tis the transverse energy of the photon. At particle level, Eiso Tis calculated as the sum of transverse energy of all particles with a decay length above 10 mm, excluding muons and neutrinos. This sum is corrected for the ambient contribution from underlying-event particles, consistent with the previous measurements [9,10]. This letter reports a measurement of the cross-section for prompt, isolated photons in p +Pb collisions at √sNN =8.16 TeV. Photons are measured with Eγ T>20 GeV, the isolation requirement detailed above, and in three nucleon–nucleon centre-of-mass pseudorapidity (η∗) regions, −2.83 <η∗<−2.02, −1.84 <η∗< 0.91, and 1.09 <η∗<1.90. In addition to the cross-section, the data are compared to a pp reference cross-section derived from a previous measurement of prompt photon production in pp collisions at √s=8TeV that used the identical isolation condition [9]. The nuclear modification factor RpPb is derived in each pseudorapidity region, using an extrapolation for the different collision energy and centre-of-mass pseudorapidity selection, and is reported in the region Eγ T>25 GeV where reference data is available. Furthermore, the ratio of RpPb in the forward region to that in the backward region is presented. The measurements are compared with next-to-leading-order (NLO) pQCD predictions from Jetphox [23]using parton distribution functions (PDF) extracted from global analyses that include nuclear modification effects analyses [24,25]. Additionally, the data are compared with predictions from a model including initial-state energy loss [4,5,26]. 2. Experimental set-up The ATLAS detector [27]is a multipurpose detector with a forward–backward symmetric cylindrical geometry. For this measurement, its relevant components include an inner tracking detector surrounded by a thin superconducting solenoid, and electromagnetic and hadronic calorimeters. The inner-detector system 1ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θas η=− lntan(θ/2)and the rapidity of the components of the beam, y, are defined in terms of their energy, E, and longitudinal momentum, pz, as y =0.5 ln E+pz E−pz. is immersed in a 2T axial magnetic field and provides chargedparticle tracking in the pseudorapidity range  ηlab <2.5in the laboratory frame. In order of closest to furthest from the beam pipe, it consists of a high-granularity silicon pixel detector, a silicon microstrip tracker, and a transition radiation tracker. Additionally, the new insertable B-layer [28] has been operating as the innermost layer of the tracking system since 2015. The calorimeter system covers the range  ηlab <4.9. In the region  ηlab < 3.2, electromagnetic calorimetry is provided by barrel and endcap high-granularity lead/liquid-argon (LAr) sampling calorimeters. An additional thin LAr presampler covers  ηlab <1.8to correct for energy loss in material before the calorimeters. The LAr calorimeters are divided into three layers in radial depth. Hadronic calorimetry is provided by a steel/scintillator-tile calorimeter, segmented into three barrel structures within  ηlab <1.7, and two copper/LAr hadronic endcap calorimeters, which cover the region 1.5 < ηlab <3.2. Finally, the forward calorimeter covers 3.2 <  ηlab <4.9 and is divided into three compartments. The first compartment is a copper/LAr electromagnetic calorimeter, while the remaining two tungsten/LAr calorimeter compartments collect the hadronic energy. During data-taking, events were initially selected using a level-1 trigger, implemented in custom electronics, based on energy deposition in the electromagnetic calorimeter. The high-level trigger [29]was then used to select events consistent with a high-Eγ T photon candidate. The high level trigger was configured with five online Eγ Tthresholds from 15 GeV to 35 GeV. Each trigger is used for an exclusive region of the Eγ Tspectrum, starting 5GeV above the trigger threshold because there the trigger is fully efficient. The highest-threshold trigger is used in the measurement over the whole Eγ Trange above 40 GeV and is unprescaled. The lower-threshold, prescaled, triggers are used to perform the measurement for Eγ Tin the range of 20–40 GeV. Data-taking was divided into two periods with different configurations of the LHC beams. In the first period, the lead ions circulated in beam 1 (clockwise) and protons circulated in beam 2, while in the second period the beams were reversed. These periods corresponded to integrated luminosities of 57 nb−1and 108 nb−1 respectively. 3. Photon reconstruction and identification Photons are reconstructed following a procedure used extensively in previous ATLAS measurements [10], of which only the main features are summarised here. Photon candidates are reconstructed from clusters of energy deposited in the electromagnetic calorimeter in three regions corresponding to the laboratory-frame (ηlab) positions of the barrel and forward and backward endcaps  ηlab <2.37. The transition region between the barrel and endcap calorimeters, 1.37 < ηlab <1.56, is excluded due to its higher level of inactive material. The measurement of the photon energy is based on the energy collected in calorimeter cells in an area of size η×φ=0.075 ×0.175 in the barrel and η×φ=0.125 ×0.125 in the endcaps. It is corrected via a dedicated energy calibration [30] which accounts for losses in the material before the calorimeter, both lateral and longitudinal leakage, and for variation of the sampling-fraction with energy and shower depth. The photons are identified using the tight calorimeter shower shape requirements described in Ref. [31]. The tight requirements select clusters which are compatible with originating from a single photon impacting the calorimeter. The information used includes that from the hadronic calorimeter, the lateral shower shape in the second layer of the electromagnetic calorimeter, and the detailed shower shape in the finely segmented first layer. 232 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 The isolation transverse energy, Eiso T, is computed from the sum of ETvalues in topological clusters of calorimeter cells [32]inside a cone of size R =0.4centred on the photon. This cone size is chosen to be compatible with a previous measurement of photon production in pp collisions at √s=8TeV[9], which is used to construct the reference spectrum for the RpPb measurement. This estimate excludes an area of η×φ=0.125 ×0.175 centred on the photon, and is corrected for the expected leakage of the photon energy from this region into the isolation cone. 4. Simulated event samples Samples of Monte Carlo (MC) simulated events were generated to study the detector performance for signal photons. Proton– proton generators were used as the source of events containing photons. To include the effects of the p +Pb underlying-event environment, these simulated pp events were combined with p +Pb events from data before reconstruction. In this way, the simulated events contain the effects of the p +Pb underlying-event identical to those observed in data. The Pythia 8.186 [33] generator was used to generate the nominal set of MC events, with the NNPDF23LO parton distribution function (PDF) set [34] and a set of generator parameters tuned to reproduce minimum-bias pp events with the same collision energy as that in the p +Pb data (“A14” tune) [35]. A centre-of-mass boost was applied to the generated events to bring them into the same laboratory frame as the data. The generator simulates the direct photon contribution and, through final-state QED radiation in 2 →2QCD processes, also includes the fragmentation photon contributions; these components are defined to be signal photons. Events were generated in six exclusive Eγ Tranges from 17 GeV to 500 GeV. An additional MC sample was used to assess the sensitivity of the measurement to this choice of generator. The Sherpa 2.2.4 [36]event generator produces fragmentation photons in a different way from Pythia and was thus chosen for the comparison. The NNPDF3.0NNLO PDF set [37]was used, and the events were generated in the same kinematic regions as the Pythia events. These events were generated with leading-order matrix elements for photon-plus-jet final states with up to three additional partons, which were merged with the Sherpa parton shower. The Sherpa sample produced results consistent with Pythia, and, thus, no correction or uncertainty is applied. The Pythia and Sherpapp events were passed through a full Geant4 simulation of the ATLAS detector [38,39]. To model the underlying event effects, each simulated event was combined with a minimum-bias p +Pb data event and the two were reconstructed together as a single event, using the same algorithms as used for the data. These events were split between the two beam configurations in a proportion matched to that in data-taking. The underlying event activity levels, as characterized by the sum of the transverse energy in the outgoing-Pb-beam side of the forward calorimeter (3.1 <|ηlab| <4.9), are different in the photoncontaining data events from the minimum-bias data events used in the simulation. Thus, the simulated events were weighted on a per-event basis to match the underlying event activity distribution in data. Furthermore, the photon shower shapes and identification efficiency in simulation were adjusted for small differences previously observed between these quantities in data and in Geant4 simulation [31]. 5. Data analysis The differential cross-section is calculated for each Eγ Tand η∗ bin as dσ dEγ T=1 Lint 1 Eγ T Nsig Psig seltrig CMC, where Lint is the integrated luminosity, Eγ Tis the width of the Eγ Tbin, Nsig is the yield of photon candidates passing identification and isolation requirements, Psig is the purity of the signal selection, sel is the combined reconstruction, identification and isolation efficiency for signal photons, trig is the trigger efficiency, and CMC is a MC derived bin-by-bin correction for the change in the Eγ Tspectrum from photons migrating between bins in the spectrum due to the width in the energy response. CMC is determined after all selection criteria at both reconstruction and particle levels are imposed. Trigger efficiencies trig are studied using events selected with minimum-bias triggers, level-1 triggers without additional requirements, and photon high-level triggers without identification requirements. They are greater than 99.5% for all triggers [29]. In this analysis they are taken as trig =1, and any uncertainty is neglected as being sub-dominant to other uncertainties. The purity Psig is determined via a double-sideband procedure used extensively in previous measurements of cross-sections for processes with a photon in the final state [9,10,40,41] and summarised here. In the procedure, four regions are defined which categorise photon candidates along two axes: (1) isolation, corresponding to an isolated and an inverted “non-isolated” selection; (2) identification, corresponding to photons that pass the tight identification requirements described in Ref. [31], and those that pass the loose requirements of Ref. [31]but fail certain components of the tight requirements, designed to mostly select background. The majority of signal photons are in the tight, isolated region, defined to be the signal region, while the other three regions are dominated by the background. Photon candidates that comprise the background are assumed to be distributed in a way that is uncorrelated along the two axes. The yields in the three non-signal sidebands are used to estimate the yield of background in the signal region and is combined with the yield in the signal region to extract the purity. The procedure also accounts for the small fraction of signal photons which are reconstructed in the non-signal sidebands; these quantities, known as leakage fractions, are determined from the simulation samples described in Section 4. The purity is typically 45% at Eγ T=20 GeV, rises to 80% at Eγ T=100 GeV and reaches 99% at Eγ T=300 GeV. Fig. 1shows example Eiso Tdistributions for identified and isolated photons, the corresponding distributions for background photons with the normalisation determined by the double-sideband method, and the resulting signal-photon distributions after background subtraction, compared with those for generator-level photons in MC simulation. The figure shows the shape of the background distribution within the signal region, and the correspondence between the background-subtracted data and the signal-only Pythia 8 distributions gives confidence that the simulations accurately represent the data. The photon selection efficiency is determined from MC simulations. Generated prompt photons are required to be isolated at the generator level, after an estimate of the underlying event has been subtracted from the isolation energy, as described above. Reconstruction efficiency is determined by requiring a photon to have been reconstructed within R =0.2of the generated photon. Reconstructed photons matching to a generated photon are further required to satisfy tight identification and isolation criteria defined in Section 1. The combined efficiency of signal photons to pass all reconstruction level selections, sel, is typically 90% at all Eγ Tand η∗, except at Eγ T≈20 GeV where it decreases to about 80%. Fig. 2summarises the different components of the total selection efficiency. The reconstruction efficiency is 96–99% everywhere, The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 233 Fig. 1. Distributions of detector-level photon isolation transverse energy Eiso Tfor identified photons in data (black points), background photons scaled to match the data at large Eiso T(blue solid line), the resulting distribution for signal photons scaled so that the maximum value is the same as that for identified photons (green dot-dashed line), and that for photons in simulation which are isolated at the generator level normalised to have the same integral as the signal photon distribution (red dashed line). Each panel shows a different pseudorapidity region, while the top and bottom panels show the low-Eγ Tand high-Eγ Trange respectively. The vertical error bars represent statistical uncertainties only. Fig. 2. Efficiency for simulated photons passing the generator-level isolation requirement, shown as a function of photon transverse energy Eγ Twith a different pseudorapidity region in each panel. The reconstruction (red circles), reconstruction plus identification (blue squares) and total selection (green triangles) efficiencies are shown separately. with the lowest values at the lowest Eγ T. The isolation efficiency is lowest at high Eγ T, most likely because the associated products of fragmentation photons are, on average, more energetic and collimated when the energy of the photon is higher. The largest inefficiency is due to the identification requirements. This identification efficiency is lowest at 20 GeV and increases with Eγ Tas higher-energy photons create larger and more identifiable showers in the calorimeter. It peaks around 100 GeV, and decreases with increasing Eγ Tdue to the difficulty of separating conversion electrons at high energy. In MC events, the ETresponse for prompt, isolated photons, defined as the ratio of the reconstructed to generator ET, is found to be within 1% of unity, with a resolution that decreases from 3% to 2% over the Eγ Trange of the measurement. The bin migration correction factors CMC are determined using the event simulations described in Section 4. They are defined as the bin-by-bin 234 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 Fig. 3. Summary of extrapolation factors applied to the measured pp √s=8TeV data to construct an approximate √s=8.16 TeV spectrum matching the shift of the centre-of-mass in p +Pb data plotted as a function of generator-level photon transverse energy. Here ηrepresents the boost of the centre-of-mass frame of 0.465. The factors determined using Jetphox (dashed lines) and Pythia 8 (solid lines) are shown for the three ηlab ranges used in the measurement (different colours). The relative difference between these two extrapolation methods is taken as a systematic uncertainty. ratio CMC =Npart MC /Nreco MC of the reconstructed, identified, and isolated photon Eγ Tspectrum, where Npart MC is the number in a given Eγ Tbin at the particle level and Nreco MC is the number in the corresponding bin at the reconstruction level. The nuclear modification factor RpPb can be expressed as a ratio of cross-sections in the following way: RpPb =(dσp+Pb→γ+X/dEγ T)/(A·dσpp→γ+X/dEγ T), (1) where the geometric factor Ais simply the number of nucleons in the Pb nucleus, 208. The reference pp spectrum is constructed using measurements of √s=8TeV pp data by ATLAS [9] that use the same particle-level isolation requirement. The 8TeV measurements in the regions |ηlab| <1.37 and 1.56 <|ηlab| <2.37 are used as the reference spectra for the central and the forward and backward rapidity data, after applying a multiplicative correction for the effects of the boost in the 8.16 TeV p +Pb system. For each kinematic region, extrapolation factors are determined as the ratio of photon cross sections from Jetphox calculations for pp collisions. The numerator has √s=8.16 TeV with a boost of the centre-of-mass corresponding to the p +Pb system, and the denominator has energy √s=8TeV with its rest frame corresponding with that of the laboratory reference frame. That is, the cross-sections in the numerator and denominator use the same ηlab regions, although in the former case this corresponds to a different centre-of-mass pseudorapidity. These factors are shown in Fig. 3and are applied as multiplicative factors to the measured 8TeV data. They are dominated by the effect from the boost of the p +Pb system, as the effect due to the difference in collision energy alone is less than 1% for all Eγ T. For −1.84 <η∗<0.91, or Eγ T<100 GeV at large rapidities, the factors are typically within a few percent of unity. However, at large Eγ T, where the rapidity distribution becomes steeper, the extrapolation factors become more sensitive to the rapidity shift from the centre-of-mass boost between the frames, and at large pseudorapidity they reach a factor of 2–3. An alternative set of factors, derived from the generator-level predictions of Pythia 8, are also shown in Fig. 3; these are used to assess the sensitivity of the extrapolation factors to the rapidity and Eγ T dependence of the model cross sections. 6. Systematic uncertainties The sources of systematic uncertainties affecting the measurement are described in this section, which is broken into two parts discussing the uncertainty in 1) the cross-section and 2) the nuclear modification factor RpPb, including its ratio between forward and backward pseudorapidity regions. 6.1. Cross-section uncertainty The major uncertainties in the cross-section can be divided into two main categories: those affecting the purity determination, which are dominant at low Eγ Twhere the sample purity is low, and those affecting the detector performance corrections, which are dominant at high Eγ T. All other sources tend to be weakly dependent on Eγ T. A summary is shown in Fig. 4. In each category, the uncertainty is the sum in quadrature of the individual components; the combined uncertainty is the sum in quadrature of all contributions, excluding those associated with the luminosity. The total uncertainties range from 15% at low and high Eγ T, where they are dominated by the purity and detector performance uncertainties respectively, to a minimum of approximately 6% at Eγ T≈100 GeV, where both of these uncertainties are modest. To assess the uncertainty in the purity determination, each boundary defining the sidebands used in the calculation is varied independently in order to understand the sensitivity of the Fig. 4. Summary of the relative sizes of major sources of systematic uncertainty in the cross-section measurement, as well as the combined uncertainty (excluding luminosity), shown as a function of photon transverse energy Eγ T. The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 235 measurement to the double sideband binning and correlation assumptions. The dominant uncertainty arises from uncertainty in the level of sideband correlation. This is estimated directly from data by dividing the non-isolated region in two subregions and calculating the ratio of identified to background-enhanced yields in each subregion. These ratios differ at the level of 10% which agrees with estimates from previous studies [10]. This ±10% variation in the sideband correlation yields a 13% uncertainty in the cross-section in the lowest Eγ Trange, decreasing to less than 1% for Eγ T>100 GeV. The inverted photon identification requirement for the background candidates is varied to be less or more restrictive about which shower shapes the background candidates are required to fail. This variation yields an uncertainty that is less than 1% for all Eγ Tin the forward and backward rapidity bins, but is significant at mid-rapidity (−1.84 <η∗<0.91) where it is 9% in the lowest Eγ Tbin and decreases to be less than 1% for Eγ T>100 GeV. Variations in the isolation energy threshold of ±1GeV have been shown to cover any difference between simulations and data [10]. These variations result in a 1–2% effect on the cross-section in the lowest Eγ Trange and less than 1% at higher Eγ T. The uncertainty associated with the inverted shower-shape was smoothed and symmetrised, however, the other uncertainties are derived asymmetrically from the positive and negative variations separately. Uncertainties associated with detector performance corrections are dominant at high Eγ T. A detailed description of the several components of the photon energy scale and resolution uncertainties are given in Ref. [10]. The impact of these on the measurement is determined by varying the reconstructed photon Eγ Tin simulation within the energy scale uncertainties and deriving alternative correction factors for positive and negative variations separately. Of these, the impact of the energy scale variation is dominant, giving a 10–15% contribution at 500 GeV in the forward and backward regions, decreasing to less than 2% at the lowest Eγ T. In the midrapidity region, the energy scale variation gives a 5% uncertainty at high Eγ T, decreasing to less than 1% at low Eγ T. Additionally, there are uncertainties associated with corrections for small differences in reconstruction, identification and isolation efficiencies observed between data and simulation [31]. These uncertainties are about 5% in the forward regions and low Eγ Tand less than 2% elsewhere. Systematic uncertainties related to modelling in simulation, luminosity, electron contamination, and other sources tend to be lower than those previously discussed. However, their combined effect is dominant in the mid-rapidity region and between 90 GeV and 250 GeV. To test the sensitivity of the measurement to the difference of isolation energy between particle-level and detector level in the simulation, the generator-level isolation definition is changed to better correspond to the reconstruction-level definition. The relative change in the cross-section after this deviation from the nominal is about 2% at low Eγ T, decreasing to about 1% at high Eγ T, for each pseudorapidity region, and is taken as a symmetric uncertainty. An uncertainty is assigned to cover the possible contribution of misreconstructed electrons, primarily from the decays of W±and Zbosons, to the selected photon yield. Based on simulation studies, and the results of previous measurements [9,10], this is assigned to be 1.3% for Eγ T<105 GeV in forward pseudorapidity regions, and 0.5% everywhere else. To test the beam orientation dependence, the cross-section is measured using the data from each beam configuration separately. The two measurements agree at the level of 1%, well above the statistical uncertainty for most Eγ Tbins. This difference is taken as a global, symmetric uncertainty in the combined results. To test the sensitivity to the relative fractions of direct and fragmentation photons in the MC samples, the simulation is weighted such that the fraction of direct photons is unity, that is, all photons in the sample are direct. This reflects a conservative difference compared with the default estimate of this fraction of about 50–80% from the MC samples. This variation gives a relative change in the cross-section of approximately 1% for all kinematic regions, which is taken as a systematic uncertainty. The uncertainty in the integrated luminosity of the combined data sample is 2.4% It is derived, following a methodology similar to that detailed in Ref. [42], and using the LUCID-2 detector for the baseline luminosity measurements [43], from calibration of the luminosity scale using x-y beam-separation scans. 6.2. RpPb uncertainty The nuclear modification factor RpPb is affected by systematic uncertainties associated with the p +Pb and pp measurements. The uncertainties in the differential cross-section of the pp reference data are obtained directly from Ref. [9]. Due to differences in photon reconstruction, energy calibration, isolation and identification procedures between the pp and p +Pb datasets, the uncertainties are treated as uncorrelated and added in quadrature. The uncertainty in the extrapolation of the pp Eγ Tspectrum at 8TeV is determined by using an alternative method to derive the multiplicative extrapolation factors. Instead of Jetphox, photon cross-sections for the 8TeV and rapidity-boosted √s=8.16 TeV kinematics are determined from Pythia 8. The extrapolation factors from both Jetphox and Pythia 8 are shown in Fig. 3. Additionally, Jetphox is run with an alternative PDF set to quantify the impact of a given PDF choice. The differences between the extrapolation factors from these two variations, which are at most a few percent in the kinematic region of the measurement and subdominant with respect to the other uncertainties in the cross-sections, are added in quadrature and used as an estimate of the uncertainty in the extrapolation procedure. For the measurement of the ratio of RpPb values (Eq. (1)) between the forward and backward pseudorapidity regions, each systematic variation affecting the purity and detector performance corrections is applied to the numerator and denominator in a coherent way, allowing them to partially cancel out in the ratio. All uncertainties in the other categories, except those from electron contamination and the beam direction difference, are treated as correlated. For this reason, they cancel out; notably the p +Pb luminosity and pp cross-section uncertainties cancel out completely. The extrapolation uncertainties are treated as independent and are added in quadrature to the other uncertainties in RpPb. The resulting uncertainty ranges from about 5% at the lowest Eγ T, where it is dominated by the uncertainty in the purity, to about 3% at mid-Eγ T, and again about 5% at high Eγ T, where it is dominated by uncertainty in the energy scale. A summary of the uncertainties in the forward-to-backward ratio is shown in Fig. 5. 7. Results Photon production cross-sections are shown in Fig. 6for photons with Eγ T>20 GeV in three pseudorapidity regions. The measured dσ/dEγ Tvalues decrease by five orders of magnitude over the complete Eγ Trange, which extends out to Eγ T≈500 GeV for photons at mid-rapidity. In Pythia 8, photons in this range typically arise from parton configurations in which the parton in the nucleus has Bjorken scale variable, xA, in the range 3 ×10−3 xA4 ×10−1. In the nuclear modified PDF (nPDF) picture, this range probes the so-called shadowing (suppression for xA0.1), anti-shadowing (enhancement for 0.1 xA0.3), and EMC (suppression for 0.3 xA0.7) regions [24]. The data are compared with an NLO pQCD calculation similar to that used in Ref. [3], where the data is similarly underestimated 236 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 Fig. 5. Summary of the relative size of major sources of systematic uncertainty in the forward-to-backward ratio of the nuclear modification factor RpPb, as well as the combined uncertainty, shown as a function of photon transverse energy Eγ T. The Reference extrapolation refers to the uncertainty related to the extrapolation of the previously measured 8TeV pp spectrum to 8.16 TeV and boosted kinematics. at low ET, but using the updated CT14 [44]PDF set for the freenucleon parton densities. Jetphox [23]is used to perform a full NLO pQCD calculation of the direct and fragmentation contributions to the cross-section. The BFG set II [45]of parton-to-photon fragmentation functions are used, the number of massless quark flavours is set to five, and the renormalisation, factorisation and fragmentation scales are chosen to be Eγ T. In addition to the calculation with the free-nucleon PDFs, separate calculations are performed with the EPPS16 [24] and nCTEQ15 [25]nPDF sets. The EPPS16 calculation uses the same free-proton PDF set, CT14, as the free-nucleon baseline to which the modifications are applied. The prediction is systematically lower than the data by up to 20% at low Eγ Tbut is closer to the data at higher Eγ T, consistent with the results of such comparisons in pp collisions at LHC energies [9,10]. A recent calculation of isolated photon production at NNLO found that the predicted cross-sections were systematically larger at low Eγ Tthan the NLO prediction [46], and thus may provide a better description of the data in this and previous measurements. Uncertainties associated with these calculations are assessed in a number of ways. Factorisation, renormalisation, and fragmentation scales are varied, up and down, by a factor of two as in Ref. [9]. The uncertainty is taken as the envelope formed by the minimum and maximum of each variation in every kinematic region and is dominant in most regions. PDF uncertainties are calculated via the standard CT14 error sets and correspond to a 68% confidence interval. Again following Ref. [9]the sensitivity to the choice of αSis evaluated by varying αSby ±0.002 around the central value of 0.118 in the calculation and PDF. Uncertainties from Fig. 6. Prompt, isolated photon cross-sections as a function of transverse energy Eγ T, shown for different centre-of-mass pseudorapidity, η∗, regions in each panel. The data are compared with Jetphox with the EPPS16 nuclear PDF set [24], with the ratio of theory to data shown in the lower panels. Yellow bands correspond to total systematic uncertainties in the data (not including the luminosity uncertainty), vertical bars correspond to the statistical uncertainties in the data, and the red bands correspond to the uncertainties in the theoretical calculation. The green box (at the far right) represents the 2.4% luminosity uncertainty. Fig. 7. A breakdown of all systematic uncertainties in the cross-section prediction from Jetphox with the EPPS16 nPDF set. The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 237 Fig. 8. Nuclear modification factor RpPb for isolated, prompt photons as a function of photon transverse energy Eγ T, shown for different centre-of-mass pseudorapidity, η∗, regions in each panel. The RpPb is measured using a reference which is a simulation-derived extrapolation from √s=8TeV pp data (see text). The data are identical in each row, but show comparisons with the expectations based on Jetphox with the EPPS16 nuclear PDF set (top) [24], with the nCTEQ15 nuclear PDF set (middle) [25], and with an initial-state energy-loss calculation (bottom) [4,5,26]. In all plots, the yellow bands and vertical bars correspond to total systematic and statistical uncertainties in the data respectively. In the top and middle panels, the red and purple bands correspond to the systematic uncertainties in the theoretical calculations. The green box (at the far right) represents the combined 2.4% p +Pb and 1.9% pp luminosity uncertainties. nPDFs are calculated from the error sets which correspond to 90% confidence intervals, as described in Ref. [24]. These are converted into uncertainty bands which correspond to a 68% confidence interval. A summary of each variation is shown in Fig. 7. Fig. 8shows the nuclear modification factor RpPb as a function of Eγ Tin different η∗regions. At forward rapidities (1.09 < η∗<1.90), the RpPb value is consistent with unity, indicating that nuclear effects are small. In Pythia 8, photons in this region typically arise from configurations with gluon partons from the Pb nucleus with xA≈10−2. Nuclear modification pulls the pQCD calculation down slightly for Eγ T<100 GeV, above which the modification reverses, indicating a crossover between shadowing and anti-shadowing regions. At mid-rapidity, nuclear effects are similarly small and consistent with unity at low Eγ T, but at higher Eγ T, there is a hint that RpPb is lower. This feature primarily reflects the different up- and down-quark composition of the nucleus relative to the proton and is more important at larger parton x. In this case, the larger relative down-quark density decreases the photon yield. This effect is evident in the Jetphox theory curve in blue dash-dotted line, which includes the proton–neutron asymmetry and the free-nucleon PDF set CT14. This effect is most pronounced at backward pseudorapidity where, in Pythia 8, the nuclear parton composition is typically a quark with xA≈0.2. Here, nPDF modification moves RpPb above the free-nucleon PDF calculation at low 238 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 Fig. 9. Ratio of the nuclear modification factor RpPb between forward and backward pseudorapidity for isolated, prompt photons as a function of photon transverse energy Eγ T. The data are identical in each panel, but show comparisons with the expectations based on Jetphox with the EPPS16 nuclear PDF set (top, left) [24]or with the nCTEQ15 nuclear PDF set (top, right) [25], and with an initial-state energy-loss calculation (bottom) [4,5,26]. The strength of the initial-state energy-loss effect is parameterised by λq, which represents the mean free path of partons in the nuclear medium and is smaller for a larger degree of energy loss. In all plots, the yellow bands and vertical bars correspond to total systematic and statistical uncertainties in the data respectively. In the left and right panels, the red and purple bands correspond to the systematic uncertainties in the calculations. Eγ Tbut below at high Eγ T, indicating the crossover from the antishadowing to the EMC region. The RpPb calculations including nPDFs consider only the nPDF uncertainty, since previous calculations have shown that the scale and PDF uncertainties cancel out almost completely in the kinematic region of the measurement [3], and no non-perturbative corrections are applied. Within the present uncertainties, the data are consistent with both the free-proton PDFs and with the small effects expected from a nuclear modification of the parton densities. The RpPb measurements are also compared with an initial-state energy-loss prediction that is calculated within the framework described in Refs. [4,5,24]. In this model, the energetic partons undergo multiple scattering in the cold nuclear medium, and thus lose energy due to this medium-induced gluon bremsstrahlung, before the hard collision. The calculation is performed with a parton–gluon momentum transfer μ =0.35 GeV and mean free path for quarks λq=1.5fm. Alternative calculations with a shorter path length (λq=1fm), and a control version with no initial-state energy loss, are also considered. The data disfavour a large suppression of the cross-section from initial-state energy-loss effects. The ratio of the RpPb values between forward and backward pseudorapidity, shown in Fig. 9, is studied as a way to reduce the effect of common systematic uncertainties and better isolate the magnitude of nuclear effects [47]. The remaining systematic uncertainty, discussed in Sec. 6.2, is dominated by the reference extrapolation and treated as uncorrelated between points. Below Eγ T≈100 GeV, this corresponds roughly to the ratio of RpPb from photons from gluon nuclear parton configurations in the shadowing xAregion to that from quark partons in the anti-shadowing region. This can be seen in the top two panels of Fig. 9, where the nuclear modification (red/purple bands) brings the Jetphox calculation below that of the free-nucleon PDF (blue curve), though the effect from EPPS16 is less significant. In contrast, the behaviour is reversed at higher Eγ Twhere the numerator probes the shadowing/anti-shadowing crossover region and the denominator moves deeper into the EMC region [24]. The data are consistent with the pQCD calculation before incorporating nuclear effects, except possibly in the region Eγ T<55 GeV, which is sensitive to the effects from gluon shadowing. At low Eγ T, the data are systematically higher than the calculations which incorporate nPDF effects, but approximately within their theoretical uncertainty. Additionally, in the lower plot of Fig. 9, the forward-to-backward ratios are compared with predictions from a model incorporating initial-state energy loss. The data show a preference for no or only a limited amount of energy loss. 8. Conclusion This letter presents a measurement of the inclusive prompt, isolated photon cross-section in p +Pb collisions at √sNN = 8.16 TeV, using a dataset corresponding to an integrated luminosity of 165 nb−1recorded by the ATLAS experiment at the LHC. The differential cross-section as a function of the photon transverse energy is reported in three pseudorapidity regions in the The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 245 P. Liu 18, Y. Liu 15a, Y.L. Liu58a, Y.W. Liu 58a, M. Livan68a,68b, A. Lleres56, J. Llorente Merino15a, S.L. Lloyd90, C.Y. Lo61b, F. Lo Sterzo41, E.M. Lobodzinska44, P. Loch 7, T. Lohse 19, K. Lohwasser146, M. Lokajicek138, J.D. Long170, R.E. Long87, L. Longo65a,65b, K.A. Looper123, J.A. Lopez144b, I. Lopez Paz98, A. Lopez Solis146, J. Lorenz112, N. Lorenzo Martinez5, M. Losada22, P.J. Lösel112, A. Lösle50, X. Lou44, X. Lou15a, A. Lounis129, J. Love 6, P.A. Love 87, J.J. Lozano Bahilo 171, H. Lu 61a, M. Lu58a, Y.J. Lu 62, H.J. Lubatti145, C. Luci 70a,70b, A. Lucotte56, C. Luedtke50, F. Luehring63, I. Luise 133, L. Luminari70a, B. Lund-Jensen151, M.S. Lutz100, P.M. Luzi133, D. Lynn 26b, R. Lysak138, E. Lytken94, F. Lyu 15a, V. Lyubushkin77, T. Lyubushkina 77, H. Ma 26b, L.L. Ma58b, Y. Ma 58b, G. Maccarrone49, A. Macchiolo113, C.M. Macdonald146, J. Machado Miguens134,137b, D. Madaffari171, R. Madar37, W.F. Mader46, N. Madysa46, J. Maeda80, K. Maekawa 160, S. Maeland 17, T. Maeno26b, M. Maerker46, A.S. Maevskiy111, V. Magerl50, D.J. Mahon38, C. Maidantchik78b, T. Maier112, A. Maio137a,137b,137d, O. Majersky29a, S. Majewski128, Y. Makida 79, N. Makovec129, B. Malaescu 133, Pa. Malecki 82, V.P. Maleev135, F. Malek 56, U. Mallik75, D. Malon6, C. Malone31, S. Maltezos10, S. Malyukov35, J. Mamuzic171, G. Mancini49, I. Mandi´ c89, J. Maneira137a, L. Manhaes de Andrade Filho78a, J. Manjarres Ramos46, K.H. Mankinen94, A. Mann112, A. Manousos74, B. Mansoulie 142, S. Manzoni66a,66b, A. Marantis159, G. Marceca30, L. March52, L. Marchese132, G. Marchiori133, M. Marcisovsky138, C. Marcon94, C.A. Marin Tobon35, M. Marjanovic37, F. Marroquim78b, Z. Marshall18, M.U.F Martensson169, S. Marti-Garcia171, C.B. Martin123, T.A. Martin 175, V.J. Martin 48, B. Martin dit Latour17, M. Martinez14,y, V.I. Martinez Outschoorn100, S. Martin-Haugh141, V.S. Martoiu28b, A.C. Martyniuk92, A. Marzin35, L. Masetti97, T. Mashimo160, R. Mashinistov108, J. Masik 98, A.L. Maslennikov120b,120a, L.H. Mason102, L. Massa71a,71b, P. Massarotti67a,67b, P. Mastrandrea152, A. Mastroberardino40b,40a, T. Masubuchi160, P. Mättig24, J. Maurer28b, B. Maˇ cek89, S.J. Maxfield 88, D.A. Maximov120b,120a, R. Mazini155, I. Maznas159, S.M. Mazza 143, S.P. Mc Kee103, A. McCarn Deiana41, T.G. McCarthy 113, L.I. McClymont92, W.P. McCormack18, E.F. McDonald 102, J.A. Mcfayden35, G. Mchedlidze51, M.A. McKay41, K.D. McLean173, S.J. McMahon 141, P.C. McNamara 102, C.J. McNicol 175, R.A. McPherson173,ac, J.E. Mdhluli32c, Z.A. Meadows100, S. Meehan145, T.M. Megy 50, S. Mehlhase112, A. Mehta88, T. Meideck56, B. Meirose42, D. Melini 171,aw, B.R. Mellado Garcia32c, J.D. Mellenthin51, M. Melo 29a, F. Meloni 44, A. Melzer24, S.B. Menary 98, E.D. Mendes Gouveia137a, L. Meng88, X.T. Meng103, S. Menke113, E. Meoni 40b,40a, S. Mergelmeyer19, S.A.M. Merkt136, C. Merlassino20, P. Mermod 52, L. Merola67a,67b, C. Meroni66a, A. Messina70a,70b, J. Metcalfe6, A.S. Mete168, C. Meyer63, J. Meyer157, J-P. Meyer 142, H. Meyer Zu Theenhausen59a, F. Miano 153, R.P. Middleton141, L. Mijovi´ c48, G. Mikenberg177, M. Mikestikova138, M. Mikuž 89, M. Milesi 102, A. Milic164, D.A. Millar90, D.W. Miller 36, A. Milov177, D.A. Milstead43a,43b, R.A. Mina150,q, A.A. Minaenko121, M. Miñano Moya171, I.A. Minashvili156b, A.I. Mincer122, B. Mindur81a, M. Mineev77, Y. Minegishi160, Y. Ming178, L.M. Mir14, A. Mirto65a,65b, K.P. Mistry134, T. Mitani 176, J. Mitrevski112, V.A. Mitsou171, M. Mittal 58c, A. Miucci20, P.S. Miyagawa146, A. Mizukami79, J.U. Mjörnmark94, T. Mkrtchyan181, M. Mlynarikova140, T. Moa 43a,43b, K. Mochizuki107, P. Mogg50, S. Mohapatra38, S. Molander43a,43b, R. Moles-Valls 24, M.C. Mondragon104, K. Mönig44, J. Monk 39, E. Monnier99, A. Montalbano149, J. Montejo Berlingen35, F. Monticelli86, S. Monzani66a, N. Morange129, D. Moreno22, M. Moreno Llácer35, P. Morettini53b, M. Morgenstern118, S. Morgenstern46, D. Mori149, M. Morii57, M. Morinaga176, V. Morisbak131, A.K. Morley35, G. Mornacchi35, A.P. Morris 92, J.D. Morris90, L. Morvaj152, P. Moschovakos10, M. Mosidze 156b, H.J. Moss146, J. Moss 150,n, K. Motohashi162, R. Mount 150, E. Mountricha35, E.J.W. Moyse100, S. Muanza99, F. Mueller113, J. Mueller136, R.S.P. Mueller112, D. Muenstermann87, G.A. Mullier94, F.J. Munoz Sanchez98, P. Murin29b, W.J. Murray 175,141, A. Murrone66a,66b, M. Muškinja89, C. Mwewa 32a, A.G. Myagkov121,an, J. Myers128, M. Myska139, B.P. Nachman18, O. Nackenhorst45, K. Nagai132, K. Nagano79, Y. Nagasaka 60, M. Nagel50, E. Nagy99, A.M. Nairz35, Y. Nakahama 115, K. Nakamura79, T. Nakamura 160, I. Nakano124, H. Nanjo130, F. Napolitano 59a, R.F. Naranjo Garcia44, R. Narayan11, D.I. Narrias Villar 59a, I. Naryshkin135, T. Naumann 44, G. Navarro22, R. Nayyar7, H.A. Neal103,∗, P.Y. Nechaeva 108, T.J. Neep142, A. Negri68a,68b, M. Negrini23b, S. Nektarijevic117, C. Nellist51, M.E. Nelson132, S. Nemecek138, P. Nemethy 122, M. Nessi35,f, M.S. Neubauer170, M. Neumann179, P.R. Newman 21, T.Y. Ng 61c, Y.S. Ng 19, Y.W.Y. Ng 168, H.D.N. Nguyen99, T. Nguyen Manh107, E. Nibigira37, R.B. Nickerson132, R. Nicolaidou 142, D.S. Nielsen39, J. Nielsen143, N. Nikiforou11, V. Nikolaenko121,an, 246 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 I. Nikolic-Audit 133, K. Nikolopoulos21, P. Nilsson26b, H.R. Nindhito52, Y. Ninomiya 79, A. Nisati70a, N. Nishu58c, R. Nisius113, I. Nitsche45, T. Nitta 176, T. Nobe 160, Y. Noguchi83, M. Nomachi130, I. Nomidis133, M.A. Nomura26b, T. Nooney90, M. Nordberg35, N. Norjoharuddeen132, T. Novak 89, O. Novgorodova46, R. Novotny 139, L. Nozka127, K. Ntekas168, E. Nurse92, F. Nuti 102, F.G. Oakham 33,av, H. Oberlack113, J. Ocariz133, A. Ochi80, I. Ochoa38, J.P. Ochoa-Ricoux144a, K. O’Connor27, S. Oda85, S. Odaka79, S. Oerdek51, A. Oh98, S.H. Oh47, C.C. Ohm151, H. Oide53b,53a, M.L. Ojeda 164, H. Okawa 166, Y. Okazaki83, Y. Okumura160, T. Okuyama 79, A. Olariu28b, L.F. Oleiro Seabra137a, S.A. Olivares Pino144a, D. Oliveira Damazio 26b, J.L. Oliver1, M.J.R. Olsson 36, A. Olszewski 82, J. Olszowska82, D.C. O’Neil149, A. Onofre137a,137e, K. Onogi115, P.U.E. Onyisi11, H. Oppen131, M.J. Oreglia36, G.E. Orellana86, Y. Oren 158, D. Orestano72a,72b, N. Orlando61b, A.A. O’Rourke44, R.S. Orr164, B. Osculati53b,53a,∗, V. O’Shea 55, R. Ospanov58a, G. Otero y Garzon30, H. Otono85, M. Ouchrif34d, J. Ouellette26a, F. Ould-Saada131, A. Ouraou142, Q. Ouyang15a, M. Owen55, R.E. Owen21, V.E. Ozcan 12c, N. Ozturk8, J. Pacalt127, H.A. Pacey31, K. Pachal149, A. Pacheco Pages14, L. Pacheco Rodriguez 142, C. Padilla Aranda14, S. Pagan Griso18, M. Paganini180, G. Palacino63, S. Palazzo48, S. Palestini35, M. Palka81b, D. Pallin37, I. Panagoulias10, C.E. Pandini35, J.G. Panduro Vazquez91, P. Pani 35, G. Panizzo64a,64c, L. Paolozzi52, K. Papageorgiou9,j, A. Paramonov6, D. Paredes Hernandez61b, S.R. Paredes Saenz 132, B. Parida163, T.H. Park 33, A.J. Parker87, K.A. Parker44, M.A. Parker31, F. Parodi 53b,53a, J.A. Parsons38, U. Parzefall 50, V.R. Pascuzzi164, J.M.P. Pasner143, E. Pasqualucci70a, S. Passaggio53b, F. Pastore 91, P. Pasuwan 43a,43b, S. Pataraia97, J.R. Pater98, A. Pathak178,k, T. Pauly 35, B. Pearson113, M. Pedersen131, L. Pedraza Diaz117, R. Pedro137a,137b, S.V. Peleganchuk120b,120a, O. Penc138, C. Peng15d, H. Peng58a, B.S. Peralva78a, M.M. Perego129, A.P. Pereira Peixoto137a, D.V. Perepelitsa26b, F. Peri 19, L. Perini66a,66b, H. Pernegger35, S. Perrella67a,67b, V.D. Peshekhonov77,∗, K. Peters44, R.F.Y. Peters98, B.A. Petersen35, T.C. Petersen 39, E. Petit56, A. Petridis1, C. Petridou159, P. Petroff 129, M. Petrov132, F. Petrucci72a,72b, M. Pettee180, N.E. Pettersson100, A. Peyaud142, R. Pezoa144b, T. Pham 102, F.H. Phillips104, P.W. Phillips141, M.W. Phipps170, G. Piacquadio152, E. Pianori 18, A. Picazio100, R.H. Pickles98, R. Piegaia30, J.E. Pilcher36, A.D. Pilkington98, M. Pinamonti71a,71b, J.L. Pinfold3, M. Pitt 177, L. Pizzimento71a,71b, M.-A. Pleier26b, V. Pleskot140, E. Plotnikova77, D. Pluth 76, P. Podberezko120b,120a, R. Poettgen94, R. Poggi52, L. Poggioli129, I. Pogrebnyak104, D. Pohl24, I. Pokharel51, G. Polesello68a, A. Poley18, A. Policicchio70a,70b, R. Polifka35, A. Polini23b, C.S. Pollard44, V. Polychronakos26b, D. Ponomarenko110, L. Pontecorvo35, G.A. Popeneciu28d, D.M. Portillo Quintero133, S. Pospisil139, K. Potamianos44, I.N. Potrap77, C.J. Potter31, H. Potti11, T. Poulsen 94, J. Poveda35, T.D. Powell 146, M.E. Pozo Astigarraga35, P. Pralavorio 99, S. Prell76, D. Price 98, M. Primavera65a, S. Prince101, M.L. Proffitt145, N. Proklova110, K. Prokofiev61c, F. Prokoshin144b, S. Protopopescu26b, J. Proudfoot6, M. Przybycien81a, A. Puri170, P. Puzo 129, J. Qian 103, Y. Qin 98, A. Quadt51, M. Queitsch-Maitland44, A. Qureshi1, P. Rados 102, F. Ragusa 66a,66b, G. Rahal95, J.A. Raine52, S. Rajagopalan26b, A. Ramirez Morales90, K. Ran15a, T. Rashid 129, S. Raspopov5, M.G. Ratti 66a,66b, D.M. Rauch44, F. Rauscher112, S. Rave97, B. Ravina146, I. Ravinovich177, J.H. Rawling98, M. Raymond35, A.L. Read131, N.P. Readioff56, M. Reale65a,65b, D.M. Rebuzzi68a,68b, A. Redelbach174, G. Redlinger26b, R. Reece143, R.G. Reed32c, K. Reeves42, L. Rehnisch19, J. Reichert134, D. Reikher158, A. Reiss97, A. Rej148, C. Rembser35, H. Ren15d, M. Rescigno70a, S. Resconi66a, E.D. Resseguie134, S. Rettie172, E. Reynolds21, O.L. Rezanova120b,120a, P. Reznicek 140, E. Ricci 73a,73b, R. Richter113, S. Richter44, E. Richter-Was 81b, O. Ricken24, M. Ridel133, P. Rieck113, C.J. Riegel179, O. Rifki 44, M. Rijssenbeek152, A. Rimoldi68a,68b, M. Rimoldi 20, L. Rinaldi23b, G. Ripellino151, B. Risti´ c87, E. Ritsch35, I. Riu14, J.C. Rivera Vergara144a, F. Rizatdinova 126, E. Rizvi 90, C. Rizzi14, R.T. Roberts98, S.H. Robertson101,ac, D. Robinson31, J.E.M. Robinson44, A. Robson55, E. Rocco97, C. Roda69a,69b, Y. Rodina 99, S. Rodriguez Bosca171, A. Rodriguez Perez14, D. Rodriguez Rodriguez171, A.M. Rodríguez Vera165b, S. Roe35, C.S. Rogan57, O. Røhne131, R. Röhrig 113, C.P.A. Roland 63, J. Roloff57, A. Romaniouk110, M. Romano23b,23a, N. Rompotis88, M. Ronzani122, L. Roos133, S. Rosati70a, K. Rosbach50, N-A. Rosien51, B.J. Rosser134, E. Rossi44, E. Rossi72a,72b, E. Rossi67a,67b, L.P. Rossi53b, L. Rossini66a,66b, J.H.N. Rosten31, R. Rosten14, M. Rotaru28b, J. Rothberg145, D. Rousseau129, D. Roy32c, A. Rozanov99, Y. Rozen 157, X. Ruan32c, F. Rubbo150, F. Rühr 50, A. Ruiz-Martinez171, Z. Rurikova50, N.A. Rusakovich77, H.L. Russell101, J.P. Rutherfoord7, E.M. Rüttinger44,l, Y.F. Ryabov 135, M. Rybar38, G. Rybkin 129, S. Ryu 6, A. Ryzhov121, G.F. Rzehorz51, The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 247 P. Sabatini51, G. Sabato118, S. Sacerdoti129, H.F-W. Sadrozinski143, R. Sadykov77, F. Safai Tehrani70a, P. Saha 119, M. Sahinsoy59a, A. Sahu179, M. Saimpert44, M. Saito160, T. Saito160, H. Sakamoto160, A. Sakharov122,am, D. Salamani52, G. Salamanna 72a,72b, J.E. Salazar Loyola144b, P.H. Sales De Bruin169, D. Salihagic113,∗, A. Salnikov150, J. Salt171, D. Salvatore40b,40a, F. Salvatore 153, A. Salvucci61a,61b,61c, A. Salzburger 35, J. Samarati35, D. Sammel50, D. Sampsonidis159, D. Sampsonidou159, J. Sánchez171, A. Sanchez Pineda 64a,64c, H. Sandaker131, C.O. Sander44, M. Sandhoff 179, C. Sandoval22, D.P.C. Sankey141, M. Sannino53b,53a, Y. Sano 115, A. Sansoni49, C. Santoni37, H. Santos137a, I. Santoyo Castillo153, A. Santra171, A. Sapronov77, J.G. Saraiva137a,137d, O. Sasaki79, K. Sato166, E. Sauvan5, P. Savard 164,av, N. Savic113, R. Sawada160, C. Sawyer141, L. Sawyer93,ak, C. Sbarra23b, A. Sbrizzi23a, T. Scanlon 92, J. Schaarschmidt145, P. Schacht 113, B.M. Schachtner112, D. Schaefer36, L. Schaefer134, J. Schaeffer97, S. Schaepe35, U. Schäfer 97, A.C. Schaffer129, D. Schaile112, R.D. Schamberger152, N. Scharmberg98, V.A. Schegelsky135, D. Scheirich140, F. Schenck19, M. Schernau168, C. Schiavi 53b,53a, S. Schier143, L.K. Schildgen24, Z.M. Schillaci27, E.J. Schioppa35, M. Schioppa40b,40a, K.E. Schleicher50, S. Schlenker35, K.R. Schmidt-Sommerfeld113, K. Schmieden 35, C. Schmitt97, S. Schmitt44, S. Schmitz97, J.C. Schmoeckel44, U. Schnoor50, L. Schoeffel142, A. Schoening59b, E. Schopf132, M. Schott97, J.F.P. Schouwenberg117, J. Schovancova35, S. Schramm52, A. Schulte97, H-C. Schultz-Coulon59a, M. Schumacher50, B.A. Schumm143, Ph. Schune142, A. Schwartzman150, T.A. Schwarz 103, Ph. Schwemling142, R. Schwienhorst104, A. Sciandra24, G. Sciolla27, M. Scornajenghi40b,40a, F. Scuri69a, F. Scutti 102, L.M. Scyboz113, C.D. Sebastiani70a,70b, P. Seema19, S.C. Seidel 116, A. Seiden143, T. Seiss 36, J.M. Seixas78b, G. Sekhniaidze67a, K. Sekhon103, S.J. Sekula41, N. Semprini-Cesari23b,23a, S. Sen47, S. Senkin37, C. Serfon 131, L. Serin129, L. Serkin64a,64b, M. Sessa58a, H. Severini125, F. Sforza 167, A. Sfyrla52, E. Shabalina 51, J.D. Shahinian143, N.W. Shaikh43a,43b, D. Shaked Renous177, L.Y. Shan15a, R. Shang170, J.T. Shank25, M. Shapiro18, A.S. Sharma1, A. Sharma132, P.B. Shatalov109, K. Shaw 153, S.M. Shaw98, A. Shcherbakova135, Y. Shen125, N. Sherafati33, A.D. Sherman25, P. Sherwood 92, L. Shi155,ar, S. Shimizu79, C.O. Shimmin180, Y. Shimogama176, M. Shimojima 114, I.P.J. Shipsey132, S. Shirabe85, M. Shiyakova77, J. Shlomi177, A. Shmeleva108, D. Shoaleh Saadi107, M.J. Shochet36, S. Shojaii102, D.R. Shope125, S. Shrestha123, E. Shulga110, P. Sicho 138, A.M. Sickles170, P.E. Sidebo 151, E. Sideras Haddad32c, O. Sidiropoulou35, A. Sidoti23b,23a, F. Siegert46, Dj. Sijacki16, J. Silva 137a, M. Silva Jr. 178, M.V. Silva Oliveira78a, S.B. Silverstein43a, S. Simion129, E. Simioni97, M. Simon 97, R. Simoniello97, P. Sinervo164, N.B. Sinev128, M. Sioli23b,23a, I. Siral103, S.Yu. Sivoklokov111, J. Sjölin 43a,43b, P. Skubic125, M. Slater21, T. Slavicek139, M. Slawinska 82, K. Sliwa167, R. Slovak140, V. Smakhtin177, B.H. Smart5, J. Smiesko29a, N. Smirnov110, S.Yu. Smirnov110, Y. Smirnov110, L.N. Smirnova111, O. Smirnova94, J.W. Smith51, M. Smizanska87, K. Smolek139, A. Smykiewicz82, A.A. Snesarev108, I.M. Snyder 128, S. Snyder26b, R. Sobie173,ac, A.M. Soffa168, A. Soffer158, A. Søgaard48, F. Sohns 51, G. Sokhrannyi89, C.A. Solans Sanchez35, M. Solar139, E.Yu. Soldatov110, U. Soldevila171, A.A. Solodkov121, A. Soloshenko77, O.V. Solovyanov121, V. Solovyev135, P. Sommer146, H. Son167, W. Song 141, W.Y. Song 165b, A. Sopczak139, F. Sopkova 29b, C.L. Sotiropoulou 69a,69b, S. Sottocornola68a,68b, R. Soualah64a,64c,i, A.M. Soukharev120b,120a, D. South44, S. Spagnolo65a,65b, M. Spalla 113, M. Spangenberg175, F. Spanò 91, D. Sperlich19, T.M. Spieker59a, R. Spighi23b, G. Spigo35, L.A. Spiller102, D.P. Spiteri55, M. Spousta140, A. Stabile66a,66b, R. Stamen59a, S. Stamm19, E. Stanecka82, R.W. Stanek6, C. Stanescu72a, B. Stanislaus132, M.M. Stanitzki44, B. Stapf118, E.A. Starchenko121, G.H. Stark143, J. Stark56, S.H Stark39, P. Staroba 138, P. Starovoitov 59a, S. Stärz101, R. Staszewski82, M. Stegler44, P. Steinberg26b, B. Stelzer149, H.J. Stelzer35, O. Stelzer-Chilton165a, H. Stenzel54, T.J. Stevenson153, G.A. Stewart35, M.C. Stockton35, G. Stoicea28b, P. Stolte 51, S. Stonjek113, A. Straessner46, J. Strandberg151, S. Strandberg43a,43b, M. Strauss125, P. Strizenec29b, R. Ströhmer174, D.M. Strom128, R. Stroynowski41, A. Strubig48, S.A. Stucci 26b, B. Stugu17, J. Stupak125, N.A. Styles44, D. Su150, J. Su 136, S. Suchek59a, Y. Sugaya130, M. Suk139, V.V. Sulin 108, M.J. Sullivan 88, D.M.S. Sultan52, S. Sultansoy4c, T. Sumida 83, S. Sun 103, X. Sun3, K. Suruliz153, C.J.E. Suster154, M.R. Sutton153, S. Suzuki79, M. Svatos138, M. Swiatlowski36, S.P. Swift2, A. Sydorenko97, I. Sykora29a, M. Sykora140, T. Sykora 140, D. Ta 97, K. Tackmann44,z, J. Taenzer158, A. Taffard168, R. Tafirout165a, E. Tahirovic 90, N. Taiblum 158, H. Takai26b, R. Takashima84, E.H. Takasugi113, K. Takeda 80, T. Takeshita147, Y. Takubo 79, M. Talby99, A.A. Talyshev120b,120a, J. Tanaka160, M. Tanaka 162, R. Tanaka129, B.B. Tannenwald123, 248 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 S. Tapia Araya170, S. Tapprogge97, A. Tarek Abouelfadl Mohamed133, S. Tarem157, G. Tarna 28b,e, G.F. Tartarelli66a, P. Tas 140, M. Tasevsky138, T. Tashiro83, E. Tassi40b,40a, A. Tavares Delgado137a,137b, Y. Tayalati 34e, A.J. Taylor48, G.N. Taylor102, P.T.E. Taylor102, W. Taylor 165b, A.S. Tee87, R. Teixeira De Lima150, P. Teixeira-Dias91, H. Ten Kate35, J.J. Teoh118, S. Terada79, K. Terashi160, J. Terron96, S. Terzo14, M. Testa49, R.J. Teuscher164,ac, S.J. Thais 180, T. Theveneaux-Pelzer44, F. Thiele39, D.W. Thomas91, J.P. Thomas21, A.S. Thompson55, P.D. Thompson21, L.A. Thomsen180, E. Thomson134, Y. Tian 38, R.E. Ticse Torres51, V.O. Tikhomirov108,ao, Yu.A. Tikhonov120b,120a, S. Timoshenko110, P. Tipton 180, S. Tisserant99, K. Todome162, S. Todorova-Nova5, S. Todt 46, J. Tojo85, S. Tokár29a, K. Tokushuku79, E. Tolley123, K.G. Tomiwa32c, M. Tomoto115, L. Tompkins150,q, K. Toms116, B. Tong 57, P. Tornambe50, E. Torrence128, H. Torres46, E. Torró Pastor145, C. Tosciri132, J. Toth99,ab, F. Touchard 99, D.R. Tovey146, C.J. Treado122, T. Trefzger174, F. Tresoldi153, A. Tricoli 26b, I.M. Trigger165a, S. Trincaz-Duvoid133, W. Trischuk 164, B. Trocmé56, A. Trofymov129, C. Troncon66a, M. Trovatelli173, F. Trovato 153, L. Truong32b, M. Trzebinski82, A. Trzupek82, F. Tsai 44, J.C-L. Tseng132, P.V. Tsiareshka105,ai, A. Tsirigotis159, N. Tsirintanis9, V. Tsiskaridze152, E.G. Tskhadadze156a, I.I. Tsukerman109, V. Tsulaia 18, S. Tsuno79, D. Tsybychev152,163, Y. Tu 61b, A. Tudorache28b, V. Tudorache28b, T.T. Tulbure 28a, A.N. Tuna57, S. Turchikhin77, D. Turgeman177, I. Turk Cakir4b,t, R.J. Turner21, R.T. Turra66a, P.M. Tuts 38, STzamarias 159, E. Tzovara97, G. Ucchielli45, I. Ueda79, M. Ughetto43a,43b, F. Ukegawa 166, G. Unal35, A. Undrus26b, G. Unel168, F.C. Ungaro 102, Y. Unno79, K. Uno160, J. Urban29b, P. Urquijo 102, G. Usai 8, J. Usui79, L. Vacavant99, V. Vacek139, B. Vachon101, K.O.H. Vadla131, A. Vaidya92, C. Valderanis112, E. Valdes Santurio43a,43b, M. Valente52, S. Valentinetti23b,23a, A. Valero171, L. Valéry44, R.A. Vallance21, A. Vallier5, J.A. Valls Ferrer171, T.R. Van Daalen14, H. Van der Graaf118, P. Van Gemmeren6, I. Van Vulpen118, M. Vanadia 71a,71b, W. Vandelli35, A. Vaniachine163, P. Vankov 118, R. Vari70a, E.W. Varnes7, C. Varni53b,53a, T. Varol 41, D. Varouchas129, K.E. Varvell154, G.A. Vasquez144b, J.G. Vasquez180, F. Vazeille 37, D. Vazquez Furelos14, T. Vazquez Schroeder35, J. Veatch51, V. Vecchio72a,72b, L.M. Veloce164, F. Veloso137a,137c, S. Veneziano70a, A. Ventura65a,65b, N. Venturi 35, V. Vercesi68a, M. Verducci72a,72b, C.M. Vergel Infante76, C. Vergis24, W. Verkerke118, A.T. Vermeulen118, J.C. Vermeulen118, M.C. Vetterli149,av, N. Viaux Maira144b, M. Vicente Barreto Pinto52, I. Vichou170,∗, T. Vickey 146, O.E. Vickey Boeriu146, G.H.A. Viehhauser132, S. Viel 18, L. Vigani132, M. Villa 23b,23a, M. Villaplana Perez66a,66b, E. Vilucchi49, M.G. Vincter33, V.B. Vinogradov 77, A. Vishwakarma44, C. Vittori23b,23a, I. Vivarelli153, S. Vlachos10, M. Vogel179, P. Vokac 139, G. Volpi 14, S.E. von Buddenbrock32c, E. Von Toerne 24, V. Vorobel140, K. Vorobev110, M. Vos 171, J.H. Vossebeld 88, N. Vranjes16, M. Vranjes Milosavljevic16, V. Vrba139, M. Vreeswijk118, T. Šfiligoj89, R. Vuillermet35, I. Vukotic36, T. Ženiš 29a, L. Živkovi´ c16, P. Wagner 24, W. Wagner 179, J. Wagner-Kuhr112, H. Wahlberg86, S. Wahrmund 46, K. Wakamiya80, V.M. Walbrecht113, J. Walder87, R. Walker112, S.D. Walker91, W. Walkowiak 148, V. Wallangen43a,43b, A.M. Wang57, C. Wang58b, F. Wang 178, H. Wang18, H. Wang3, J. Wang 154, J. Wang59b, P. Wang 41, Q. Wang 125, R.-J. Wang133, R. Wang 58a, R. Wang6, S.M. Wang155, W.T. Wang 58a, W. Wang 15c,ad, W.X. Wang 58a,ad, Y. Wang 58a,al, Z. Wang58c, C. Wanotayaroj44, A. Warburton101, C.P. Ward31, D.R. Wardrope92, A. Washbrook48, P.M. Watkins21, A.T. Watson21, M.F. Watson21, G. Watts145, S. Watts98, B.M. Waugh92, A.F. Webb11, S. Webb97, C. Weber180, M.S. Weber20, S.A. Weber33, S.M. Weber 59a, A.R. Weidberg132, J. Weingarten45, M. Weirich97, C. Weiser50, P.S. Wells35, T. Wenaus 26b, T. Wengler 35, S. Wenig 35, N. Wermes24, M.D. Werner76, P. Werner35, M. Wessels59a, T.D. Weston20, K. Whalen 128, N.L. Whallon 145, A.M. Wharton87, A.S. White103, A. White8, M.J. White1, R. White144b, D. Whiteson168, B.W. Whitmore87, F.J. Wickens 141, W. Wiedenmann 178, M. Wielers141, C. Wiglesworth39, L.A.M. Wiik-Fuchs50, F. Wilk 98, H.G. Wilkens35, L.J. Wilkins91, H.H. Williams134, S. Williams 31, C. Willis 104, S. Willocq100, J.A. Wilson21, I. Wingerter-Seez5, E. Winkels153, F. Winklmeier128, O.J. Winston153, B.T. Winter50, M. Wittgen150, M. Wobisch93, A. Wolf97, T.M.H. Wolf 118, R. Wolff99, J. Wollrath50, M.W. Wolter82, H. Wolters137a,137c, V.W.S. Wong172, N.L. Woods143, S.D. Worm21, B.K. Wosiek82, K.W. Wo´zniak 82, K. Wraight 55, M. Wu36, S.L. Wu178, X. Wu52, Y. Wu 58a, T.R. Wyatt 98, B.M. Wynne48, S. Xella39, Z. Xi 103, L. Xia 175, D. Xu15a, H. Xu58a,e, L. Xu26b, T. Xu 142, W. Xu 103, Z. Xu150, B. Yabsley154, S. Yacoob32a, K. Yajima130, D.P. Yallup92, D. Yamaguchi162, Y. Yamaguchi162, A. Yamamoto79, T. Yamanaka 160, F. Yamane 80, M. Yamatani160, T. Yamazaki 160, Y. Yamazaki 80, Z. Yan25, H.J. Yang58c,58d, H.T. Yang18, S. Yang75, The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 249 Y. Yang 160, Z. Yang17, W-M. Yao18, Y.C. Yap 44, Y. Yasu 79, E. Yatsenko58c,58d, J. Ye41, S. Ye26b, I. Yeletskikh77, E. Yigitbasi25, E. Yildirim 97, K. Yorita 176, K. Yoshihara134, C.J.S. Young35, C. Young150, J. Yu8, J. Yu 76, X. Yue59a, S.P.Y. Yuen24, B. Zabinski 82, G. Zacharis10, E. Zaffaroni52, R. Zaidan14, A.M. Zaitsev121,an, T. Zakareishvili156b, N. Zakharchuk33, S. Zambito57, D. Zanzi 35, D.R. Zaripovas55, S.V. Zeißner45, C. Zeitnitz179, G. Zemaityte132, J.C. Zeng170, Q. Zeng150, O. Zenin 121, D. Zerwas 129, M. Zgubiˇ c132, D.F. Zhang58b, D. Zhang103, F. Zhang 178, G. Zhang58a, G. Zhang15b, H. Zhang15c, J. Zhang6, L. Zhang 15c, L. Zhang 58a, M. Zhang170, P. Zhang 15c, R. Zhang 58a, R. Zhang 24, X. Zhang58b, Y. Zhang 15d, Z. Zhang129, P. Zhao 47, Y. Zhao 58b,129,aj, Z. Zhao 58a, A. Zhemchugov77, Z. Zheng103, D. Zhong170, B. Zhou 103, C. Zhou 178, M.S. Zhou 15d, M. Zhou152, N. Zhou58c, Y. Zhou 7, C.G. Zhu 58b, H.L. Zhu58a, H. Zhu 15a, J. Zhu 103, Y. Zhu 58a, X. Zhuang15a, K. Zhukov108, V. Zhulanov120b,120a, A. Zibell174, D. Zieminska63, N.I. Zimine77, S. Zimmermann 50, Z. Zinonos113, M. Ziolkowski148, G. Zobernig178, A. Zoccoli23b,23a, K. Zoch51, T.G. Zorbas 146, R. Zou 36, M. Zur Nedden19, L. Zwalinski35 1Department of Physics, University of Adelaide, Adelaide, Australia 2Physics Department, SUNY Albany, Albany, NY, United States of America 3Department of Physics, University of Alberta, Edmonton, AB, Canada 4(a)Department of Physics, Ankara University, Ankara; (b)Istanbul Aydin University, Istanbul; (c)Division of Physics, TOBB University of Economics and Technology, Ankara, Turkey 5LAPP, Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS/IN2P3, Annecy, France 6High Energy Physics Division, Argonne National Laboratory, Argonne, IL, United States of America 7Department of Physics, University of Arizona, Tucson, AZ, United States of America 8Department of Physics, University of Texas at Arlington, Arlington, TX, United States of America 9Physics Department, National and Kapodistrian University of Athens, Athens, Greece 10 Physics Department, National Technical University of Athens, Zografou, Greece 11 Department of Physics, University of Texas at Austin, Austin, TX, United States of America 12 (a)Bahcesehir University, Faculty of Engineering and Natural Sciences, Istanbul; (b)Istanbul Bilgi University, Faculty of Engineering and Natural Sciences, Istanbul; (c)Department of Physics, Bogazici University, Istanbul; (d)Department of Physics Engineering, Gaziantep University, Gaziantep, Turkey 13 Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan 14 Institut de Física d’Altes Energies (IFAE), Barcelona Institute of Science and Technology, Barcelona, Spain 15 (a)Institute of High Energy Physics, Chinese Academy of Sciences, Beijing; (b)Physics Department, Tsinghua University, Beijing; (c)Department of Physics, Nanjing University, Nanjing; (d)University of Chinese Academy of Science (UCAS), Beijing, China 16 Institute of Physics, University of Belgrade, Belgrade, Serbia 17 Department for Physics and Technology, University of Bergen, Bergen, Norway 18 Physics Division, Lawrence Berkeley National Laboratory and University of California, Berkeley, CA, United States of America 19 Institut für Physik, Humboldt Universität zu Berlin, Berlin, Germany 20 Albert Einstein Center for Fundamental Physics and Laboratory for High Energy Physics, University of Bern, Bern, Switzerland 21 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 22 Centro de Investigaciónes, Universidad Antonio Nariño, Bogota, Colombia 23 (a)INFN Bologna and Universita’ di Bologna, Dipartimento di Fisica; (b)INFN Sezione di Bologna, Italy 24 Physikalisches Institut, Universität Bonn, Bonn, Germany 25 Department of Physics, Boston University, Boston, MA, United States of America 26 (a)University of Colorado Boulder, Department of Physics, CO; (b)Physics Department, Brookhaven National Laboratory, Upton, NY, United States of America 27 Department of Physics, Brandeis University, Waltham, MA, United States of America 28 (a)Transilvania University of Brasov, Brasov; (b)Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest; (c)Department of Physics, Alexandru Ioan Cuza University of Iasi, Iasi; (d)National Institute for Research and Development of Isotopic and Molecular Technologies, Physics Department, Cluj-Napoca; (e)University Politehnica Bucharest, Bucharest; (f)West University in Timisoara, Timisoara, Romania 29 (a)Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava; (b)Department of Subnuclear Physics, Institute of Experimental Physics of the Slovak Academy of Sciences, Kosice, Slovak Republic 30 Departamento de Física, Universidad de Buenos Aires, Buenos Aires, Argentina 31 Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 32 (a)Department of Physics, University of Cape Town, Cape Town; (b)Department of Mechanical Engineering Science, University of Johannesburg, Johannesburg; (c)School of Physics, University of the Witwatersrand, Johannesburg, South Africa 33 Department of Physics, Carleton University, Ottawa, ON, Canada 34 (a)Faculté des Sciences Ain Chock, Réseau Universitaire de Physique des Hautes Energies – Université Hassan II, Casablanca; (b)Centre National de l’Energie des Sciences Techniques Nucleaires (CNESTEN), Rabat; (c)Faculté des Sciences Semlalia, Université Cadi Ayyad, LPHEA, Marrakech; (d)Faculté des Sciences, Université Mohamed Premier and LPTPM, Oujda; (e)Faculté des sciences, Université Mohammed V, Rabat, Morocco 35 CERN, Geneva, Switzerland 36 Enrico Fermi Institute, University of Chicago, Chicago, IL, United States of America 37 LPC, Université Clermont Auvergne, CNRS/IN2P3, Clermont-Ferrand, France 38 Nevis Laboratory, Columbia University, Irvington, NY, United States of America 39 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 40 (a)Dipartimento di Fisica, Università della Calabria, Rende; (b)INFN Gruppo Collegato di Cosenza, Laboratori Nazionali di Frascati, Italy 41 Physics Department, Southern Methodist University, Dallas, TX, United States of America 42 Physics Department, University of Texas at Dallas, Richardson, TX, United States of America 43 (a)Department of Physics, Stockholm University; (b)Oskar Klein Centre, Stockholm, Sweden 44 Deutsches Elektronen-Synchrotron DESY, Hamburg and Zeuthen, Germany 45 Lehrstuhl für Experimentelle Physik IV, Technische Universität Dortmund, Dortmund, Germany 46 Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany 47 Department of Physics, Duke University, Durham, NC, United States of America 48 SUPA – School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 49 INFN e Laboratori Nazionali di Frascati, Frascati, Italy 50 Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany 51 II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany 52 Département de Physique Nucléaire et Corpusculaire, Université de Genève, Genève, Switzerland 250 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 53 (a)Dipartimento di Fisica, Università di Genova, Genova; (b)INFN Sezione di Genova, Italy 54 II. Physikalisches Institut, Justus-Liebig-Universität Giessen, Giessen, Germany 55 SUPA – School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 56 LPSC, Université Grenoble Alpes, CNRS/IN2P3, Grenoble INP, Grenoble, France 57 Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge, MA, United States of America 58 (a)Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei; (b)Institute of Frontier and Interdisciplinary Science and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Qingdao; (c)School of Physics and Astronomy, Shanghai Jiao Tong University, KLPPAC-MoE, SKLPPC, Shanghai; (d)Tsung-Dao Lee Institute, Shanghai, China 59 (a)Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Heidelberg; (b)Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 60 Faculty of Applied Information Science, Hiroshima Institute of Technology, Hiroshima, Japan 61 (a)Department of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong; (b)Department of Physics, University of Hong Kong, Hong Kong; (c)Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China 62 Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 63 Department of Physics, Indiana University, Bloomington, IN, United States of America 64 (a)INFN Gruppo Collegato di Udine, Sezione di Trieste, Udine; (b)ICTP, Trieste; (c)Dipartimento Politecnico di Ingegneria e Architettura, Università di Udine, Udine, Italy 65 (a)INFN Sezione di Lecce; (b)Dipartimento di Matematica e Fisica, Università del Salento, Lecce, Italy 66 (a)INFN Sezione di Milano; (b)Dipartimento di Fisica, Università di Milano, Milano, Italy 67 (a)INFN Sezione di Napoli; (b)Dipartimento di Fisica, Università di Napoli, Napoli, Italy 68 (a)INFN Sezione di Pavia; (b)Dipartimento di Fisica, Università di Pavia, Pavia, Italy 69 (a)INFN Sezione di Pisa; (b)Dipartimento di Fisica E. Fermi, Università di Pisa, Pisa, Italy 70 (a)INFN Sezione di Roma; (b)Dipartimento di Fisica, Sapienza Università di Roma, Roma, Italy 71 (a)INFN Sezione di Roma Tor Vergata; (b)Dipartimento di Fisica, Università di Roma Tor Vergata, Roma, Italy 72 (a)INFN Sezione di Roma Tre; (b)Dipartimento di Matematica e Fisica, Università Roma Tre, Roma, Italy 73 (a)INFN-TIFPA; (b)Università degli Studi di Trento, Trento, Italy 74 Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität, Innsbruck, Austria 75 University of Iowa, Iowa City, IA, United States of America 76 Department of Physics and Astronomy, Iowa State University, Ames, IA, United States of America 77 Joint Institute for Nuclear Research, Dubna, Russia 78 (a)Departamento de Engenharia Elétrica, Universidade Federal de Juiz de Fora (UFJF), Juiz de Fora; (b)Universidade Federal do Rio De Janeiro COPPE/EE/IF, Rio de Janeiro; (c)Universidade Federal de São João del Rei (UFSJ), São João del Rei; (d)Instituto de Física, Universidade de São Paulo, São Paulo, Brazil 79 KEK, High Energy Accelerator Research Organization, Tsukuba, Japan 80 Graduate School of Science, Kobe University, Kobe, Japan 81 (a)AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow; (b)Marian Smoluchowski Institute of Physics, Jagiellonian University, Krakow, Poland 82 Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland 83 Faculty of Science, Kyoto University, Kyoto, Japan 84 Kyoto University of Education, Kyoto, Japan 85 Research Center for Advanced Particle Physics and Department of Physics, Kyushu University, Fukuoka , Japan 86 Instituto de Física La Plata, Universidad Nacional de La Plata and CONICET, La Plata, Argentina 87 Physics Department, Lancaster University, Lancaster, United Kingdom 88 Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 89 Department of Experimental Particle Physics, Jožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, Slovenia 90 School of Physics and Astronomy, Queen Mary University of London, London, United Kingdom 91 Department of Physics, Royal Holloway University of London, Egham, United Kingdom 92 Department of Physics and Astronomy, University College London, London, United Kingdom 93 Louisiana Tech University, Ruston, LA, United States of America 94 Fysiska institutionen, Lunds universitet, Lund, Sweden 95 Centre de Calcul de l’Institut National de Physique Nucléaire et de Physique des Particules (IN2P3), Villeurbanne, France 96 Departamento de Física Teorica C-15 and CIAFF, Universidad Autónoma de Madrid, Madrid, Spain 97 Institut für Physik, Universität Mainz, Mainz, Germany 98 School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 99 CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France 100 Department of Physics, University of Massachusetts, Amherst, MA, United States of America 101 Department of Physics, McGill University, Montreal, QC, Canada 102 School of Physics, University of Melbourne, Victoria, Australia 103 Department of Physics, University of Michigan, Ann Arbor, MI, United States of America 104 Department of Physics and Astronomy, Michigan State University, East Lansing, MI, United States of America 105 B.I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, Minsk, Belarus 106 Research Institute for Nuclear Problems of Byelorussian State University, Minsk, Belarus 107 Group of Particle Physics, University of Montreal, Montreal, QC, Canada 108 P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia 109 Institute for Theoretical and Experimental Physics of the National Research Centre Kurchatov Institute, Moscow, Russia 110 National Research Nuclear University MEPhI, Moscow, Russia 111 D.V. Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Moscow, Russia 112 Fakultät für Physik, Ludwig-Maximilians-Universität München, München, Germany 113 Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), München, Germany 114 Nagasaki Institute of Applied Science, Nagasaki, Japan 115 Graduate School of Science and Kobayashi–Maskawa Institute, Nagoya University, Nagoya, Japan 116 Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM, United States of America 117 Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen/Nikhef, Nijmegen, Netherlands 118 Nikhef National Institute for Subatomic Physics and University of Amsterdam, Amsterdam, Netherlands 119 Department of Physics, Northern Illinois University, DeKalb, IL, United States of America 120 (a)Budker Institute of Nuclear Physics and NSU, SB RAS, Novosibirsk; (b)Novosibirsk State University Novosibirsk, Russia 121 Institute for High Energy Physics of the National Research Centre Kurchatov Institute, Protvino, Russia 122 Department of Physics, New York University, New York, NY, United States of America 123 Ohio State University, Columbus, OH, United States of America 124 Faculty of Science, Okayama University, Okayama, Japan 125 Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK, United States of America 126 Department of Physics, Oklahoma State University, Stillwater, OK, United States of America The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 251 127 Palacký University, RCPTM, Joint Laboratory of Optics, Olomouc, Czech Republic 128 Center for High Energy Physics, University of Oregon, Eugene, OR, United States of America 129 LAL, Université Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay, France 130 Graduate School of Science, Osaka University, Osaka, Japan 131 Department of Physics, University of Oslo, Oslo, Norway 132 Department of Physics, Oxford University, Oxford, United Kingdom 133 LPNHE, Sorbonne Université, Paris Diderot Sorbonne Paris Cité, CNRS/IN2P3, Paris, France 134 Department of Physics, University of Pennsylvania, Philadelphia, PA, United States of America 135 Konstantinov Nuclear Physics Institute of National Research Centre “Kurchatov Institute”, PNPI, St. Petersburg, Russia 136 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA, United States of America 137 (a)Laboratório de Instrumentação e Física Experimental de Partículas – LIP; (b)Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisboa; (c)Departamento de Física, Universidade de Coimbra, Coimbra; (d)Centro de Física Nuclear da Universidade de Lisboa, Lisboa; (e)Departamento de Física, Universidade do Minho, Braga; (f)Universidad de Granada, Granada (Spain); (g)Dep. Física and CEFITEC of Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal 138 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 139 Czech Technical University in Prague, Prague, Czech Republic 140 Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic 141 Particle Physics Department, Rutherford Appleton Laboratory, Didcot, United Kingdom 142 IRFU, CEA, Université Paris-Saclay, Gif-sur-Yvette, France 143 Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz, CA, United States of America 144 (a)Departamento de Física, Pontificia Universidad Católica de Chile, Santiago; (b)Departamento de Física, Universidad Técnica Federico Santa María, Valparaíso, Chile 145 Department of Physics, University of Washington, Seattle, WA, United States of America 146 Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom 147 Department of Physics, Shinshu University, Nagano, Japan 148 Department Physik, Universität Siegen, Siegen, Germany 149 Department of Physics, Simon Fraser University, Burnaby, BC, Canada 150 SLAC National Accelerator Laboratory, Stanford, CA, United States of America 151 Physics Department, Royal Institute of Technology, Stockholm, Sweden 152 Departments of Physics and Astronomy, Stony Brook University, Stony Brook, NY, United States of America 153 Department of Physics and Astronomy, University of Sussex, Brighton, United Kingdom 154 School of Physics, University of Sydney, Sydney, Australia 155 Institute of Physics, Academia Sinica, Taipei, Taiwan 156 (a)E. Andronikashvili Institute of Physics, Iv. Javakhishvili Tbilisi State University, Tbilisi; (b)High Energy Physics Institute, Tbilisi State University, Tbilisi, Georgia 157 Department of Physics, Technion, Israel Institute of Technology, Haifa, Israel 158 Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel 159 Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece 160 International Center for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo, Japan 161 Graduate School of Science and Technology, Tokyo Metropolitan University, Tokyo, Japan 162 Department of Physics, Tokyo Institute of Technology, Tokyo, Japan 163 Tomsk State University, Tomsk, Russia 164 Department of Physics, University of Toronto, Toronto, ON, Canada 165 (a)TRIUMF, Vancouver, BC; (b)Department of Physics and Astronomy, York University, Toronto, ON, Canada 166 Division of Physics and Tomonaga Center for the History of the Universe, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan 167 Department of Physics and Astronomy, Tufts University, Medford, MA, United States of America 168 Department of Physics and Astronomy, University of California Irvine, Irvine, CA, United States of America 169 Department of Physics and Astronomy, University of Uppsala, Uppsala, Sweden 170 Department of Physics, University of Illinois, Urbana, IL, United States of America 171 Instituto de Física Corpuscular (IFIC), Centro Mixto Universidad de Valencia – CSIC, Valencia, Spain 172 Department of Physics, University of British Columbia, Vancouver, BC, Canada 173 Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada 174 Fakultät für Physik und Astronomie, Julius-Maximilians-Universität Würzburg, Würzburg, Germany 175 Department of Physics, University of Warwick, Coventry, United Kingdom 176 Waseda University, Tokyo, Japan 177 Department of Particle Physics, Weizmann Institute of Science, Rehovot, Israel 178 Department of Physics, University of Wisconsin, Madison, WI, United States of America 179 Fakultät für Mathematik und Naturwissenschaften, Fachgruppe Physik, Bergische Universität Wuppertal, Wuppertal, Germany 180 Department of Physics, Yale University, New Haven, CT, United States of America 181 Yerevan Physics Institute, Yerevan, Armenia aAlso at Borough of Manhattan Community College, City University of New York, NY; United States of America. bAlso at California State University, East Bay; United States of America. cAlso at Centre for High Performance Computing, CSIR Campus, Rosebank, Cape Town; South Africa. dAlso at CERN, Geneva; Switzerland. eAlso at CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille; France. fAlso at Département de Physique Nucléaire et Corpusculaire, Université de Genève, Genève; Switzerland. gAlso at Departament de Fisica de la Universitat Autonoma de Barcelona, Barcelona; Spain. hAlso at Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisboa; Portugal. iAlso at Department of Applied Physics and Astronomy, University of Sharjah, Sharjah; United Arab Emirates. jAlso at Department of Financial and Management Engineering, University of the Aegean, Chios; Greece. kAlso at Department of Physics and Astronomy, University of Louisville, Louisville, KY; United States of America. lAlso at Department of Physics and Astronomy, University of Sheffield, Sheffield; United Kingdom. mAlso at Department of Physics, California State University, Fresno CA; United States of America. nAlso at Department of Physics, California State University, Sacramento CA; United States of America. oAlso at Department of Physics, King’s College London, London; United Kingdom. pAlso at Department of Physics, St. Petersburg State Polytechnical University, St. Petersburg; Russia. qAlso at Department of Physics, Stanford University; United States of America. rAlso at Department of Physics, University of Fribourg, Fribourg; Switzerland. sAlso at Department of Physics, University of Michigan, Ann Arbor MI; United States of America. 252 The ATLAS Collaboration / Physics Letters B 796 (2019) 230–252 tAlso at Giresun University, Faculty of Engineering, Giresun; Turkey. uAlso at Graduate School of Science, Osaka University, Osaka; Japan. vAlso at Hellenic Open University, Patras; Greece. wAlso at Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest; Romania. xAlso at II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen; Germany. yAlso at Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona; Spain. zAlso at Institut für Experimentalphysik, Universität Hamburg, Hamburg; Germany. aa Also at Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen/Nikhef, Nijmegen; Netherlands. ab Also at Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Budapest; Hungary. ac Also at Institute of Particle Physics (IPP); Canada. ad Also at Institute of Physics, Academia Sinica, Taipei; Taiwan. ae Also at Institute of Physics, Azerbaijan Academy of Sciences, Baku; Azerbaijan. af Also at Institute of Theoretical Physics, Ilia State University, Tbilisi; Georgia. ag Also at Instituto de Física Teórica de la Universidad Autónoma de Madrid; Spain. ah Also at Istanbul University, Dept. of Physics, Istanbul; Turkey. ai Also at Joint Institute for Nuclear Research, Dubna; Russia. aj Also at LAL, Université Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay; France. ak Also at Louisiana Tech University, Ruston LA; United States of America. al Also at LPNHE, Sorbonne Université, Paris Diderot Sorbonne Paris Cité, CNRS/IN2P3, Paris; France. am Also at Manhattan College, New York NY; United States of America. an Also at Moscow Institute of Physics and Technology State University, Dolgoprudny; Russia. ao Also at National Research Nuclear University MEPhI, Moscow; Russia. ap Also at Physics Dept, University of South Africa, Pretoria; South Africa. aq Also at Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg; Germany. ar Also at School of Physics, Sun Yat-sen University, Guangzhou; China. as Also at The City College of New York, New York NY; United States of America. at Also at The Collaborative Innovation Center of Quantum Matter (CICQM), Beijing; China. au Also at Tomsk State University, Tomsk, and Moscow Institute of Physics and Technology State University, Dolgoprudny; Russia. av Also at TRIUMF, Vancouver BC; Canada. aw Also at Universidad de Granada, Granada (Spain); Spain. ax Also at Universita di Napoli Parthenope, Napoli; Italy. ∗Deceased.