Direct photon production at low transverse momentum in proton-proton collisions at √s = 2.76 and 8 TeV

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Direct photon production at low transverse momentum in proton-proton collisions at √s = 2.76 and 8 TeV Published version ALICE Collaboration ALICE Collaboration. (2019). Direct photon production at low transverse momentum in protonproton collisions at √s = 2.76 and 8 TeV. Physical Review C, 99(2), Article 024912. https://doi.org/10.1103/PhysRevC.99.024912 2019 PHYSICAL REVIEW C 99, 024912 (2019) Direct photon production at low transverse momentum in proton-proton collisions at √s=2.76 and 8 TeV S. Acharya et al.∗ (ALICE Collaboration) (Received 30 March 2018; published 21 February 2019) Measurements of inclusive and direct photon production at midrapidity in pp collisions at √s=2.76 and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of 0.4<pT<10 GeV/cand 0.3<pT<16 GeV/c, respectively. Photons are detected with the electromagnetic calorimeter (EMCal) and via reconstruction of e+e−pairs from conversions in the ALICE detector material using the central tracking system. For the final measurement of the inclusive photon spectra the results are combined in the overlapping pTinterval of both methods. Direct photon spectra, or their upper limits at 90% C.L. are extracted using the direct photon excess ratio Rγ, which quantifies the ratio of inclusive photons over decay photons generated with a decay-photon simulation. An additional hybrid method, combining photons reconstructed from conversions with those identified in the EMCal, is used for the combination of the direct photon excess ratio Rγ, as well as the extraction of direct photon spectra or their upper limits. While no significant signal of direct photons is seen over the full pTrange, Rγfor pT>7 GeV/cis at least one σabove unity and consistent with expectations from next-to-leading order pQCD calculations. DOI: 10.1103/PhysRevC.99.024912 I. INTRODUCTION Major experimental efforts are undertaken at the Relativistic Heavy Ion Collider (RHIC) [1–4] and the Large Hadron Collider (LHC) [5–13] to study the conditions for the creation and the properties of the quark-gluon plasma (QGP), a deconfined partonic state predicted by the theory of strong interaction, quantum chromodynamics (QCD) [14,15]. Direct photons, which are defined as all photons that are produced directly in scattering processes and therefore do not originate from hadronic decays, are a powerful tool for exploring the QGP. They are produced during all stages of the collision and are basically unaffected by final-state interactions as they only participate in electromagnetic interactions [16]. Hence, they are sensitive to the early stages of the collision’s evolution. Since a variety of QGP signatures are also present in high multiplicity p-Pb or pp collision at the LHC [17], it is interesting to study if a direct photon signal at low pTcan be observed already in minimum bias pp collisions, as predicted for √s= 7TeV[18]. Experimentally, however, the main challenge for direct photon measurements is to distinguish them from the large background of decay photons. Depending on their production mechanism, direct photons are usually classified into two main categories: prompt and thermal photons. Prompt photons carry information about ∗Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. parton distributions in nuclei [19,20] as they are produced in hard scatterings of incoming partons, such as Compton scattering q+g→q+γor annihilation q+q→g+γ,as well as bremsstrahlung emission from quarks, which undergo a hard scattering [21–23]. These processes are described by perturbative QCD (pQCD) in leading and next-to-leading order, which are dominant at LHC energies. One of the purposes of direct photon measurements is to improve the accuracy of such calculations in various collision systems. At RHIC at center of mass energies per nucleon-nucleon pair of √sNN = 0.2 TeV and at the LHC at √sNN =2.76 TeV, direct photons with transverse momenta (pT) above about 3 and 15 GeV/c, respectively, were found to be dominated by prompt photons and to follow a power-law spectral shape in small systems (pp, pA,dA)[24–27] as well as in heavy-ion collisions [27–30], as described by pQCD. In heavy-ion collisions, additionally, thermal photons are expected to be radiated off the locally thermalized, hot QGP and hadronic matter, which provide information about the temperature, collective expansion, as well as the space-time evolution of the medium [31], and are expected to dominate the direct photon spectrum at low transverse momenta (pT3GeV/c)[32,33]. Further direct photon production mechanisms, such as interactions of hard scattered partons with dense partonic matter (jet-photon conversion) [21,22], as well as production of photons from nonequilibrated phases [34], may also play a role in the low and intermediate pT region from 3 to ∼10 GeV/c. In the following, we present first results from the measurement of direct photon production at midrapidity in 0.4< pT<10 GeV/cand 0.3<pT<16 GeV/cin ppcollisions at √s=2.76 and 8 TeV, respectively. These data, which are the first direct photon data below 15 GeV/cin ppcollisions at the 2469-9985/2019/99(2)/024912(19) 024912-1 ©2019 CERN, for the ALICE Collaboration S. ACHARYA et al. PHYSICAL REVIEW C 99, 024912 (2019) LHC, enable pQCD calculations to be tested in this low pT regime. Furthermore, they provide an important baseline for the interpretation of initialand final-state effects observed in direct photon data from heavy-ion collisions [35,36], because event generators and perturbative calculations are generally not reliable at pT3. The direct photon yield is extracted by comparing the measured inclusive photon spectrum to the spectrum of photons from hadron decays via a double ratio, obtained from the so-called direct photon excess ratio Rγ[37,38]. The double ratio, defined on the level of fully corrected quantities, can be written as Rγ=Yγincl Yγdecay ≈Yγincl Yπ0measYγdecay Yπ0sim ,(1) where the numerator denotes the measured inclusive photon yield, Yγincl , divided by the measured neutral pion yield, Yπ0, and the denominator is constructed in the same way, but with the photon yield obtained by a decay-photon simulation and a parametrization of the neutral pion yield. With Rγ, the direct photon yield can then be obtained from the inclusive photon yield as Yγdir =Yγincl −Yγdecay =1−1 RγYγincl .(2) The yields in Eqs. (1) and (2) are implicitly defined at midrapidty as a function of pTof the corresponding particle. The cross sections can be obtained by replacing the inclusive photon yield with the inclusive photon cross section in Eq. (2). The advantage of using Rγ(rather than trying to directly quantify the difference of inclusive and decay photons) is the partial or full cancellation of several systematic uncertainties in the double ratio. The photon reconstruction is performed independently using either conversions in the inner detector material reconstructed with the central tracking system and for the first time with the Electromagnetic Calorimeter (EMCal). Combined inclusive and direct photon spectra are determined based on the individual inclusive photon spectra and direct photon excess ratios. The direct photon spectra, or respectively their upper limits at 90% C.L., are finally compared to next-to-leading order pQCD calculations. The paper is structured as follows. Section II describes the photon-decay simulation at generator level, usually known as cocktail simulation. Section III describes the relevant ALICE detectors for the photon and neutral meson measurements, the data taking conditions, and the event selection. Section IV describes the data analysis with emphasis on the photon reconstruction via the Photon Conversion Method (PCM) and using the EMCal. The systematic uncertainties are summarized in Sec. V, whereas Sec. VI presents the results. Section VII concludes with a short summary. II. GENERATOR-LEVEL DECAY-PHOTON SIMULATION The decay-photon spectra are obtained by a particle decay simulation, also called cocktail simulation, needed for the secondary decay-photon correction as well as for the calculation of Rγ. The decay simulation is based on the PYTHIA6.4 particle decayer [43] with random generation of mother particles uniform in azimuth and pT. Parametrizations of the transverse momentum spectra of the mother particles measured by ALICE are used as weights in order to obtain the correct abundances. For the √s=2.76 TeV cocktail, measured pTdifferential yields per inelastic event of π0[39], K±,p[40], φ[41], and ρ0[42], as well as the η/π0[39] ratio are parametrized as inputs, and are shown in the left panel of Fig. 1. Neutral kaons, which constitute an important background for secondary decay photons, are approximated by the average of the charged kaon yields. The particle decay simulation for √s=8TeV uses measured pTdifferential yields from π0and η[44]as input. Furthermore, pTdifferential yields for K±,φ, and p are extrapolated using the measured spectra at √s=2.76 and 7 TeV [45–47] as inputs. The extrapolation is done on a bin-by-bin basis in pTassuming a power-law evolution of the particle yields with increasing center-of-mass energy. For the parametrization, the pTdifferential particle yields are fitted with a modified Hagedorn function [48,49] whose functional form is given by d2N dydpT=pT·A·exp apT+bp2 T+pT p0−n .(3) In order to obtain a stable parametrization for the ηparticle yields up to high pT,theη/π0ratios at √s=2.76 and 8 TeV are fitted with an empirical function that describes contributions from soft and hard processes [49], given as η π0(pT)= A·exp βpT−mη T T√1−β2+N·B·1+pT p02−n exp βpT−mπ0 T T√1−β2+B·1+pT p02−n, (4) with a relative normalization factor Bbetween the soft and hard part of the parametrization and the constant ratio value Nbetween the two particle species that is approached at high pT. All spectra are described by their parametrization within a maximum of 10% deviation over the full transverse momentum range. For particles that are neither measured nor extrapolated, the parametrization is obtained via transverse mass scaling, mT=√p2 T+m2 0, with the neutral pion as basis (B) for mesons and the proton as basis for baryons. The mTscaling factors, CX mT=(dNX/dmT)/(dNB/dmT), for each particle X are derived from the respective spectra in PYTHIA. These particles are η(CmT=0.4), (CmT=1.0), 0(CmT=0.49), 0,+(CmT=1.0), and ω(CmT=0.85) at √s=2.76 TeV and additionally ρ0(CmT=1.0) at √s=8 TeV. The limitations of transverse mass scaling [49]atlowpTcan be neglected for this measurement as the transverse mass scaled particles contribute only a tiny fraction to the total decay-photon yield as seen in Fig. 1. For the particle decay simulation, particles are generated uniformly in the transverse momentum range of 0 ⩽pT⩽ 50 GeV/c, for the rapidity range of |y|<1.0aswellasthefull azimuth of 0 <φ<2π. For each particle, the full decay chain is simulated, which allows the decay simulation to be used for the secondary photon correction for photons produced following weak decays of primary hadrons as well as the 024912-2 DIRECT PHOTON PRODUCTION AT LOW TRANSVERSE … PHYSICAL REVIEW C 99, 024912 (2019) )c (GeV/ T p 1 10 decay γ / source γ 6− 10 5− 10 4− 10 3− 10 2− 10 1− 10 1 10 0 π η ω ’η0 ρ± ρ φ0 ΣS 0 KL 0 KΛ0/+ Δ ALICE simulation = 2.76 TeVspp, fromγ (b) )c (GeV/ T p 1 10 ) -1 )c ((GeV/ yd T pd 2 Nd inel N 1 9− 10 8− 10 7− 10 6− 10 5− 10 4− 10 3− 10 2− 10 1− 10 1mod. Hagedorn fit 0 π η )/2 - π+ + π( )/2 - +K + (K )/2p(p+ φ 0* K0 ρ = 2.76 TeVsALICE, pp, (a) FIG. 1. (a) Measured identified particle yields per inelastic event in pp collisions at √s=2.76 TeV [39–42] including their modified Hagedorn parametrization used as input for the cocktail simulation. Statistical uncertainties are shown with vertical lines and systematic uncertainties with boxes. See the text for references to the data. (b) Ratio of primary decay photons from different sources to all primary decay photons in the decay-photon simulation for pp collisions at √s=2.76 TeV. From top to bottom at high pTthe different sources are π0,η,ω, η,ρ0,φ,K 0 L,ρ±,K 0 S,0/+,0,and. extraction of the decay-photon spectrum. After generation, only decay photons are kept, which fulfill |y|<0.9, to match the hadron rapidity range that is used in the measurements. Furthermore, the mother particles as well as all decay products are weighted with the parametrization of the mother particle. The contribution of each individual decay-photon source to all decay photons of the cocktail simulation is shown in the right panel of Fig. 1. Decay photons originating from π0 decays are the dominant contribution with ∼86% of total decay photons at high pT. Contributions from the ηmeson decay photons represent ∼10% whereas decay photons from ωand ηmesons contribute below 3% and 1.5%, respectively. All other remaining sources are basically negligible as shown in Fig. 1. III. EXPERIMENTAL SETUP AND DATA TAKING CONDITIONS Two different methods using independent detector systems of ALICE [50] are employed to measure photons in this analysis. In the first method, PCM, photons are reconstructed from e+e−pairs, which are created by photon conversions in the inner detector material. The inner material includes the full active and passive material of the beam pipe, the Inner Tracking System (ITS), as well as the inner field cage vessel of the Time Projection Chamber (TPC) and part of the TPC gas. The main tracking systems in ALICE at midrapidity, the ITS and the TPC, are used for the reconstruction of these electronpositron pairs, which originate from secondary vertices (V0). In the second photon reconstruction method, EMC, the energy deposit in the EMCal is used to measure photons. With PCM, a high momentum resolution at low pTis achieved, but the method is limited by statistics at high transverse momenta. The EMC method benefits from large statistics up to high pT but has a decreasing resolution towards low pT. The necessary detector systems are described in the following with emphasis on the detector configurations in both pp data taking periods of √s=2.76 TeV in 2011 and √s=8 TeV in 2012. The ITS [51] consists of three subdetectors each with two layers to measure the trajectories of charged particles and to reconstruct primary [52] and secondary vertices [53]. The two innermost layers are the Silicon Pixel Detectors (SPD) positioned at radial distances of 3.9 cm and 7.6 cm relative to the beam line, followed by two layers of the Silicon Drift Detectors (SDD) at 15.0 cm and 23.9 cm, and completed by two layers of the Silicon Strip Detectors (SSD) at 38 cm and 43 cm. The two layers of SPD cover pseudorapidity ranges of |η|<2 and |η|<1.4, respectively. The SDD and SSD cover |η|<0.9 and |η|<1.0, accordingly. The TPC [54]isalarge(90m 3) cylindrical drift detector filled with Ne-CO2(90%–10%) gas mixture. It covers a pseudorapidity range of |η|<0.9 over the full azimuth, providing up to 159 reconstructed space points per track. A magnetic field of B=0.5 T is generated by a large solenoidal magnet surrounding the central barrel detectors. Charged tracks originating from the primary vertex can be reconstructed down to pT≈100 MeV/cand charged secondaries down to pT≈ 50 MeV/cwith a tracking efficiency of ≈80% for tracks with 024912-3 S. ACHARYA et al. PHYSICAL REVIEW C 99, 024912 (2019) pT>1GeV/c[55]. In addition, the TPC provides particle identification via the measurement of energy loss dE/dx with a resolution of ≈5%. The ITS and TPC are complemented by the Transition Radiation Detector (TRD) [56] and the Time-Of-Flight (TOF) [57] detector. The EMCal detector [58] is an electromagnetic sampling calorimeter covering φ =100◦in azimuth and |η|<0.7in pseudorapidity, located at a radial distance of 4.28 m from the nominal collision vertex. During the data taking periods in 2011 and 2012, it consisted of a total of 11520 active elements, or cells, each of which comprise 77 alternating layers of lead and plastic scintillator providing a radiation length of 20.1X0. Attached perpendicular to the face of each cell are wavelength shifting fibers that collect the scintillation light in each layer. Avalanche photo diodes (APDs) with an active area of 5 ×5mm 2are connected to the fibres to detect the generated scintillation light. The size of each cell is η ×φ =0.0143 ×0.0143 rad (≈6.0×6.0cm 2), corresponding to approximately twice the Molière radius. The EMCal consists of ten supermodules, where each supermodule is composed of 12 ×24 modules, consisting of 2 ×2 cells apiece. It has an intrinsic energy resolution of σE/E= 4.8%/E⊕11.3%/√E⊕1.7% where the energy Eis given in units of GeV [59]. The energy calibration of the detector is performed by measuring, in each cell, the reconstructed π0mass in the two-photon invariant mass distribution with one photon associated with the given cell. An estimated calibration level of better than 3% is achieved with this method, which adds up quadratically to the constant term of the energy resolution. Between 2011 and 2012 an additional TRD module in front of EMCal was installed, which results in a slightly different outer material budget between the 2.76 and 8 TeV data sets. The material budget differences due to the TRD will be studied in detail for the estimation of the associated systematic uncertainties. As trigger for minimum bias pp collisions and to reduce beam-induced background and pileup events the V0 detector [60] is used. It consists of two scintillator arrays (V0A and V0C) covering 2.8<η<5.1 and −3.7<η<−1.7. The probability of collision pileup per triggered event was below 2.5% and below 1% at √s=2.76 and 8 TeV, respectively. Background events from beam-gas interactions or detector noise are rejected based on the timing information from V0A and V0C [55]. Events containing more than one pp collision within a single bunch crossing are rejected based on the information reconstructed in the SPD. In these events, either multiple primary vertices could be reconstructed within the acceptance [55] or an excess of SPD clusters with respect to the number of SPD tracklets could be observed. In addition, the primary vertex is required to be reconstructed within |z|<10 cm from the nominal interaction point. In 2011, the minimum bias trigger condition required a hit in either the SPD, the V0A or the V0C (MBOR condition), whereas in 2012 a hit in the V0A and the V0C (MBAND condition) was required. The latter was necessary due to the higher beam intensities in 2012. The corresponding cross section for the minimum bias triggers are obtained from van der Meer scans [61] yielding σMBOR =55.4±3.9stat+syst mb [62] and σMBAND =55.8±1.2stat ±1.5syst mb [63] for the data taking campaigns at √s=2.76 TeV and √s=8 TeV, respectively. For the conversion-based measurements, integrated luminosities of Lint =0.96 ±0.07norm nb−1at √s=2.76 TeV and Lint =2.17 ±0.06norm nb−1at √s=8 TeV are analyzed. The calorimeter-based measurements sample 50% and, respectively, 10.6% smaller integrated luminosities, since the EMCal was not always active during data taking. Further information about the performance of these and other detector systems can be found in Ref. [55]. IV. PHOTON RECONSTRUCTION Inclusive photons are reconstructed in two ways; either using photon conversions between 0.4 (0.3) and 8 (16) GeV/c or using the EMCal between 1.5 and 10 (16) GeV/cfor √s=2.76 (8) TeV. Photons convert within the inner detector material of ALICE with a probability of about 8.9%, and are reconstructed with the PCM method as follows: (i) tracking of charged particles and secondary vertex finding [53]; (ii) particle identification; and (iii) photon candidate reconstruction and subsequent selection. The secondary vertices used in this analysis are obtained during data reconstruction by employing the full tracking capabilities of ITS and TPC. For the daughter tracks, a minimum of 60% of the maximum possible findable TPC clusters, that a particle track can create in the TPC along its path, and a minimum track pT of 50 MeV/care required. The contamination from Dalitz decays is reduced by rejecting conversion candidates with reconstructed vertices with a radial distance of less than 5 cm with respect to the nominal center of the detector. Furthermore, only secondary tracks and vertices with |η|< 0.9 are accepted. In addition, we restrict the geometrical η distribution of the V0s in order to remove photon candidates that would otherwise appear outside the angular dimensions of the detector. To do so, the condition Rconv >|Zconv|SZR −7cm is applied with SZR =tan {2arctan[exp(−ηmax)]}≈0.974 for ηmax =0.9, where Rconv and Zconv denote the radial and longitudinal coordinate of the conversion point, respectively. The coordinates Rconv and Zconv are determined with respect to the center of the detector and are set to Rconv <180 cm and |Zconv|<240 cm to ensure a high quality secondary track reconstruction inside the TPC. Electrons and positrons are identified via their energy deposit in the TPC, dE/dx, by employing the difference of the measured dE/dx to the expected value for electrons and positrons [55]. For the measurement at √s=2.76 TeV, the dE/dx of the charged tracks is required to be within −4<nσe<5 of the expected electron/positron energy loss, where nσe=(dE/dx −dE/dxe)/σeis pTdependent with the average energy loss of the electron/positron, dE/dxe, and the Gaussian width of the fit to the measured dE/dx distribution, σe. This condition is tightened for the measurement at √s=8TeVto−3<nσe<5. To reduce the contamination from pions, an additional selection based on the separation from the charged pion energy loss hypothesis is required in nσπ. A rejection of tracks with energy losses closer to the pion line than |nσπ|<1isapplieduptoapTof 3.5 GeV/c. In the √s=2.76 TeV analysis, this rejection is continued above pT>3.5GeV/cwith an |nσπ|<0.5 in order reduce 024912-4 DIRECT PHOTON PRODUCTION AT LOW TRANSVERSE … PHYSICAL REVIEW C 99, 024912 (2019) the contamination even further in the momentum region where the two dE/dx bands of the pion and electron merge. Further contamination from nonphotonic V0candidates is suppressed by a triangular two-dimensional selection range of |pair|< pair,max(1 −χ2 red/χ2 red,max) with χ2 red,max =30 and pair,max =0.1 rad. As explained in Ref. [39], this selection is based on the reduced χ2of the Kalman-Filter hypothesis [64,65]forthee+e−pair and on the angle pair between the plane perpendicular to the magnetic field of the ALICE magnet and the e+e−pair plane extrapolated 50 cm beyond the reconstructed conversion point. An additional selection based on the cosine of the pointing angle with cos(θPA )>0.85 is applied, where the pointing angle, θPA, is the angle between the reconstructed photon momentum vector and the vector joining the collision vertex and the conversion point. A selection in the Armenteros-Podolanski plot [66], which contains the distribution of qT=pdaughter ×sin θmother−daugther versus the longitudinal momentum asymmetry [α=(p+ L−p− L)/(p+ L+p− L)] with qT<qT,max1−α2/α2 max where qT,max =0.05 GeV/c and αmax =0.95 removes the remaining contamination from K0 S,, and . Additionally, as explained in Ref. [67], an outof-bunch pileup correction is required for the PCM measurement, which estimates the contamination of photon candidates from multiple overlapping events in the TPC. The correction is obtained from a study of the longitudinal distance of closest approach (DCA) of the conversion photon candidates, which is the smallest distance in beam direction (z) between the primary vertex and the momentum vector of the photon candidate. Photon candidates from different events generate a broad underlying Gaussian-like DCA distribution, which is described with a background estimator to describe the out-of-bunch pileup contribution. This correction is found to be transverse momentum dependent and ranges from 12% at low pT(≈0.5GeV/c)to4%athighpT(≈7GeV/c) at both center-of-mass energies. Photons and electrons/positrons produce electromagnetic showers as they enter an electromagnetic calorimeter and their deposited energy can be measured. By design, these showers usually spread over several adjacent calorimeter cells in the EMCal. Therefore, the reconstruction of the full energy of particles requires the grouping of such adjacent cells into clusters, for which a clusterization algorithm is used. The cell with the highest deposited energy, exceeding a given seed energy, Eseed, is used by the algorithm as a starting point. The cluster is then formed by addition of all adjacent cells with individual energy above a minimum energy, Emin. This aggregation of cells continues as long as the energy of an adjacent cell is smaller than the energy of the previous cell. Otherwise the clusterization algorithm stops the aggregation process. The clustering procedure is repeated until all cells are grouped into clusters. The energy deposited in the individual cells of the cluster is summed to obtain the total cluster energy. For the presented EMC analyses, the values of Eseed =500 MeV and Emin =100 MeV are chosen, which are determined to suppress out-of-bunch background, as well as the general noise level of the front-end electronics. Finally, a correction for the difference of relative energy scale and position of the EMCal between data and simulation is applied, which was obtained by reconstructing in data and simulation the average neutral pion mass peak as a function of the EMCal photon energy, pairing photon candidates from PCM with those of the EMCal [39,44]. To select true photon candidates from the sample of reconstructed clusters, photon identification criteria are applied. Clusters are required to have a minimum energy Ecluster > 0.7 GeV and should consist of at least two cells. EMCal clusters are accepted only if they are within |η|<0.67 and 1.40 rad <ϕ<3.15 rad. A cluster timing selection relative to the collision time of −35 <tcluster <30 ns at √s=8TeV (|tcluster|<50 ns at √s=2.76 TeV) is imposed to remove pileup from multiple events that may occur within the readout interval of the front-end electronics. This constraint removes photon candidates from different bunch crossings with an efficiency of better than 99%. Clusters, which may have a significant contribution from energy deposited by charged hadrons, are rejected by propagating charged particle tracks to the EMCal surface and associating them to clusters based on geometrical criteria, generally called track matching in what follows. Track matching is applied in ηand ϕdepending on track momentum, from |η|<0.04 and |ϕ|<0.09 rad for lowest pTto |η|<0.01 and |ϕ|<0.015 rad at highest pT. Parameterized as |η|<0.01 +(pT+4.07)−2.5and |ϕ|<0.015 + (pT+3.65)−2rad, with pTin units of GeV/c, these criteria result in a track matching efficiency of more than 95% over the full pTrange. Furthermore, the photon purity is significantly improved by the application of a cluster shape selection of 0.1<σ 2 long <(0.32 +0.0072 ·E2 clus/GeV2)for Eclus ⩽5 GeV and 0.1<σ 2 long <0.5forEclus >5GeV, used to suppress the contamination caused by overlapping clusters. Here, σ2 long stands for the larger eigenvalue of the dispersion matrix of the shower shape ellipse defined by the corresponding cell indices in the supermodule and their energy contributions to the cluster [39,68]. In addition, by applying σ2 long >0.1 the contamination caused by neutrons hitting the APDs of the readout electronics is removed. Corrections for reconstruction efficiencies, conversion probability and purity are evaluated using the PYTHIA8[69] and PHOJET [70] MC event generators. Particles generated by the event generator are propagated through the ALICE detector using GEANT3[71]. The same reconstruction algorithms and analysis selection ranges are applied as those in data. The correction factors for both MC productions are found to be consistent, and are therefore combined to reduce the statistical uncertainties. Before the efficiency correction, pT-scale and resolution effects are corrected using Bayesian unfolding [72] with the detector response is used to convert from the reconstructed to the true pTof the photons. Consequently, the reconstruction efficiency is calculated as a function of the true transverse momentum by dividing the reconstructed Monte Carlo (MC) validated photon spectrum by all photons from the simulation. The reconstruction efficiency is found to be largest at pT≈3GeV/cwith 73% for PCM and 56% at pT≈5GeV/cfor EMC in the respective detector acceptance, decreasing with lower and higher pTfor both methods. For the photons reconstructed with PCM, a further correction based on MC information is applied to account for the conversion 024912-5 S. ACHARYA et al. PHYSICAL REVIEW C 99, 024912 (2019) TABLE I. Summary of relative systematic uncertainties in percent for selected pTbins for the reconstruction of inclusive photons and the Rγmeasurement at √s=2.76 TeV. The hybrid method PCM-EMC is abbreviated as P-E in this table. The statistical uncertainties are given in addition to the total systematic uncertainty as well as the uncertainties after combination of the independent measurements. The visible cross section uncertainty for σMBOR of 2.5% is independent from reported measurements and is separately indicated in the figures below. pTinterval (GeV/c)0.4–0.61.6–1.86.0–8.0 Method PCM PCM P-E EMC PCM P-E EMC Measurement Yγincl RγYγincl RγRγYγincl RγYγincl RγRγYγincl Rγ Inner material 4.5 4.5 4.5 4.5 – – – 4.5 4.5 – – – Outer material – – – – 2.1 2.1 3.0 – – 2.1 2.1 3.0 PCM track rec. 0.3 3.3 0.3 1.6 1.3 – – 0.3 8.4 1.3 – – PCM electron PID 0.5 1.4 0.6 1.8 0.4 – – 1.3 13.4 3.8 – – PCM photon PID 0.4 5.4 0.6 2.5 1.1 – – 2.2 11.4 3.1 – – Cluster description – – – – 2.6 2.7 4.1 – – 5.5 2.7 4.0 Cluster energy calib. – – – – 2.0 1.4 2.0 – – 2.6 2.0 2.5 Track match to cluster – – – – 1.5 0.7 0.7 – – 5.7 0.7 1.4 Efficiency – – – – 2.0 1.5 2.5 – – 2.0 1.5 2.5 Signal extraction π0– 5.0 – 2.7 2.1 – 2.5 – 4.9 3.7 – 2.4 Cocktail – 0.9 – 2.2 1.3 – 1.4 – 3.4 3.1 – 2.3 Pileup 2.4 2.6 1.1 1.2 0.9 0.3 – 1.6 1.7 0.9 0.3 – Total syst. uncertainty 5.1 9.7 4.7 6.7 5.6 4.0 6.7 5.4 20.9 11.3 4.3 7.1 Statistical uncertainty 0.2 7.8 0.7 4.3 4.4 0.5 5.0 6.3 23.1 18.3 4.6 8.6 Measurement Yγincl RγYγincl RγYγincl Rγ Comb syst. uncertainty 5.1 9.7 3.1 4.6 2.9 7.1 Comb stat. uncertainty 0.2 7.8 0.4 2.7 3.9 7.2 probability of the photons in the detector material, which increases from 5.6% at the lowest to 8.9% at the highest measured pT, mainly due to the minimum electron track momentum requirement. A correction based on MC information for the contamination of the photon sample from falsely identified and subsequently combined tracks of electrons, pions, kaons, or muons is applied for the conversion method. The purity of the photon sample reconstructed with PCM is found to be 99% up to 3 GeV/cand decreases down to 96% at high transverse momentum due to the increasing contamination from electron-pion pairs. For the calorimeter-based method, a similar purity correction is applied but for falsely identified photon candidates mainly from clusters created by neutrons and antineutrons at low pTand by neutral kaons at high pT. The purity correction is the largest for pT<3GeV/cwhere purities between 87% and 97% rising with pTwere found, while at high pTthe purities reach values of 97%. For the inclusive photon measurements, contributions of secondary photons from weak decays and hadronic interactions are estimated and removed. The main source of photons from weak decays are K0 Sdecays, however, contributions from K0 Land are also considered. The correction uses the decay-photon cocktail simulation described in Sec. II, which provides the secondary photon yields. Taking into account the detector response and detection efficiency for the different reconstruction techniques, these photons are removed from the photon sample. The remaining correction factor for secondary photons, for example due to interactions with the detector material, are obtained purely from MC information. The secondary corrections are of the order of 1–3% for K0 S, 0.05–0.2% for K0 L,0.02% for and 0.1–2.5% for material interactions depending on pTand on the photon reconstruction technique within the given ranges. In general the correction factors tend to be larger for the EMC reconstruction technique, due to the worse pointing resolution of the photons. The neutral pion and ηmeson measurements, which are needed to extract Rγfrom Eq. (1), are described in detail in Refs. [39,44]. The meson yields are obtained for PCM, EMC, and a hybrid method (PCM-EMC), in which photon candidates reconstructed with PCM are paired with those reconstructed in the EMCal. For the measurement of Rγwith the PCM-EMC method, it is beneficial to measure the inclusive photons with PCM. However, to be consistent with the corresponding meson measurements [44], a wider selection range of −4<nσe<5 on the energy loss hypothesis of the electron/positron in the √s=8 TeV measurement is used, and the charged pion dE/dx-based rejection is applied independent of pTin both collision systems for the corresponding inclusive photon measurement with PCM. V. SYSTEMATIC UNCERTAINTIES Systematic uncertainties are summarized for the measurements of Yγincl and Rγin Table Ifor √s=2.76 TeV and in Table II for √s=8TeV and shown for three transverse momentum bins used in the analyses. The uncertainties are given in percent and for each reconstruction method individually. The detailed description of uncertainties related to the π0 meson measurements that enter into the calculation of the direct photon excess ratios Rγcan be found in Ref. [39] for √s=2.76 TeV and in Ref. [44]for√s=8TeV. All 024912-6 DIRECT PHOTON PRODUCTION AT LOW TRANSVERSE … PHYSICAL REVIEW C 99, 024912 (2019) TABLE II. Summary of relative systematic uncertainties in percent for selected pTbins for the reconstruction of inclusive photons and the Rγmeasurement at √s=8 TeV. The hybrid method PCM-EMC is abbreviated as P-E in this table. The statistical uncertainties are given in addition to the total systematic uncertainty as well as the uncertainties after combination of the independent measurements. The visible cross section uncertainty of 2.6% is independent from reported uncertainties and is separately indicated in the figures below. pTinterval (GeV/c)0.4–0.61.6–1.89.0–12.0 Method PCM PCM P-E EMC PCM P-E EMC Measurement Yγincl RγYγincl RγRγYγincl RγYγincl RγRγYγincl Rγ Inner material 4.5 4.5 4.5 4.5 – – – 4.5 4.5 – – – Outer material – – – – 2.1 2.1 3.0 – – 2.1 2.1 3.0 PCM track rec. 0.2 0.5 0.1 0.5 0.2 – – 0.1 0.5 0.2 – – PCM electron PID 1.1 2.4 0.6 0.8 0.3 – – 0.7 0.8 0.8 – – PCM photon PID 1.8 1.2 1.3 1.0 1.0 – – 2.3 5.5 1.7 – – Cluster description – – – – 2.5 2.6 3.0 – – 3.0 2.6 1.9 Cluster energy calib. – – – – 2.3 1.4 2.3 – – 1.8 0.9 1.8 Track match to cluster – – – – 0.2 1.8 1.5 – – 1.9 1.8 1.6 Efficiency 0.5 0.5 0.5 0.5 2.1 1.8 2.7 0.5 0.5 2.1 1.8 2.7 Signal extraction π0– 4.9 – 1.6 1.8 – 2.7 – 6.6 3.1 – 1.9 Cocktail 0.2 1.7 0.1 0.7 1.0 0.3 0.8 0.1 0.5 1.9 0.3 1.4 Pileup 3.8 4.3 2.7 4.2 2.7 0.1 – 4.3 4.4 3.0 0.1 – Total syst. uncertainty 6.3 8.5 5.4 6.6 5.7 4.4 6.4 6.4 10.7 7.1 4.3 5.6 Statistical uncertainty 0.1 4.3 0.3 2.2 2.1 0.2 2.7 3.3 17.2 9.9 2.1 4.7 Measurement Yγincl RγYγincl RγYγincl Rγ Comb syst. uncertainty 6.3 8.5 3.5 4.5 3.6 5.9 Comb stat. uncertainty 0.1 4.3 0.2 1.4 1.8 4.3 uncertainties are evaluated on the fully corrected spectra of Yγincl or directly on Rγ. In case of Rγ, the systematic uncertainties therefore also contain the effects of the systematic variations on the measured neutral pion spectrum, thus benefiting from partial cancellations of common uncertainties. For the PCM measurements, the material budget uncertainty is the main contributor to the total uncertainty and its value of 4.5% was previously determined in Refs. [55,73]. Systematic uncertainties associated with track reconstruction are the uncertainties that are estimated from variations of required TPC clusters as well as minimum transverse momentum requirements of tracks. Particle identification (PID) uncertainties are determined by variation of the PID selection ranges of electrons and photons as described in Sec. IV. A systematic uncertainty is estimated for the pileup corrections that are applied in the analyses. It is dominated by the contribution from the DCA background description for the out-of-bunch pileup estimation but also contains the uncertainty from the SPD in-bunch pileup rejection due to its limited efficiency. The systematic uncertainty of the EMC measurement contains a large contribution from the limited knowledge of the outer material budget, which is composed by all detector components from the radial center of the TPC up to the EMCal. This uncertainty is determined by comparing the effects on the corrected spectra using inputs from data taking campaigns with and without TRD modules in front of the EMCal. This could be done as the EMCal was masked only partially by the TRD during the data taking in 2011 and 2012. The material budgets of TRD and TOF are roughly similar and therefore the quoted uncertainty is taken as √2 times the difference of the corrected spectra with and without TRD modules in front of the EMCal. Systematic uncertainties contributing to the cluster description category are the uncertainties associated to the description of clusters in simulation, which influence the reconstruction efficiencies. The associated variables are the minimum cluster energy, shower shape, number of cells, time, and clusterization seed, as well as minimum energy selection variations. The uncertainty of nonlinearity effects as well as the energy scale of clusters are incorporated in the cluster energy calibration. To assess this uncertainty different parametrizations for the MC π0mass peak position correction are considered to account for the residual differences between data and MC. The efficiency uncertainty reflects the differences between the MC generators that are used for the efficiency calculation. The pileup systematic uncertainty reflects the finite efficiency of the SPD for in-bunch pileup rejection. The hybrid method PCM-EMC requires the same evaluation of uncertainties as its individual stand-alone methods. However, most systematics show a different size or behavior on Rγas the contained inclusive photon measurement is PCM based whereas for the neutral pion one photon candidate of each reconstruction approach is used. In addition, the track matching to cluster uncertainty includes the uncertainties associated with the matching ofV0tracks or primary tracks with the cluster, which is an important ingredient for the hybrid method. The uncertainty on the decay-photon simulation is obtained by varying the parametrizations of the neutral pion and ηmeson for each reconstruction technique within the pT-uncorrelated systematic and statistical uncertainties. This leads to an associated uncertainty of 0.9–3% and 0.5–2% for pp collisions at √s=2.76 and 8 TeV, respectively, which 024912-7 S. ACHARYA et al. PHYSICAL REVIEW C 99, 024912 (2019) strongly depends on pT. Furthermore, a variation of the mTscaling constants has been considered for the remaining mesons, which yields an uncertainty below 0.1%. Partial systematic uncertainty cancellations are present for the direct photon excess ratio Rγ. The material budget uncertainty in the PCM measurement, which enters once in the inclusive photon measurement and twice in the neutral pion measurement, cancels once in Rγ. A similar cancellation is present in the EMC measurement, where the outer material budget uncertainty cancels partially in the double ratio as well. For the hybrid method, the inner material budget uncertainty cancels fully in the double ratio and only the outer material budget uncertainty enters once in the total uncertainty, which is the main advantage of using this reconstruction method for Rγ. The final estimated systematic uncertainties on the inclusive photon cross section amount to 5–7% for the conversion method and 4–9% for the EMC measurement in the measured pTrange. The material budget uncertainty of the conversion method is the dominant source, whereas the calorimeter-based method shows a strong dependence on the cluster description in the simulation and the associated efficiency estimates. With statistical uncertainties below 1% for pT<3GeV/c the inclusive photon measurement is therefore limited by the systematic uncertainties. The systematic uncertainties on the direct photon excess ratio Rγare larger than for the inclusive photons due to the addition of the neutral-pion-related uncertainties. For PCM, the systematic uncertainties amount to 6–20% dominated by the material budget uncertainty and the neutral pion signal extraction uncertainties at low and high pT. Systematic uncertainties for the EMC measurements are smaller at high transverse momentum compared to PCM with values of 7–9% at √s=2.76 TeV and 6–8% at √s=8 TeV with dominant contributions of the outer material budget, the cluster description and neutral pion signal extraction uncertainties. Mostly due to the cancellation of the inner material budget uncertainty, the hybrid method PCM-EMC exhibits the smallest systematic uncertainty at intermediate pTwith values of 6% at √s=2.76 TeV and 5.4% at √s=8TeV. VI. RESULTS The invariant cross sections for inclusive photons at midrapidity (|y|<0.9) are given as Ed3σpp→γ+X dp 3=1 2πpT 1 Lint pur PconvrecA Fpile-up ·Nγ−Nγ sec ypT , (5) where pur,Pconv, and rec are the purity, conversion probability and reconstruction efficiency correction factors, respectively, and Lint is the integrated luminosity. The conversion probability as well as the out-of-bunch pileup correction factor (Fpile-up) only apply for the PCM measurement. The acceptance correction factor, A, is only applied for EMCal to account for the limited azimuth coverage. In addition, the inclusive photon raw yield is given by Nγand the summed secondary photon raw yields by Nγ sec. Furthermore, the interval ranges in rapidity and transverse momentum are given by ypT. The double ratios are measured by combining the individual inclusive photon and neutral pion spectra from the same reconstruction methods with a cocktail simulation based on the same neutral pion spectrum. In this way, possible biases can be removed, since they would affect both the inclusive photon and the neutral pion measurements. The individually measured inclusive photon invariant differential cross sections of the PCM and EMC as well as the double ratios of the PCM, PCM-EMC, and EMC reconstruction methods are combined to obtain the final spectra and double ratios, respectively. For the combination, the best linear unbiased estimates (BLUE) method [82–86] with full treatment of statistical and systematic uncertainty correlations was used. For the inclusive photon measurement, the EMC measurement is assumed to be fully independent of the PCM measurement both statistically and systematically. However, for Rγthe statistical uncertainties show partial correlation between the PCM and PCM-EMC, which are determined to be ∼20–50% depending on pT, since both measurements are based on the PCM inclusive photon measurement using different subsets of the data; however, the statistical uncertainties of the neutral pion measurements are fully independent due to their different reconstruction methods. The systematic uncertainty correlations were approximated via pTdependent correlation factors. It has been found that the largest correlations of the systematic uncertainties are among the PCM-EMC and the PCM or EMC methods, respectively. The fraction of correlation among the systematic uncertainties of the PCM-EMC and EMC method has been estimated to be between 60–80%, which can be attributed to the common uncertainty regarding the cluster reconstruction and efficiency uncertainties as well as the outer material budget. For the PCM and PCM-EMC methods the uncertainties regarding the PCM photon identification and selection are largely correlated and thus the correlation factor ranges between 45–70% depending on transverse momentum. The combined invariant cross sections of inclusive and direct photons, as well as the direct photon excess ratios Rγ, cover transverse momentum ranges of 0.4<pT<10 GeV/c and 0.3<pT<16 GeV/cfor √s=2.76 and 8 TeV, respectively. The combined inclusive photon spectra are shown in Fig. 5together with a two-component model (TCM) fit [87], whose functional form is a combination of an exponential function at low pTand a power law at high pT, given as Ed3σ dp 3=Aeexp −pT Te+A1+p2 T T2n−n ,(6) with the free parameters Ae,A,Te,T, and n. The twocomponent model is fitted to the inclusive photon spectra by using the total uncertainties of the spectra, obtained by quadratic combination of statistical and systematic uncertainties. It is used only to facilitate a comparison of the methods in the ratio to the fit. The ratios of the inclusive photon spectra measured individually by PCM and EMC relative to the TCM fit are shown in Fig. 2, demonstrating that the inclusive spectra measured with PCM and EMC agree within the uncertainties. 024912-8 DIRECT PHOTON PRODUCTION AT LOW TRANSVERSE … PHYSICAL REVIEW C 99, 024912 (2019) R. S. Camacho,2P. Camerini,27 A. A. Capon,110 F. Carena,36 W. Carena,36 F. Carnesecchi,29,11 J. Castillo Castellanos,134 A. J. Castro,127 E. A. R. Casula,54 C. Ceballos Sanchez,9S. Chandra,138 B. Chang,124 W. Chang,7S. Chapeland,36 M. Chartier,125 S. Chattopadhyay,138 S. Chattopadhyay,106 A. Chauvin,114,102 C. Cheshkov,132 B. Cheynis,132 V. Chibante Barroso,36 D. D. Chinellato,119 S. Cho,60 P. Chochula,36 T. Chowdhury,131 P. Christakoglou,89 C. H. Christensen,88 P. Christiansen,80 T. Chujo,130 S. U. Chung,20 C. Cicalo,54 L. Cifarelli,11,29 F. Cindolo,53 J. Cleymans,122 F. Colamaria,52 D. Colella,65,52,36 A. Collu,79 M. Colocci,29 M. Concas,58,aG. Conesa Balbastre,78 Z. Conesa del Valle,61 J. G. Contreras,38 T. M. Cormier,94 Y. Corrales Morales,58 P. Cortese,34 M. R. Cosentino,120 F. Costa,36 S. Costanza,135 J. Crkovská,61 P. Crochet,131 E. Cuautle,70 L. Cunqueiro,94,141 T. Dahms,102,114 A. Dainese,56 M. C. Danisch,101 A. Danu,68 D. Das,106 I. Das,106 S. Das,4A. Dash,85 S. Dash,48 S. De,49 A. De Caro,32 G. de Cataldo,52 C. de Conti,118 J. de Cuveland,40 A. De Falco,26 D. De Gruttola,11,32 N. De Marco,58 S. De Pasquale,32 R. D. De Souza,119 H. F. Degenhardt,118 A. Deisting,103,101 A. Deloff,84 S. Delsanto,28 C. Deplano,89 P. Dhankher,48 D. Di Bari,35 A. Di Mauro,36 B. Di Ruzza,56 R. A. Diaz,9T. Dietel,122 P. Dillenseger,69 Y. Ding,7R. Divià,36 Ø. Djuvsland,24 A. Dobrin,36 D. Domenicis Gimenez,118 B. Dönigus,69 O. Dordic,23 L. V. R. Doremalen,63 A. K. Dubey,138 A. Dubla,103 L. Ducroux,132 S. Dudi,97 A. K. Duggal,97 M. Dukhishyam,85 P. Dupieux,131 R. J. Ehlers,143 D. Elia,52 E. Endress,108 H. Engel,74 E. Epple,143 B. Erazmus,111 F. Erhardt,96 M. R. Ersdal,24 B. Espagnon,61 G. Eulisse,36 J. Eum,20 D. Evans,107 S. Evdokimov,90 L. Fabbietti,102,114 M. Faggin,31 J. Faivre,78 A. Fantoni,51 M. Fasel,94 L. Feldkamp,141 A. Feliciello,58 G. Feofilov,137 A. Fernández Téllez,2A. Ferretti,28 A. Festanti,31,36 V. J. G. Feuillard,134,131 J. Figiel,115 M. A. S. Figueredo,118 S. Filchagin,105 D. Finogeev,62 F. M. Fionda,24 G. Fiorenza,52 M. Floris,36 S. Foertsch,73 P. Foka,103 S. Fokin,87 E. Fragiacomo,59 A. Francescon,36 A. Francisco,111 U. Frankenfeld,103 G. G. Fronze,28 U. Fuchs,36 C. Furget,78 A. Furs,62 M. Fusco Girard,32 J. J. Gaardhøje,88 M. Gagliardi,28 A. M. Gago,108 K. Gajdosova,88 M. Gallio,28 C. D. Galvan,117 P. Ganoti,83 C. Garabatos,103 E. Garcia-Solis,12 K. Garg,30 C. Gargiulo,36 P. Gasik,102,114 E. F. Gauger,116 M. B. Gay Ducati,71 M. Germain,111 J. Ghosh,106 P. Ghosh,138 S. K. Ghosh,4P. Gianotti,51 P. Giubellino,58,103 P. Giubilato,31 P. Glässel,101 D. M. Goméz Coral,72 A. Gomez Ramirez,74 V. Gonzalez,103 P. González-Zamora,2S. Gorbunov,40 L. Görlich,115 S. Gotovac,126 V. Grabski,72 L. K. Graczykowski,139 K. L. Graham,107 L. Greiner,79 A. Grelli,63 C. Grigoras,36 V. Grigoriev,91 A. Grigoryan,1S. Grigoryan,75 J. M. Gronefeld,103 F. Grosa,33 J. F. Grosse-Oetringhaus,36 R. Grosso,103 R. Guernane,78 B. Guerzoni,29 M. Guittiere,111 K. Gulbrandsen,88 T. Gunji,129 A. Gupta,98 R. Gupta,98 I. B. Guzman,2R. Haake,36 M. K. Habib,103 C. Hadjidakis,61 H. Hamagaki,81 G. Hamar,142 J. C. Hamon,133 M. R. Haque,63 J. W. Harris,143 A. Harton,12 H. Hassan,78 D. Hatzifotiadou,53,11 S. Hayashi,129 S. T. Heckel,69 E. Hellbär,69 H. Helstrup,37 A. Herghelegiu,47 E. G. Hernandez,2G. Herrera Corral,10 F. Herrmann,141 K. F. Hetland,37 T. E. Hilden,44 H. Hillemanns,36 C. Hills,125 B. Hippolyte,133 B. Hohlweger,102 D. Horak,38 S. Hornung,103 R. Hosokawa,130,78 P. Hristov,36 C. Hughes,127 P. Huhn,69 T. J. Humanic,19 H. Hushnud,106 N. Hussain,42 T. Hussain,18 D. Hutter,40 D. S. Hwang,21 J. P. Iddon,125 S. A. Iga Buitron,70 R. Ilkaev,105 M. Inaba,130 M. Ippolitov,87 M. S. Islam,106 M. Ivanov,103 V. Ivanov,95 V. Izucheev,90 B. Jacak,79 N. Jacazio,29 P. M. Jacobs,79 M. B. Jadhav,48 S. Jadlovska,113 J. Jadlovsky,113 S. Jaelani,63 C. Jahnke,118,114 M. J. Jakubowska,139 M. A. Janik,139 C. Jena,85 M. Jercic,96 R. T. Jimenez Bustamante,103 M. Jin,123 P. G. Jones,107 A. Jusko,107 P. Kalinak,65 A. Kalweit,36 J. H. Kang,144 V. Kaplin,91 S. Kar,7A. Karasu Uysal,77 O. Karavichev,62 T. Karavicheva,62 P. Karczmarczyk,36 E. Karpechev,62 U. Kebschull,74 R. Keidel,46 D. L. D. Keijdener,63 M. Keil,36 B. Ketzer,43 Z. Khabanova,89 S. Khan,18 S. A. Khan,138 A. Khanzadeev,95 Y. Kharlov,90 A. Khatun,18 A. Khuntia,49 M. M. Kielbowicz,115 B. Kileng,37 B. Kim,130 D. Kim,144 D. J. Kim,124 E. J. Kim,14 H. Kim,144 J. S. Kim,41 J. Kim,101 M. Kim,60,101 S. Kim,21 T. Kim,144 T. Kim,144 S. Kirsch,40 I. Kisel,40 S. Kiselev,64 A. Kisiel,139 J.L. Klay,6C. Klein,69 J. Klein,36,58 C. Klein-Bösing,141 S. Klewin,101 A. Kluge,36 M. L. Knichel,101,36 A. G. Knospe,123 C. Kobdaj,112 M. Kofarago,142 M. K. Köhler,101 T. Kollegger,103 N. Kondratyeva,91 E. Kondratyuk,90 A. Konevskikh,62 M. Konyushikhin,140 O. Kovalenko,84 V. Kovalenko,137 M. Kowalski,115 I. Králik,65 A. Kravˇ cáková,39 L. Kreis,103 M. Krivda,65,107 F. Krizek,93 M. Krüger,69 E. Kryshen,95 M. Krzewicki,40 A. M. Kubera,19 V. Ku ˇ cera,93,60 C. Kuhn,133 P. G. Kuijer,89 J. Kumar,48 L. Kumar,97 S. Kumar,48 S. Kundu,85 P. Kurashvili,84 A. Kurepin,62 A. B. Kurepin,62 A. Kuryakin,105 S. Kushpil,93 M. J. Kweon,60 Y. Kwon,144 S. L. La Pointe,40 P. La Rocca,30 Y. S. Lai,79 I. Lakomov,36 R. Langoy,121 K. Lapidus,143 C. Lara,74 A. Lardeux,23 P. Larionov,51 A. Lattuca,28 E. Laudi,36 R. Lavicka,38 R. Lea,27 L. Leardini,101 S. Lee,144 F. Lehas,89 S. Lehner,110 J. Lehrbach,40 R. C. Lemmon,92 E. Leogrande,63 I. León Monzón,117 P. Lévai,142 X. Li,13 X. L. Li,7J. Lien,121 R. Lietava,107 B. Lim,20 S. Lindal,23 V. Lindenstruth,40 S. W. Lindsay,125 C. Lippmann,103 M. A. Lisa,19 V. Litichevskyi,44 A. Liu,79 H. M. Ljunggren,80 W. J. Llope,140 D. F. Lodato,63 V. Loginov,91 C. Loizides,94,79 P. Loncar,126 X. Lopez,131 E. López Torres,9A. Lowe,142 P. Luettig,69 J. R. Luhder,141 M. Lunardon,31 G. Luparello,59 M. Lupi,36 A. Maevskaya,62 M. Mager,36 S. M. Mahmood,23 A. Maire,133 R. D. Majka,143 M. Malaev,95 L. Malinina,75,bD. Mal’Kevich,64 P. Malzacher,103 A. Mamonov,105 V. Manko,87 F. Manso,131 V. Manzari,52 Y. Mao,7M. Marchisone,132,128,73 J. Mareš,67 G. V. Margagliotti,27 A. Margotti,53 J. Margutti,63 A. Marín,103 C. Markert,116 M. Marquard,69 N. A. Martin,103 P. Martinengo,36 M. I. Martínez,2 G. Martínez García,111 M. Martinez Pedreira,36 S. Masciocchi,103 M. Masera,28 A. Masoni,54 L. Massacrier,61 E. Masson,111 A. Mastroserio,52 A. M. Mathis,102,114 P. F. T. Matuoka,118 A. Matyja,115,127 C. Mayer,115 M. Mazzilli,35 M. A. Mazzoni,57 F. Meddi,25 Y. Melikyan,91 A. Menchaca-Rocha,72 E. Meninno,32 J. Mercado Pérez,101 M. Meres,15 C. S. Meza,108 S. Mhlanga,122 Y. Miake,130 L. Micheletti,28 M. M. Mieskolainen,44 D. L. Mihaylov,102 K. Mikhaylov,64,75 A. Mischke,63 A. N. Mishra,70 D. Mi´ skowiec,103 J. Mitra,138 C. M. Mitu,68 N. Mohammadi,36,63 A. P. Mohanty,63 B. Mohanty,85 M. Mohisin Khan,18,cD. A. Moreira De Godoy,141 L. A. P. Moreno,2S. Moretto,31 A. Morreale,111 A. Morsch,36 024912-15 S. ACHARYA et al. PHYSICAL REVIEW C 99, 024912 (2019) V. Muccifora,51 E. Mudnic,126 D. Mühlheim,141 S. Muhuri,138 M. Mukherjee,4J. D. Mulligan,143 M. G. Munhoz,118 K. Münning,43 M. I. A. Munoz,79 R. H. Munzer,69 H. Murakami,129 S. Murray,73 L. Musa,36 J. Musinsky,65 C. J. Myers,123 J. W. Myrcha,139 B. Naik,48 R. Nair,84 B. K. Nandi,48 R. Nania,53,11 E. Nappi,52 A. Narayan,48 M. U. Naru,16 H. Natal da Luz,118 C. Nattrass,127 S. R. Navarro,2K. Nayak,85 R. Nayak,48 T. K. Nayak,138 S. Nazarenko,105 R. A. Negrao De Oliveira,69,36 L. Nellen,70 S. V. Nesbo,37 G. Neskovic,40 F. Ng,123 M. Nicassio,103 J. Niedziela,139,36 B. S. Nielsen,88 S. Nikolaev,87 S. Nikulin,87 V. Nikulin,95 F. Noferini,11,53 P. Nomokonov,75 G. Nooren,63 J. C. C. Noris,2J. Norman,78,125 A. Nyanin,87 J. Nystrand,24 H. Oh,144 A. Ohlson,101 J. Oleniacz,139 A. C. Oliveira Da Silva,118 M. H. Oliver,143 J. Onderwaater,103 C. Oppedisano,58 R. Orava,44 M. Oravec,113 A. Ortiz Velasquez,70 A. Oskarsson,80 J. Otwinowski,115 K. Oyama,81 Y. Pachmayer,101 V. Pacik,88 D. Pagano,136 G. Pai´ c,70 P. Palni,7J. Pan,140 A. K. Pandey,48 S. Panebianco,134 V. Papikyan,1P. Pareek,49 J. Park,60 J. E. Parkkila,124 S. Parmar,97 A. Passfeld,141 S. P. Pathak,123 R. N. Patra,138 B. Paul,58 H. Pei,7T. Peitzmann,63 X. Peng,7L. G. Pereira,71 H. Pereira Da Costa,134 D. Peresunko,87 E. Perez Lezama,69 V. Peskov,69 Y. Pestov,5V. Petrᡠcek,38 M. Petrovici,47 C. Petta,30 R. P. Pezzi,71 S. Piano,59 M. Pikna,15 P. Pillot,111 L. O. D. L. Pimentel,88 O. Pinazza,53,36 L. Pinsky,123 S. Pisano,51 D. B. Piyarathna,123 M. Płosko´ n,79 M. Planinic,96 F. Pliquett,69 J. Pluta,139 S. Pochybova,142 P. L. M. Podesta-Lerma,117 M. G. Poghosyan,94 B. Polichtchouk,90 N. Poljak,96 W. Poonsawat,112 A. Pop,47 H. Poppenborg,141 S. Porteboeuf-Houssais,131 V. Pozdniakov,75 S. K. Prasad,4R. Preghenella,53 F. Prino,58 C. A. Pruneau,140 I. Pshenichnov,62 M. Puccio,28 V. Punin,105 J. Putschke,140 S. Raha,4S. Rajput,98 J. Rak,124 A. Rakotozafindrabe,134 L. Ramello,34 F. Rami,133 R. Raniwala,99 S. Raniwala,99 S. S. Räsänen,44 B. T. Rascanu,69 V. Ratza,43 I. Ravasenga,33 K. F. Read,127,94 K. Redlich,84,dA. Rehman,24 P. Reichelt,69 F. Reidt,36 X. Ren,7R. Renfordt,69 A. Reshetin,62 J.-P. Revol,11 K. Reygers,101 V. Riabov,95 T. Richert,63,80 M. Richter,23 P. Riedler,36 W. Riegler,36 F. Riggi,30 C. Ristea,68 M. Rodríguez Cahuantzi,2K. Røed,23 R. Rogalev,90 E. Rogochaya,75 D. Rohr,36 D. Röhrich,24 P. S. Rokita,139 F. Ronchetti,51 E. D. Rosas,70 K. Roslon,139 P. Rosnet,131 A. Rossi,31,56 A. Rotondi,135 F. Roukoutakis,83 C. Roy,133 P. Roy,106 O.V. Rueda,70 R. Rui,27 B. Rumyantsev,75 A. Rustamov,86 E. Ryabinkin,87 Y. Ryabov,95 A. Rybicki,115 S. Saarinen,44 S. Sadhu,138 S. Sadovsky,90 K. Šafaˇ rík,36 S. K. Saha,138 B. Sahoo,48 P. Sahoo,49 R. Sahoo,49 S. Sahoo,66 P. K. Sahu,66 J. Saini,138 S. Sakai,130 M. A. Saleh,140 S. Sambyal,98 V. Samsonov,95,91 A. Sandoval,72 A. Sarkar,73 D. Sarkar,138 N. Sarkar,138 P. Sarma,42 M. H. P. Sas,63 E. Scapparone,53 F. Scarlassara,31 B. Schaefer,94 H. S. Scheid,69 C. Schiaua,47 R. Schicker,101 C. Schmidt,103 H. R. Schmidt,100 M.O. Schmidt,101 M. Schmidt,100 N. V. Schmidt,94,69 J. Schukraft,36 Y. Schutz,36,133 K. Schwarz,103 K. Schweda,103 G. Scioli,29 E. Scomparin,58 M. Šefˇ cík,39 J. E. Seger,17 Y. Sekiguchi,129 D. Sekihata,45 I. Selyuzhenkov,91,103 K. Senosi,73 S. Senyukov,133 E. Serradilla,72 P. Sett,48 A. Sevcenco,68 A. Shabanov,62 A. Shabetai,111 R. Shahoyan,36 W. Shaikh,106 A. Shangaraev,90 A. Sharma,97 A. Sharma,98 N. Sharma,97 A. I. Sheikh,138 K. Shigaki,45 M. Shimomura,82 S. Shirinkin,64 Q. Shou,7,109 K. Shtejer,28 Y. Sibiriak,87 S. Siddhanta,54 K. M. Sielewicz,36 T. Siemiarczuk,84 D. Silvermyr,80 G. Simatovic,89 G. Simonetti,102,36 R. Singaraju,138 R. Singh,85 V. Singhal,138 T. Sinha,106 B. Sitar,15 M. Sitta,34 T. B. Skaali,23 M. Slupecki,124 N. Smirnov,143 R. J. M. Snellings,63 T. W. Snellman,124 J. Song,20 F. Soramel,31 S. Sorensen,127 F. Sozzi,103 I. Sputowska,115 J. Stachel,101 I. Stan,68 P. Stankus,94 E. Stenlund,80 D. Stocco,111 M. M. Storetvedt,37 P. Strmen,15 A. A. P. Suaide,118 T. Sugitate,45 C. Suire,61 M. Suleymanov,16 M. Suljic,36,27 R. Sultanov,64 M. Šumbera,93 S. Sumowidagdo,50 K. Suzuki,110 S. Swain,66 A. Szabo,15 I. Szarka,15 U. Tabassam,16 J. Takahashi,119 G. J. Tambave,24 N. Tanaka,130 M. Tarhini,61,111 M. Tariq,18 M. G. Tarzila,47 A. Tauro,36 G. Tejeda Muñoz,2A. Telesca,36 C. Terrevoli,31 B. Teyssier,132 D. Thakur,49 S. Thakur,138 D. Thomas,116 F. Thoresen,88 R. Tieulent,132 A. Tikhonov,62 A. R. Timmins,123 A. Toia,69 N. Topilskaya,62 M. Toppi,51 S. R. Torres,117 S. Tripathy,49 S. Trogolo,28 G. Trombetta,35 L. Tropp,39 V. Trubnikov,3 W. H. Trzaska,124 T. P. Trzcinski,139 B. A. Trzeciak,63 T. Tsuji,129 A. Tumkin,105 R. Turrisi,56 T. S. Tveter,23 K. Ullaland,24 E. N. Umaka,123 A. Uras,132 G. L. Usai,26 A. Utrobicic,96 M. Vala,113 J. W. Van Hoorne,36 M. van Leeuwen,63 P. Vande Vyvre,36 D. Varga,142 A. Vargas,2M. Vargyas,124 R. Varma,48 M. Vasileiou,83 A. Vasiliev,87 A. Vauthier,78 O. Vázquez Doce,102,114 V. Vechernin,137 A. M. Veen,63 A. Velure,24 E. Vercellin,28 S. Vergara Limón,2L. Vermunt,63 R. Vernet,8R. Vértesi,142 L. Vickovic,126 J. Viinikainen,124 Z. Vilakazi,128 O. Villalobos Baillie,107 A. Villatoro Tello,2 A. Vinogradov,87 L. Vinogradov,137 T. Virgili,32 V. Vislavicius,80 A. Vodopyanov,75 M. A. Völkl,100 K. Voloshin,64 S. A. Voloshin,140 G. Volpe,35 B. von Haller,36 I. Vorobyev,114,102 D. Voscek,113 D. Vranic,103,36 J. Vrláková,39 B. Wagner,24 H. Wang,63 M. Wang,7Y. Watanabe,130,129 M. Weber,110 S. G. Weber,103 A. Wegrzynek,36 D. F. Weiser,101 S. C. Wenzel,36 J. P. Wessels,141 U. Westerhoff,141 A. M. Whitehead,122 J. Wiechula,69 J. Wikne,23 G. Wilk,84 J. Wilkinson,53 G. A. Willems,141,36 M. C. S. Williams,53 E. Willsher,107 B. Windelband,101 W. E. Witt,127 R. Xu,7S. Yalcin,77 K. Yamakawa,45 S. Yano,45 Z. Yin,7H. Yokoyama,130,78 I.-K. Yoo,20 J. H. Yoon,60 V. Yurchenko,3V. Zaccolo,58 A. Zaman,16 C. Zampolli,36 H. J. C. Zanoli,118 N. Zardoshti,107 A. Zarochentsev,137 P. Závada,67 N. Zaviyalov,105 H. Zbroszczyk,139 M. Zhalov,95 X. Zhang,7Y. Zhang,7Z. Zhang,131,7C. Zhao,23 N. Zhigareva,64 D. Zhou,7Y. Zhou,88 Z. Zhou,24 H. Zhu,7J. Zhu,7Y. Zhu,7 A. Zichichi,29,11 M. B. Zimmermann,36 G. Zinovjev,3J. Zmeskal,110 and S. Zou7 (ALICE Collaboration) 1A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2Benemérita Universidad Autónoma de Puebla, Puebla, Mexico 3Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kiev, Ukraine 024912-16 DIRECT PHOTON PRODUCTION AT LOW TRANSVERSE … PHYSICAL REVIEW C 99, 024912 (2019) 4Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5Budker Institute for Nuclear Physics, Novosibirsk, Russia 6California Polytechnic State University, San Luis Obispo, California, United States 7Central China Normal University, Wuhan, China 8Centre de Calcul de l’IN2P3, Villeurbanne, Lyon, France 9Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 10Centro de Investigación y de Estudios Avanzados (CINVESTAV), Mexico City and Mérida, Mexico 11Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi’, Rome, Italy 12Chicago State University, Chicago, Illinois, United States 13China Institute of Atomic Energy, Beijing, China 14Chonbuk National University, Jeonju, Republic of Korea 15Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia 16COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan 17Creighton University, Omaha, Nebraska, United States 18Department of Physics, Aligarh Muslim University, Aligarh, India 19Department of Physics, Ohio State University, Columbus, Ohio, United States 20Department of Physics, Pusan National University, Pusan, Republic of Korea 21Department of Physics, Sejong University, Seoul, Republic of Korea 22Department of Physics, University of California, Berkeley, California, United States 23Department of Physics, University of Oslo, Oslo, Norway 24Department of Physics and Technology, University of Bergen, Bergen, Norway 25Dipartimento di Fisica dell’Università ’La Sapienza’ and Sezione INFN, Rome, Italy 26Dipartimento di Fisica dell’Università and Sezione INFN, Cagliari, Italy 27Dipartimento di Fisica dell’Università and Sezione INFN, Trieste, Italy 28Dipartimento di Fisica dell’Università and Sezione INFN, Turin, Italy 29Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Bologna, Italy 30Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Catania, Italy 31Dipartimento di Fisica e Astronomia dell’Università and Sezione INFN, Padova, Italy 32Dipartimento di Fisica ‘E.R. Caianiello’ dell’Università and Gruppo Collegato INFN, Salerno, Italy 33Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 34Dipartimento di Scienze e Innovazione Tecnologica dell’Università del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy 35Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 36European Organization for Nuclear Research (CERN), Geneva, Switzerland 37Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 38Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 39Faculty of Science, P.J. Šafárik University, Košice, Slovakia 40Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 41Gangneung-Wonju National University, Gangneung, Republic of Korea 42Gauhati University, Department of Physics, Guwahati, India 43Helmholtz-Institut für Strahlenund Kernphysik, Rheinische Friedrich-Wilhelms-Universität Bonn, Bonn, Germany 44Helsinki Institute of Physics (HIP), Helsinki, Finland 45Hiroshima University, Hiroshima, Japan 46Hochschule Worms, Zentrum für Technologietransfer und Telekommunikation (ZTT), Worms, Germany 47Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 48Indian Institute of Technology Bombay (IIT), Mumbai, India 49Indian Institute of Technology Indore, Indore, India 50Indonesian Institute of Sciences, Jakarta, Indonesia 51INFN, Laboratori Nazionali di Frascati, Frascati, Italy 52INFN, Sezione di Bari, Bari, Italy 53INFN, Sezione di Bologna, Bologna, Italy 54INFN, Sezione di Cagliari, Cagliari, Italy 55INFN, Sezione di Catania, Catania, Italy 56INFN, Sezione di Padova, Padova, Italy 57INFN, Sezione di Roma, Rome, Italy 58INFN, Sezione di Torino, Turin, Italy 59INFN, Sezione di Trieste, Trieste, Italy 60Inha University, Incheon, Republic of Korea 61Institut de Physique Nucléaire d’Orsay (IPNO), Institut National de Physique Nucléaire et de Physique des Particules (IN2P3/CNRS), Université de Paris-Sud, Université Paris-Saclay, Orsay, France 024912-17 S. ACHARYA et al. PHYSICAL REVIEW C 99, 024912 (2019) 62Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 63Institute for Subatomic Physics, Utrecht University/Nikhef, Utrecht, Netherlands 64Institute for Theoretical and Experimental Physics, Moscow, Russia 65Institute of Experimental Physics, Slovak Academy of Sciences, Košice, Slovakia 66Institute of Physics, Bhubaneswar, India 67Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 68Institute of Space Science (ISS), Bucharest, Romania 69Institut für Kernphysik, Johann Wolfgang Goethe-Universität Frankfurt, Frankfurt, Germany 70Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, Mexico 71Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 72Instituto de Física, Universidad Nacional Autónoma de México, Mexico City, Mexico 73iThemba LABS, National Research Foundation, Somerset West, South Africa 74Johann-Wolfgang-Goethe Universität Frankfurt Institut für Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 75Joint Institute for Nuclear Research (JINR), Dubna, Russia 76Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 77KTO Karatay University, Konya, Turkey 78Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 79Lawrence Berkeley National Laboratory, Berkeley, California, United States 80Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 81Nagasaki Institute of Applied Science, Nagasaki, Japan 82Nara Women’s University (NWU), Nara, Japan 83National and Kapodistrian University of Athens, School of Science, Department of Physics, Athens, Greece 84National Centre for Nuclear Research, Warsaw, Poland 85National Institute of Science Education and Research, HBNI, Jatni, India 86National Nuclear Research Center, Baku, Azerbaijan 87National Research Centre Kurchatov Institute, Moscow, Russia 88Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 89Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 90NRC Kurchatov Institute IHEP, Protvino, Russia 91NRNU Moscow Engineering Physics Institute, Moscow, Russia 92Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 93Nuclear Physics Institute of the Czech Academy of Sciences, ˇ Rež u Prahy, Czech Republic 94Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 95Petersburg Nuclear Physics Institute, Gatchina, Russia 96Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 97Physics Department, Panjab University, Chandigarh, India 98Physics Department, University of Jammu, Jammu, India 99Physics Department, University of Rajasthan, Jaipur, India 100Physikalisches Institut, Eberhard-Karls-Universität Tübingen, Tübingen, Germany 101Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 102Physik Department, Technische Universität München, Munich, Germany 103Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany 104Rudjer Boškovi´c Institute, Zagreb, Croatia 105Russian Federal Nuclear Center (VNIIEF), Sarov, Russia 106Saha Institute of Nuclear Physics, Kolkata, India 107School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 108Sección Física, Departamento de Ciencias, Pontificia Universidad Católica del Perú, Lima, Peru 109Shanghai Institute of Applied Physics, Shanghai, China 110Stefan Meyer Institut für Subatomare Physik (SMI), Vienna, Austria 111SUBATECH, IMT Atlantique, Université de Nantes, CNRS-IN2P3, Nantes, France 112Suranaree University of Technology, Nakhon Ratchasima, Thailand 113Technical University of Košice, Košice, Slovakia 114Technische Universität München, Excellence Cluster ’Universe’, Munich, Germany 115The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 116The University of Texas at Austin, Austin, Texas, United States 117Universidad Autónoma de Sinaloa, Culiacán, Mexico 118Universidade de São Paulo (USP), São Paulo, Brazil 119Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 120Universidade Federal do ABC, Santo Andre, Brazil 024912-18 DIRECT PHOTON PRODUCTION AT LOW TRANSVERSE … PHYSICAL REVIEW C 99, 024912 (2019) 121University College of Southeast Norway, Tonsberg, Norway 122University of Cape Town, Cape Town, South Africa 123University of Houston, Houston, Texas, United States 124University of Jyväskylä, Jyväskylä, Finland 125University of Liverpool, Department of Physics Oliver Lodge Laboratory, Liverpool, United Kingdom 126University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, Split, Croatia 127University of Tennessee, Knoxville, Tennessee, United States 128University of the Witwatersrand, Johannesburg, South Africa 129University of Tokyo, Tokyo, Japan 130University of Tsukuba, Tsukuba, Japan 131Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 132Université de Lyon, Université Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, Lyon, France 133Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 134Université Paris-Saclay Centre d’Études de Saclay (CEA), IRFU, Department de Physique Nucléaire (DPhN), Saclay, France 135Università degli Studi di Pavia, Pavia, Italy 136Università di Brescia, Brescia, Italy 137V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia 138Variable Energy Cyclotron Centre, Kolkata, India 139Warsaw University of Technology, Warsaw, Poland 140Wayne State University, Detroit, Michigan, United States 141Westfälische Wilhelms-Universität Münster, Institut für Kernphysik, Münster, Germany 142Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary 143Yale University, New Haven, Connecticut, United States 144Yonsei University, Seoul, Republic of Korea aDipartimento DET del Politecnico di Torino, Turin, Italy. bM.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear, Physics, Moscow, Russia. cDepartment of Applied Physics, Aligarh Muslim University, Aligarh, India. dInstitute of Theoretical Physics, University of Wroclaw, Poland. 024912-19