Coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at √sNN = 5.02 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/ Coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at √sNN = 5.02 TeV © 2020 CERN Published version ALICE collaboration ALICE collaboration. (2020). Coherent photoproduction of ρ0 vector mesons in ultra-peripheral Pb-Pb collisions at √sNN = 5.02 TeV. Journal of High Energy Physics, 2020(6), Article 35. https://doi.org/10.1007/JHEP06(2020)035 2020 JHEP06(2020)035 Published for SISSA by Springer Received:March 1, 2020 Revised:April 2, 2020 Accepted:May 11, 2020 Published:June 4, 2020 Coherent photoproduction of ρ0vector mesons in ultra-peripheral Pb-Pb collisions at √sNN = 5.02 TeV The ALICE collaboration E-mail: [email protected] Abstract: Cross sections for the coherent photoproduction of ρ0vector mesons in ultraperipheral Pb-Pb collisions at √sNN = 5.02 TeV are reported. The measurements, which rely on the π+π−decay channel, are presented in three regions of rapidity covering the range |y|<0.8. For each rapidity interval, cross sections are shown for different nuclearbreakup classes defined according to the presence of neutrons measured in the zero-degree calorimeters. The results are compared with predictions based on different models of nuclear shadowing. Finally, the observation of a coherently produced resonance-like structure with a mass around 1.7 GeV/c2and a width of about 140 MeV/c2is reported and compared with similar observations from other experiments. Keywords: Heavy Ion Experiments, Heavy-ion collision, Particle and resonance production ArXiv ePrint: 2002.10897 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. https://doi.org/10.1007/JHEP06(2020)035 JHEP06(2020)035 Contents 1 Introduction 1 2 Experimental set-up 3 3 Analysis procedure 4 3.1 Event selection 4 3.2 Background subtraction and corrections for experimental effects 5 3.3 Signal extraction 7 3.4 Signal extraction at large invariant masses 9 3.5 Systematic uncertainties 9 4 Results 12 4.1 Coherent photoproduction of ρ0vector mesons 12 4.2 Contributions from continuum production 15 4.3 Observation of a resonance-like structure 15 5 Summary and outlook 16 The ALICE collaboration 21 1 Introduction The electromagnetic field of a fast charged particle, such as those circulating in the Large Hadron Collider (LHC), is strongly Lorentz-contracted and its strength is dominated by the component perpendicular to the direction of motion, such that it can be described as a flux of quasi-real photons. The intensity of this photon flux is proportional to the square of the electric charge of the particle; thus when lead ions circulate in the LHC there are, in addition to the standard hadronic collisions, also copious photonuclear interactions. Ultraperipheral collisions (UPC) are defined as those for which the impact parameter is larger than the sum of the radii of the incoming particles, in which case the occurrence of hadronic processes is strongly suppressed due to the short range nature of quantum chromodynamics (QCD), and photon-induced processes dominate the interaction rate. The physics of UPC and recent results obtained at the LHC are reviewed in [1,2]. The photonuclear production of a ρ0vector meson in Pb-Pb UPC at the LHC is particularly interesting, because its large cross section makes it a good tool to study the approach to the black-disk limit of QCD [3]. This process can be pictured as follows: a quasi-real photon, emitted by one of the Pb ions, fluctuates into a QCD object which – 1 – JHEP06(2020)035 then interacts elastically either with the other lead nucleus (coherent interaction) or with one of its nucleons (incoherent interaction) and produces a ρ0vector meson. The QCD object can be taken as a vector meson [4], as a quark-antiquark colour dipole [5–7], or one could consider intermediate diffractive hadronic states as done in the Gribov-Glauber approach [8]. In these processes, the mean transverse momentum of the produced vector meson is related to the size of the target in the impact parameter plane by a Fourier transformation; hence, it is restricted to be in the order of 60 (500) MeV/cfor coherent (incoherent) interactions. In the coherent case the target nucleus remains intact, but in UPC of heavy nuclei the photon fluxes are so intense that further photon exchanges between the same nuclei may occur independently of the production of the vector meson and produce neutrons at beam rapidities due to electromagnetic excitation of one or both of the incoming nuclei [9]. The experimental signature of coherent ρ0photonuclear production is then the presence of a single ρ0vector meson with fairly low transverse momentum in the detector, accompanied sometimes by one or few neutrons at beam rapidities. The coherent photonuclear production of a ρ0vector meson at midrapidity was extensively studied in Au-Au UPC at the Relativistic Heavy Ion Collider (RHIC) at three different centre-of-mass energies per nucleon pair √sNN = 62.4 GeV [10], √sNN = 130 GeV [11], and √sNN = 200 GeV [12,13]. It was also studied by ALICE at the LHC in Pb-Pb UPC at √sNN = 2.76 TeV [14]. A model based on a Glauber description [3] predicts cross sections twice larger than those measured at energies of 200 GeV [12] and 2.76 TeV [14] even though it is compatible with lower-energy data [10,11]. The STARlight model [15,16], which is also based on a Glauber-like eikonal formalism, but does not take into account the elastic part of the elementary ρ0-nucleon cross section, successfully describes all the data mentioned above. The inclusion of photon inelastic diffraction into large-mass intermediate hadronic states within the Gribov-Glauber framework of nuclear shadowing provides a better comparison with data than the model based only on a Glauber description [8]. Nonetheless, the photoproduction of ρ0off nuclei is not yet satisfactorily described in all of its aspects and new measurements, particularly at higher energies, are needed to gain a better understanding. This article reports the first measurement of coherent photonuclear production of ρ0 vector mesons in Pb-Pb UPC at √sNN = 5.02 TeV. The measurement was performed by the ALICE Collaboration with data recorded in the 2015 Pb-Pb run. The cross section for this process is measured as a function of the rapidity of the vector meson (y) in the range |y|<0.8. At each rapidity, the cross sections are reported for the following nuclear-breakup classes defined by the appearance of neutrons at beam rapidities: 0n0n (no neutrons), 0nXn (neutrons are measured only on one beam side, either at positive or negative rapidity), and XnXn (neutrons are detected in both beam directions). In the following, they are denoted in general as forward-neutron classes. Furthermore, the observation of a resonance-like structure in the π+π−invariant mass spectrum at a mass around 1.7 GeV/c2is reported and compared with similar observations from other experiments. – 2 – JHEP06(2020)035 2 Experimental set-up The analysed data were recorded by ALICE towards the end of 2015 when the LHC provided Pb-Pb collisions at √sNN = 5.02 TeV. A full description of ALICE systems is given in [17] and the performance of the detector is discussed in [18]. Here, only the components relevant for the analysis are briefly described. The ρ0meson is reconstructed through its decay into a π+π−pair using the Inner Tracking System (ITS) and the Time Projection Chamber (TPC) to measure the pion tracks. Vetoes on the presence of other particles to ensure that only the ρ0meson is produced are imposed with the V0 and the ALICE Diffractive (AD) detectors. The neutrons at beam rapidities are measured with the Zero Degree Calorimeters (ZDC). The ITS [19] is the innermost detector system of ALICE. It consists of six cylindrical layers of silicon detectors, positioned coaxially with the direction of the incoming beams, which defines the z-axis. This detector covers the full azimuthal angle and the pseudorapidity range |η|<0.9. All six layers contribute to track reconstruction. The Silicon Pixel Detector (SPD) makes up the first two layers of the ITS, closest to the beam, and is particularly important for this analysis because it participates in the trigger definition. The SPD has 9.8×106pixels of reverse-biased silicon diodes, which are read out by 400 (800) chips in the inner (outer) layer. Each of the readout chips fires a trigger if at least one of its pixels has a signal. When projected into the transverse plane, the chips define 20 (40) azimuthal regions in the inner (outer) layer. The TPC [20] is the main tracking detector. It is a large cylindrical gas detector with a central membrane at high voltage and readout planes, composed of multi-wire proportional chambers, at each of the two end caps. It covers the full azimuthal range and |η|<0.9 for tracks which fully traverse it. It provides up to 159 space points for track reconstruction and for particle identification by measuring the ionisation energy loss. Both the ITS and the TPC are inside a large solenoid magnet, which creates a uniform 0.5 T magnetic field parallel to the z-axis. The V0 [21] is a set of two segmented scintillator counters, V0A and V0C. The V0A covers the range 2.8< η < 5.1, while the V0C covers −3.7< η < −1.7. The AD [22] is also a set of two arrays of scintillator detectors, ADA and ADC, placed further away from the nominal interaction point and covering 4.7< η < 6.3 and −6.9< η < −4.9, respectively. Both V0 and AD detectors participate in the first level trigger, and both detectors have timing resolution less than 1 ns. There are two ZDC detectors, ZNA and ZNC, dedicated to the measurement of neutrons at beam rapidity [23]. They are located at either side of the nominal interaction point at ±112.5 m along the z-axis. These calorimeters determine the arrival time of the particles allowing beam-beam and beam-gas interactions to be separated. Furthermore, they have a good efficiency to detect neutrons with |η|>8.8 and have a relative energy resolution of around 20% for single neutrons, which allows for a clear separation of events with either zero or a few neutrons at beam rapidities. This is illustrated in figure 1, where the concentration of events corresponds to the cases of zero, one, two or more, neutrons detected. – 3 – JHEP06(2020)035 1 10 2 10 3 10 2−0 2 4 6 8 ZNA energy (TeV) 2− 0 2 4 6 8 ZNC energy (TeV) = 5.02 TeV NN sALICE Pb-Pb UPC 2−0 2 4 6 8 10 12 ZN energy (TeV) 1 10 2 10 3 10 4 10 Events ZNA energy ZNC energy = 5.02 TeV NN sALICE Pb-Pb UPC Figure 1. (Colour online). Correlation between the energy distributions of the ZNA and ZNC detectors for events selected for the analysis (left). Energy distribution in each single detector (right). The trigger used to obtain the data sample for the measurements described below is composed of five signals. Four of them veto any activity within the time windows for nominal beam-beam interactions in ADA, ADC, V0A and V0C. In addition, the SPD provides a topological trigger formed by four SPD triggered chips. These chips form two pairs, each pair with two chips falling in compatible azimuthal regions, but in different SPD layers. The trigger selects events with at least two pairs of chips having an opening angle in azimuth larger than 153 degrees. The reason to request this topology is that the coherently produced ρ0has very small transverse momentum, and thus the two pions from its decay are produced almost back-to-back in azimuth. The integrated luminosity is determined using a reference trigger based on the multiplicity of the V0A and V0C detectors. The corresponding cross section is obtained using a Glauber model for hadronic Pb-Pb collisions [24]. The integrated luminosity for the measurements presented below is 485 mb−1with a relative systematic uncertainty of 5%. 3 Analysis procedure 3.1 Event selection Events that fulfil the trigger criteria described above are selected for further analysis if they contain exactly two tracks of opposite electric charge. To ensure a proper measurement, each track is required to have at least 50 space points in the TPC and one associated hit in each layer of the SPD. These SPD hits have to be matched to a triggered readout chip. Furthermore, each track has to have a distance of closest approach to the event interaction vertex of less than 2 cm in the z-axis direction and less than 0.0182 + 0.0350/(ptrk T)1.01 cm in the plane transverse to the beam direction. Here ptrk Tdenotes the transverse momentum of the track in GeV/c. – 4 – JHEP06(2020)035 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 ) 2 c (GeV/m 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 3 10× 2 cCounts per 5 MeV/ c < 0.2 GeV/ T p | < 0.8y| Opposite-sign pairs Like-sign pairs = 5.02 TeV NN sALICE Pb-Pb UPC 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 )c (GeV/ T p 1 10 2 10 3 10 4 10 cCounts per 10 MeV/ 2 c < 1.4 GeV/m < 2 c0.55 GeV/ | < 0.8y| Opposite-sign pairs Like-sign pairs = 5.02 TeV NN sALICE Pb-Pb UPC Figure 2. (Colour online). Invariant mass (left) and transverse momentum (right) distributions for opposite-sign (blue) and like-sign (red) pairs. The energy loss of each reconstructed track is measured in units of the standard deviation (σπ) with respect to Bethe expectations for a pion passing the TPC. The track pair is accepted if n2 σπ++n2 σπ−<52. This criterion rejects, in the considered mass range, the contribution from electrons, while there remains a small background from muon pairs which is discussed below. The four momentum of the track pair is computed under the assumption of each track being a pion. A pair is accepted if its rapidity (y), transverse momentum (pT) and mass (m) are within |y|<0.8, pT<0.2 GeV/cand 0.55 <m<1.4 GeV/c2. To veto activity in the pseudorapidity range covered by the AD and V0 detectors, their offline signals are studied. The offline reconstruction in these detectors is more precise than the online information, because it uses larger time windows than the trigger electronics and a more refined algorithm to quantify the signal. Events showing a reconstructed signal in any of ADA, ADC, V0A or V0C are rejected. The invariant mass distribution for pT<0.2 GeV/cand transverse momentum distribution for 0.55 <m<1.4 GeV/c2of the selected track pairs are shown in figure 2. The mass distribution shows the shape expected from a ρ0spectrum, while a diffraction dip is clearly seen in the transverse momentum distribution. In total, the signal sample contains almost 57 thousand events which passed all selection criteria. The signal sample is further subdivided in forward-neutron classes. The assignment of an event to a class is based on the timing capabilities of the ZNA and ZNC detectors. Events in which the timing of the energy deposition in the calorimeter is consistent within ±2 ns with the neutron having been produced in a beam-beam collision are classified as having a forward neutron in the corresponding calorimeter. 3.2 Background subtraction and corrections for experimental effects In this section the procedure to determine the corrections used in the measurement is presented. The correction factors are quoted with their corresponding uncertainties, which are discussed in section 3.5 and summarised in tables 1to 3. – 5 – JHEP06(2020)035 As a tool to quantify some of the remaining background contributions a special sample of events is selected fulfilling all criteria mentioned in section 3.1, except that both tracks have the same electric charge. The invariant mass and transverse momentum distributions of this sample are shown in figure 2. The distributions of same-charge pairs are used as an estimation of the amount and shape of the background from events with a measured opposite-charge pair and other charged tracks outside the acceptance of the detector. The contribution of same-charge pairs is at the level of 1% and is statistically subtracted from the signal sample. Another potential background comes from events with two tracks with opposite electric charge and a neutral particle. The main contribution is expected from three-body decays of the ωvector meson. Dedicated Monte Carlo (MC) simulations of coherent ωphotoproduction followed by the ω→π+π−π0decay demonstrate that the signal from such π+π−pairs from three-body ωdecays concentrates at lower masses and higher transverse momenta than those considered for the signal-extraction procedure described below. This study, as well as all studies involving MC, uses generated MC events, in this case from STARlight, passed through a detailed simulation of the ALICE detector. The contribution from ρ0vector mesons produced in incoherent interactions is estimated by fitting a template produced by STARlight of the transverse momentum distribution. The template is fitted in the region of transverse momentum 0.25 < pT<0.9 GeV/c to obtain its proper normalisation. The normalised template is used to estimate this contribution for pT<0.2 GeV/c. The final yield of ρ0mesons is corrected by subtracting this contribution, which is (4 ±0.5)%. The efficiency of the SPD readout chips participating in the trigger is measured with a data-driven approach using a minimum bias trigger. Tracks selected without requiring two hits in the different SPD layers are matched to the readout chips they cross. A chip inefficiency affects each track, and thus each event differently. The efficiency maps obtained from data are incorporated into the Monte Carlo simulation of the signal and applied eventby-event. The overall effect corresponds to a global correction of about (17 ±1)%. The efficiency of the ZNA and ZNC to detect neutrons is estimated with two different methods. In the first method, a sample of MC events generated with the RELDIS program [25,26] is used. The other method relies on a simple probabilistic model [27] applied directly to the raw data. Both methods yield compatible results, namely an efficiency of about (93 ±1)% each for the ZNA and ZNC to detect neutron activity. The propagation of this effect, and the one discussed next, into the value of the measured cross sections is discussed in section 3.5. Good events in the 0nXn and XnXn classes are rejected when, in addition to the forward neutrons, other particles are created at large rapidities and leave a signal either in the AD or the V0 detectors. These extra particles come from the different possibilities of dissociation of nuclei, e.g. neutron emission, multi-fragmentation or pion production, and the corresponding cross sections are expected to be large [28]. The amount of good events with neutrons which are lost due to vetoes by AD and V0 is estimated using control triggers. The corrections amount to (26 ±4)% for events with a signal either in ZNA or in ZNC, while it is (43 ±5)% for events with a signal in both ZNA and ZNC. – 6 – JHEP06(2020)035 Good events are also rejected if another interaction creates a signal in one of the veto detectors, an effect known as pile-up. The main pile-up comes from purely electromagnetic interactions producing a low mass electron-positron pair. The probability of the occurrence of pile-up is correlated with the average number of inelastic hadronic collisions per bunch crossing (µ), which for the data used in this analysis varied from µ= 0.0002 to µ= 0.0015. The effect of pile-up is estimated using two different methods. One method uses an event sample obtained with an unbiased trigger based only on the timing of bunches crossing the interaction region. This sample is separated into periods with specific µvalues. The probability of a signal in each of the veto detectors is computed for each value of µin otherwise empty events using the unbiased sample. This probability exhibits a linear behaviour as a function of µ. The veto inefficiencies are determined by weighting the corresponding veto rejection probabilities over periods with different µ, taking the luminosity of each period as a weight. The correlation between the online and offline vetoes is taken into account. The second method divides the signal sample described in section 3.1 into subsets of events with a specific range of µvalues. Each one of these sub-samples is subjected to the full analysis chain. The final cross sections show a linear dependence on µ. The intercept at µ= 0 is taken as the pile-up corrected cross section in this method. The two approaches produce slightly different results. The average of both results is used as the final correction factor of (11.1±3.8)%. Pile-up also affects the classification on forward-neutron classes. Electromagnetic dissociation processes [23] have a large cross section and produce neutrons at beam rapidities. Using the same unbiased sample as described above, the average pile-up probability is measured to be (3.3±0.3)% in both ZNA and ZNC. Finally, the product of the acceptance times efficiency to measure the coherently produced ρ0vector meson is determined using event samples generated with STARlight. Two different samples are used: one of pure coherent ρ0photoproduction and the other produced with a flat mass distribution. Both approaches yield similar correction functions for the invariant mass spectrum. The acceptance times efficiency rises smoothly from 15% to 19% in the mass range from 0.6 GeV/c2to 1.2 GeV/c2and remains constant for larger masses. 3.3 Signal extraction The invariant mass distribution, corrected by all effects described above and normalised by the luminosity of the sample, is fitted to the sum of a S¨oding formula [29] and a term M to account for the contribution of the γγ →µ+µ−process: dσ dmdy=|A·BWρ+B|2+M, (3.1) where Ais the normalisation factor of the ρ0Breit-Wigner (BWρ) function, and Bis the non-resonant amplitude. The relativistic Breit-Wigner function of the ρ0vector meson is BWρ=pm·mρ0·Γ(m) m2−m2 ρ0+imρ0·Γ(m),(3.2) – 7 – JHEP06(2020)035 No forward-neutron selection Cross section (mb) stat. (mb) syst. (mb) |y|<0.2 537.0 4.6 +46.1 −42.0 0.2<|y|<0.45 538.6 4.4 +46.2 −42.1 0.45 <|y|<0.8 547.0 4.9 +46.9 −42.8 0n0n |y|<0.2 431.1 4.0 +36.8 −33.6 0.2<|y|<0.45 433.8 3.8 +37.0 −33.8 0.45 <|y|<0.8 436.7 4.2 +37.3 −34.0 0nXn |y|<0.2 90.2 1.9 +10.5 −9.5 0.2<|y|<0.45 87.7 1.8 +10.2 −9.3 0.45 <|y|<0.8 89.9 2.0 +10.4 −9.5 XnXn |y|<0.2 24.4 1.3 +3.4 −2.9 0.2<|y|<0.45 24.5 1.2 +3.4 −3.0 0.45 <|y|<0.8 25.6 1.3 +3.5 −3.1 Table 4. Numerical values of the cross section for the coherent photoproduction of ρ0vector mesons in Pb-Pb UPC at √sNN = 5.02 TeV. The systematic uncertainties are obtained by adding in quadrature the contributions listed in tables 1to 3. The modification of the photon flux due to the emission of the forward neutrons is carried out in the first three models as proposed in [9]. The fourth model uses the nO On afterburner described in [41]. Figure 5shows that the lower limit of the GKZ model gives a good description of the 0n0n cross section and underestimates a little bit the 0nXn and XnXn cross sections while the upper limit of the same model overestimates the 0n0n, slightly underestimates the 0nXn and describes the XnXn cross sections. The STARlight predictions underestimate all the cross section at around the 2 sigma level, except XnXn where the difference is smaller. The behaviour of the CCKT model based on hot spots is quite similar to the upper limit of GKZ; the CCKT (nuclear) variant of this model is some 10% larger than the predictions of the CCKT model with hot spots. Finally, the GMMNS model predicts cross sections larger than STARlight, but still underestimating the measurements except in the XnXn class. Taking into account the spread of the models and the uncertainties of data the agreement between the models and the measurement appears in most cases satisfactory, particularly for the predictions of the GKZ model. This overall description of data by models suggests that the method to obtain the individual photonuclear contributions to the coherent production of ρ0using forward-neutron classes [9,42] may be applied to the – 14 – JHEP06(2020)035 data, specially once the uncertainties in the measurements are reduced and the spread on the theoretical predictions is better understood. 4.2 Contributions from continuum production The |B/A|ratio, see eq. (3.1), quantifies the contribution of the continuum in relation to the resonance production cross section. The value found at midrapidity for no forward-neutron selection is 0.57 ±0.01 (stat.)±0.02 (syst.) (GeV/c2)− 1 2, where it has been checked that most of the effects cancel in the ratio and the only remaining contribution to the systematic uncertainty are the variations in the fit procedure. The measured value can be compared with that found for the same process at √sNN = 2.76 TeV: 0.50 ±0.04 (stat.)+0.10 −0.04 (syst.) (GeV/c2)− 1 2[14]. Within the current systematic uncertainties, the ratio can be taken as constant both as a function of rapidity and for the different forward-neutron classes. Nonetheless data seems to indicate a small decrease of the ratio with rapidity for the no forward selection case: |B/A|= 0.56 ±0.01 (stat.)±0.02 (syst.) (GeV/c2)− 1 2and |B/A|= 0.52 ±0.01 (stat.)±0.01 (syst.) (GeV/c2)− 1 2for the 0.2<|y|<0.45 and 0.45 <|y|<0.8 intervals, respectively. It would be interesting if such a trend is observed with the large data sample and the improved precision, expected from the LHC Run 3 and 4 [43]. The corresponding ratio in coherent Au-Au UPC measured by STAR at √sNN = 200 GeV is 0.79 ±0.01 (stat.)±0.08 (syst.) (GeV/c2)− 1 2[13]. These results for production off heavy nuclear targets, can be compared with those from exclusive ρ0photoproduction off protons. Note that value of |B/A|might depend on the range in |t|selected to perform the measurement, where tis the square of the four momentum transfer at the target vertex. The CMS Collaboration measured 0.50 ±0.06 (stat.) (GeV/c2)− 1 2in p-Pb UPC at √sNN = 5.02 TeV [44] for |t|<0.5 GeV2. The ZEUS Collaboration, using a sample of positron-proton collisions at a centre-of-mass energy of 300 GeV, reports 0.67 ±0.02 (stat.)±0.04 (syst.) (GeV/c2)− 1 2for their full analysed sample, and ≈0.8 (GeV/c2)− 1 2for tvalues similar to those of coherent ρ0production in Pb-Pb UPC [45]. Overall, the ratio of the continuum to the resonance production of π+π−pairs seems to be sensitive to both the kinematics of the interaction and the type of target, but no clear picture has yet emerged. 4.3 Observation of a resonance-like structure As shown in figure 4, there seems to be a resonance-like structure in the region m > 1.2 GeV/c2. The model of eq. (3.4) yields a mass of (1725 ±17) MeV/c2and width (143 ± 21) MeV/c2, where the quoted uncertainties correspond to statistical fluctuations only. As shown in the same figure, this resonance-like object has very low transverse momentum as expected from a coherent-production process. Such an object is also seen by the STAR Collaboration [33] albeit at a slightly lower mass of 1.65 GeV/c2, but with a similar width. ZEUS reports a peak around 1.8 GeV/c2 for exclusive electroproduction of π+π−pairs [46]. More recently, H1 reports a peak at 1.6 GeV/c2in the exclusive photoproduction of the ρ0meson [47]. As suggested in [33], this resonance is also compatible with the ρ3(1690) listed in the PDG, which has a total angular momentum J= 3 [32]. – 15 – JHEP06(2020)035 The large data samples expected in Run 3 and Run 4 at the LHC [43] may help to shed light on the origin and structure of this object. 5 Summary and outlook The rapidity dependence of the coherent ρ0vector meson production cross section in Pb-Pb UPC at √sNN = 5.02 TeV has been presented. In each rapidity range, the cross section is measured for different classes of events defined by the presence of neutrons at beam rapidities. The cross sections are compared with the main available models of this process. The measurements of coherent ρ0photoproduction are in good agreement both with models following the parton-based colour-dipole approach and with the framework of Gribov-Glauber shadowing based on hadronic degrees of freedom. The models [9,41] of electromagnetic nuclear dissociation accompanying vector meson photoproduction provide a satisfactory description of the measured cross sections for different neutron emission classes. This observation suggests that the method proposed in [42] to decouple the low-photon-energy from the high-photon-energy contribution to the UPC cross section using neutron-differential measurements might also be applicable at forward rapidities, which is specially important in view of the expected data samples to be recorded at the LHC during the Run 3 and 4 [43]. In addition, the coherent photoproduction of a resonance-like object with a mass around 1.7 GeV/c2which decays into a π+π−pair is reported and compared with similar observations from other experiments. Acknowledgments The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467N36] and Nationalstiftung f¨ur Forschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq), Financiadora de Estudos e Projetos (Finep), Funda¸c˜ao de Amparo `a Pesquisa do Estado de S˜ao Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of Education of China (MOEC) , Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation, Croatia; Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Cubaenerg´ıa, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; Czech Science Foundation; The Danish – 16 – JHEP06(2020)035 Council for Independent Research — Natural Sciences, the VILLUM FONDEN and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat `a l’Energie Atomique (CEA), Institut National de Physique Nucl´eaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS) and R´egion des Pays de la Loire, France; Bundesministerium f¨ur Bildung und Forschung (BMBF) and GSI Helmholtzzentrum f¨ur Schwerionenforschung GmbH, Germany; General Secretariat for Research and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE), Department of Science and Technology, Government of India (DST), University Grants Commission, Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology , Nagasaki Institute of Applied Science (IIST), Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnolog´ıa, through Fondo de Cooperaci´on Internacional en Ciencia y Tecnolog´ıa (FONCICYT) and Direcci´on General de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Cat´olica del Per´u, Peru; Ministry of Science and Higher Education and National Science Centre, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Ministry of Research and Innovation and Institute of Atomic Physics, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation, National Research Centre Kurchatov Institute, Russian Science Foundation and Russian Foundation for Basic Research, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSDTA) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America. Open Access. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. – 17 – JHEP06(2020)035 References [1] A.J. Baltz, The Physics of Ultraperipheral Collisions at the LHC,Phys. Rept. 458 (2008) 1 [arXiv:0706.3356] [INSPIRE]. [2] J.G. Contreras and J.D. Tapia Takaki, Ultra-peripheral heavy-ion collisions at the LHC,Int. J. Mod. Phys. A 30 (2015) 1542012 [INSPIRE]. [3] L. Frankfurt, M. Strikman and M. Zhalov, Signals for black body limit in coherent ultraperipheral heavy ion collisions,Phys. Lett. B 537 (2002) 51 [hep-ph/0204175] [INSPIRE]. [4] T.H. Bauer, R.D. Spital, D.R. Yennie and F.M. Pipkin, The Hadronic Properties of the Photon in High-Energy Interactions,Rev. Mod. Phys. 50 (1978) 261 [Erratum ibid. 51 (1979) 407] [INSPIRE]. [5] N.N. Nikolaev and B.G. Zakharov, Color transparency and scaling properties of nuclear shadowing in deep inelastic scattering,Z. Phys. C 49 (1991) 607 [INSPIRE]. [6] N. Nikolaev and B.G. Zakharov, Pomeron structure function and diffraction dissociation of virtual photons in perturbative QCD,Z. Phys. C 53 (1992) 331 [INSPIRE]. [7] A.H. Mueller, Small x Behavior and Parton Saturation: A QCD Model,Nucl. Phys. B 335 (1990) 115 [INSPIRE]. [8] L. Frankfurt, V. Guzey, M. Strikman and M. Zhalov, Nuclear shadowing in photoproduction of ρmesons in ultraperipheral nucleus collisions at RHIC and the LHC,Phys. Lett. B 752 (2016) 51 [arXiv:1506.07150] [INSPIRE]. [9] A.J. Baltz, S.R. Klein and J. Nystrand, Coherent vector meson photoproduction with nuclear breakup in relativistic heavy ion collisions,Phys. Rev. Lett. 89 (2002) 012301 [nucl-th/0205031] [INSPIRE]. [10] STAR collaboration, ρ0Photoproduction in AuAu Collisions at √sN N = 62.4GeV with STAR,Phys. Rev. C 85 (2012) 014910 [arXiv:1107.4630] [INSPIRE]. [11] STAR collaboration, Coherent ρ0production in ultraperipheral heavy ion collisions,Phys. Rev. Lett. 89 (2002) 272302 [nucl-ex/0206004] [INSPIRE]. [12] STAR collaboration, ρ0photoproduction in ultraperipheral relativistic heavy ion collisions at √sN N = 200 GeV,Phys. Rev. C 77 (2008) 034910 [arXiv:0712.3320] [INSPIRE]. [13] STAR collaboration, Coherent diffractive photoproduction of ρ0mesons on gold nuclei at 200 GeV/nucleon-pair at the Relativistic Heavy Ion Collider,Phys. Rev. C 96 (2017) 054904 [arXiv:1702.07705] [INSPIRE]. [14] ALICE collaboration, Coherent ρ0photoproduction in ultra-peripheral Pb-Pb collisions at √sNN = 2.76 TeV,JHEP 09 (2015) 095 [arXiv:1503.09177] [INSPIRE]. [15] S. Klein and J. Nystrand, Exclusive vector meson production in relativistic heavy ion collisions,Phys. Rev. C 60 (1999) 014903 [hep-ph/9902259] [INSPIRE]. [16] S.R. Klein, J. Nystrand, J. Seger, Y. Gorbunov and J. Butterworth, STARlight: A Monte Carlo simulation program for ultra-peripheral collisions of relativistic ions,Comput. Phys. Commun. 212 (2017) 258 [arXiv:1607.03838] [INSPIRE]. [17] ALICE collaboration, The ALICE experiment at the CERN LHC,2008 JINST 3S08002 [INSPIRE]. – 18 – JHEP06(2020)035 [18] ALICE collaboration, Performance of the ALICE Experiment at the CERN LHC,Int. J. Mod. Phys. A 29 (2014) 1430044 [arXiv:1402.4476] [INSPIRE]. [19] ALICE collaboration, Alignment of the ALICE Inner Tracking System with cosmic-ray tracks,2010 JINST 5P03003 [arXiv:1001.0502] [INSPIRE]. [20] J. Alme et al., The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high multiplicity events,Nucl. Instrum. Meth. A 622 (2010) 316 [arXiv:1001.1950] [INSPIRE]. [21] ALICE collaboration, Performance of the ALICE VZERO system,2013 JINST 8P10016 [arXiv:1306.3130] [INSPIRE]. [22] LHC Forward Physics Working Group collaboration, LHC Forward Physics,J. Phys. G 43 (2016) 110201 [arXiv:1611.05079] [INSPIRE]. [23] ALICE collaboration, Measurement of the Cross Section for Electromagnetic Dissociation with Neutron Emission in Pb-Pb Collisions at √sN N = 2.76 TeV,Phys. Rev. Lett. 109 (2012) 252302 [arXiv:1203.2436] [INSPIRE]. [24] C. Loizides, J. Kamin and D. d’Enterria, Improved Monte Carlo Glauber predictions at present and future nuclear colliders,Phys. Rev. C 97 (2018) 054910 [arXiv:1710.07098] [INSPIRE]. [25] I.A. Pshenichnov, J.P. Bondorf, I.N. Mishustin, A. Ventura and S. Masetti, Mutual heavy ion dissociation in peripheral collisions at ultrarelativistic energies,Phys. Rev. C 64 (2001) 024903 [nucl-th/0101035] [INSPIRE]. [26] I.A. Pshenichnov, Electromagnetic excitation and fragmentation of ultrarelativistic nuclei, Phys. Part. Nucl. 42 (2011) 215 [INSPIRE]. [27] U. Dmitrieva and I. Pshenichnov, On the performance of Zero Degree Calorimeters in detecting multinucleon events,Nucl. Instrum. Meth. A 906 (2018) 114 [arXiv:1805.01792] [INSPIRE]. [28] I.A. Pshenichnov, I.N. Mishustin, J.P. Bondorf, A.S. Botvina and A.S. Ilinov, Particle emission following Coulomb excitation in ultrarelativistic heavy ion collisions,Phys. Rev. C 60 (1999) 044901 [nucl-th/9901061] [INSPIRE]. [29] P. Soding, On the Apparent shift of the rho meson mass in photoproduction,Phys. Lett. 19 (1966) 702 [INSPIRE]. [30] J.D. Jackson, Remarks on the phenomenological analysis of resonances,Nuovo Cim. 34 (1964) 1644 [INSPIRE]. [31] ALICE collaboration, Overview of ALICE results on ultra-peripheral collisions,EPJ Web Conf. 204 (2019) 01011 [INSPIRE]. [32] Particle Data Group collaboration, Review of Particle Physics,Phys. Rev. D 98 (2018) 030001 [INSPIRE]. [33] STAR collaboration, Ultra-Peripheral Collisions with gold ions in STAR,PoS(DIS2016)188 (2016) [arXiv:1606.02754] [INSPIRE]. [34] M.H. Ross and L. Stodolsky, Photon dissociation model for vector meson photoproduction, Phys. Rev. 149 (1966) 1172 [INSPIRE]. – 19 – JHEP06(2020)035 [35] V. Guzey, E. Kryshen and M. Zhalov, Coherent photoproduction of vector mesons in ultraperipheral heavy ion collisions: Update for run 2 at the CERN Large Hadron Collider, Phys. Rev. C 93 (2016) 055206 [arXiv:1602.01456] [INSPIRE]. [36] V.P. Gon¸calves, M.V.T. Machado, B.D. Moreira, F.S. Navarra and G.S. dos Santos, Color dipole predictions for the exclusive vector meson photoproduction in pp ,pPb and PbPb collisions at run 2 LHC energies,Phys. Rev. D 96 (2017) 094027 [arXiv:1710.10070] [INSPIRE]. [37] E. Iancu, K. Itakura and S. Munier, Saturation and BFKL dynamics in the HERA data at small x,Phys. Lett. B 590 (2004) 199 [hep-ph/0310338] [INSPIRE]. [38] J. Cepila, J.G. Contreras and J.D. Tapia Takaki, Energy dependence of dissociative J/ψ photoproduction as a signature of gluon saturation at the LHC,Phys. Lett. B 766 (2017) 186 [arXiv:1608.07559] [INSPIRE]. [39] J. Cepila, J.G. Contreras, M. Krelina and J.D. Tapia Takaki, Mass dependence of vector meson photoproduction off protons and nuclei within the energy-dependent hot-spot model, Nucl. Phys. B 934 (2018) 330 [arXiv:1804.05508] [INSPIRE]. [40] N. Armesto, A Simple model for nuclear structure functions at small x in the dipole picture, Eur. Phys. J. C 26 (2002) 35 [hep-ph/0206017] [INSPIRE]. [41] M. Broz, J.G. Contreras and J.D.T. Takaki, A generator of forward neutrons for ultra-peripheral collisions: nO On, Comput. Phys. Commun. (2020) 107181 [arXiv:1908.08263] [INSPIRE]. [42] V. Guzey, M. Strikman and M. Zhalov, Disentangling coherent and incoherent quasielastic J/ψ photoproduction on nuclei by neutron tagging in ultraperipheral ion collisions at the LHC,Eur. Phys. J. C 74 (2014) 2942 [arXiv:1312.6486] [INSPIRE]. [43] Z. Citron et al., Future physics opportunities for high-density QCD at the LHC with heavy-ion and proton beams, in HL/HE-LHC Workshop: Workshop on the Physics of HL-LHC, and Perspectives at HE-LHC, Geneva Switzerland (2018) [arXiv:1812.06772] [INSPIRE]. [44] CMS collaboration, Measurement of exclusive ρ(770)0photoproduction in ultraperipheral pPb collisions at √sNN = 5.02 TeV,Eur. Phys. J. C 79 (2019) 702 [arXiv:1902.01339] [INSPIRE]. [45] ZEUS collaboration, Elastic and proton dissociative ρ0photoproduction at HERA,Eur. Phys. J. C 2 (1998) 247 [hep-ex/9712020] [INSPIRE]. [46] ZEUS collaboration, Exclusive electroproduction of two pions at HERA,Eur. Phys. J. C 72 (2012) 1869 [arXiv:1111.4905] [INSPIRE]. [47] H1 collaboration, Measurement of Exclusive ρ0Meson Photoproduction at HERA, in XXXIX International Conference on High Energy Physics, ICHEP 2018, Seoul South Korea (2018), DESY Report H1prelim 18-012 (2018). – 20 – JHEP06(2020)035 The ALICE collaboration S. Acharya142, D. Adamov´a95, A. Adler74, J. Adolfsson81, M.M. Aggarwal100, G. Aglieri Rinella34, M. Agnello30, N. Agrawal10,54, Z. Ahammed142, S. Ahmad16, S.U. Ahn76, A. Akindinov92, M. Al-Turany107, S.N. Alam142, D.S.D. Albuquerque123, D. Aleksandrov88, B. Alessandro59, H.M. Alfanda6, R. Alfaro Molina71, B. Ali16, Y. Ali14, A. Alici10,26,54, A. Alkin2, J. Alme21, T. Alt68, L. Altenkamper21, I. Altsybeev113, M.N. Anaam6, C. Andrei48, D. Andreou34, H.A. Andrews111, A. Andronic145, M. Angeletti34, V. Anguelov104, C. Anson15, T. Antiˇci´c108, F. Antinori57, P. Antonioli54, N. Apadula80, L. Aphecetche115, H. Appelsh¨auser68, S. Arcelli26, R. Arnaldi59, M. Arratia80, I.C. Arsene20, M. Arslandok104, A. Augustinus34, R. Averbeck107, S. Aziz78, M.D. Azmi16, A. Badal`a56, Y.W. Baek41, S. Bagnasco59, X. Bai107, R. Bailhache68, R. Bala101, A. Balbino30, A. Baldisseri138, M. Ball43, S. Balouza105, D. Banerjee3, R. Barbera27, L. Barioglio25, G.G. Barnaf¨oldi146, L.S. Barnby94, V. Barret135, P. Bartalini6, K. Barth34, E. Bartsch68, F. Baruffaldi28, N. Bastid135, S. Basu144, G. Batigne115, B. Batyunya75, D. Bauri49, J.L. Bazo Alba112, I.G. Bearden89, C. Beattie147, C. Bedda63, N.K. Behera61, I. Belikov137, A.D.C. Bell Hechavarria145, F. Bellini34, R. Bellwied126, V. Belyaev93, G. Bencedi146, S. Beole25, A. Bercuci48, Y. Berdnikov98, D. Berenyi146, R.A. Bertens131, D. Berzano59, M.G. Besoiu67, L. Betev34, A. Bhasin101, I.R. Bhat101, M.A. Bhat3, H. Bhatt49, B. Bhattacharjee42, A. Bianchi25, L. Bianchi25, N. Bianchi52, J. Bielˇc´ık37, J. Bielˇc´ıkov´a95, A. Bilandzic105,118, G. Biro146, R. Biswas3, S. Biswas3, J.T. Blair120, D. Blau88, C. Blume68, G. Boca140, F. Bock34,96, A. Bogdanov93, S. Boi23, L. Boldizs´ar146, A. Bolozdynya93, M. Bombara38, G. Bonomi141, H. Borel138, A. Borissov93, H. Bossi147, E. Botta25, L. Bratrud68, P. Braun-Munzinger107, M. Bregant122, M. Broz37, E. Bruna59, G.E. Bruno106, M.D. Buckland128, D. Budnikov109, H. Buesching68, S. Bufalino30, O. Bugnon115, P. Buhler114, P. Buncic34, Z. Buthelezi72,132, J.B. Butt14, J.T. Buxton97, S.A. Bysiak119, D. Caffarri90, A. Caliva107, E. Calvo Villar112, R.S. Camacho45, P. Camerini24, A.A. Capon114, F. Carnesecchi10,26, R. Caron138, J. Castillo Castellanos138, A.J. Castro131, E.A.R. Casula55, F. Catalano30, C. Ceballos Sanchez53, P. Chakraborty49, S. Chandra142, W. Chang6, S. Chapeland34, M. Chartier128, S. Chattopadhyay142, S. Chattopadhyay110, A. Chauvin23, C. Cheshkov136, B. Cheynis136, V. Chibante Barroso34, D.D. Chinellato123, S. Cho61, P. Chochula34, T. Chowdhury135, P. Christakoglou90, C.H. Christensen89, P. Christiansen81, T. Chujo134, C. Cicalo55, L. Cifarelli10,26, F. Cindolo54, G. Clai54,ii, J. Cleymans125, F. Colamaria53, D. Colella53, A. Collu80, M. Colocci26, M. Concas59,iii, G. Conesa Balbastre79, Z. Conesa del Valle78, G. Contin24,60, J.G. Contreras37, T.M. Cormier96, Y. Corrales Morales25, P. Cortese31, M.R. Cosentino124, F. Costa34, S. Costanza140, P. Crochet135, E. Cuautle69, P. Cui6, L. Cunqueiro96, D. Dabrowski143, T. Dahms105,118, A. Dainese57, F.P.A. Damas115,138, M.C. Danisch104, A. Danu67, D. Das110, I. Das110, P. Das86, P. Das3, S. Das3, A. Dash86, S. Dash49, S. De86, A. De Caro29, G. de Cataldo53, J. de Cuveland39, A. De Falco23, D. De Gruttola10, N. De Marco59, S. De Pasquale29, S. Deb50, H.F. Degenhardt122, K.R. Deja143, A. Deloff85, S. Delsanto25,132, W. Deng6, D. Devetak107, P. Dhankher49, D. Di Bari33, A. Di Mauro34, R.A. Diaz8, T. Dietel125, P. Dillenseger68, Y. Ding6, R. Divi`a34, D.U. Dixit19, Ø. Djuvsland21, U. Dmitrieva62, A. Dobrin67, B. D¨onigus68, O. Dordic20, A.K. Dubey142, A. Dubla107, S. Dudi100, M. Dukhishyam86, P. Dupieux135, R.J. Ehlers96,147, V.N. Eikeland21, D. Elia53, E. Epple147, B. Erazmus115, F. Erhardt99, A. Erokhin113, M.R. Ersdal21, B. Espagnon78, G. Eulisse34, D. Evans111, S. Evdokimov91, L. Fabbietti105,118, M. Faggin28, J. Faivre79, F. Fan6, A. Fantoni52, M. Fasel96, P. Fecchio30, A. Feliciello59, G. Feofilov113, A. Fern´andez T´ellez45, A. Ferrero138, A. Ferretti25, A. Festanti34, V.J.G. Feuillard104, J. Figiel119, S. Filchagin109, D. Finogeev62, F.M. Fionda21, G. Fiorenza53, F. Flor126, S. Foertsch72, – 21 – JHEP06(2020)035 P. Foka107, S. Fokin88, E. Fragiacomo60, U. Frankenfeld107, U. Fuchs34, C. Furget79, A. Furs62, M. Fusco Girard29, J.J. Gaardhøje89, M. Gagliardi25, A.M. Gago112, A. Gal137, C.D. Galvan121, P. Ganoti84, C. Garabatos107, E. Garcia-Solis11, K. Garg115, C. Gargiulo34, A. Garibli87, K. Garner145, P. Gasik105,118, E.F. Gauger120, M.B. Gay Ducati70, M. Germain115, J. Ghosh110, P. Ghosh142, S.K. Ghosh3, M. Giacalone26, P. Gianotti52, P. Giubellino59,107, P. Giubilato28, P. Gl¨assel104, A. Gomez Ramirez74, V. Gonzalez107,144, L.H. Gonz´alez-Trueba71, S. Gorbunov39, L. G¨orlich119, A. Goswami49, S. Gotovac35, V. Grabski71, L.K. Graczykowski143, K.L. Graham111, L. Greiner80, A. Grelli63, C. Grigoras34, V. Grigoriev93, A. Grigoryan1, S. Grigoryan75, O.S. Groettvik21, F. Grosa30, J.F. Grosse-Oetringhaus34, R. Grosso107, R. Guernane79, M. Guittiere115, K. Gulbrandsen89, T. Gunji133, A. Gupta101, R. Gupta101, I.B. Guzman45, R. Haake147, M.K. Habib107, C. Hadjidakis78, H. Hamagaki82, G. Hamar146, M. Hamid6, R. Hannigan120, M.R. Haque63,86, A. Harlenderova107, J.W. Harris147, A. Harton11, J.A. Hasenbichler34, H. Hassan96, D. Hatzifotiadou10,54, P. Hauer43, S. Hayashi133, S.T. Heckel68,105, E. Hellb¨ar68, H. Helstrup36, A. Herghelegiu48, T. Herman37, E.G. Hernandez45, G. Herrera Corral9, F. Herrmann145, K.F. Hetland36, H. Hillemanns34, C. Hills128, B. Hippolyte137, B. Hohlweger105, J. Honermann145, D. Horak37, A. Hornung68, S. Hornung107, R. Hosokawa15, P. Hristov34, C. Huang78, C. Hughes131, P. Huhn68, T.J. Humanic97, H. Hushnud110, L.A. Husova145, N. Hussain42, S.A. Hussain14, D. Hutter39, J.P. Iddon34,128, R. Ilkaev109, H. Ilyas14, M. Inaba134, G.M. Innocenti34, M. Ippolitov88, A. Isakov95, M.S. Islam110, M. Ivanov107, V. Ivanov98, V. Izucheev91, B. Jacak80, N. Jacazio34, P.M. Jacobs80, S. Jadlovska117, J. Jadlovsky117, S. Jaelani63, C. Jahnke122, M.J. Jakubowska143, M.A. Janik143, T. Janson74, M. Jercic99, O. Jevons111, M. Jin126, F. Jonas96,145, P.G. Jones111, J. Jung68, M. Jung68, A. Jusko111, P. Kalinak64, A. Kalweit34, V. Kaplin93, S. Kar6, A. Karasu Uysal77, O. Karavichev62, T. Karavicheva62, P. Karczmarczyk34, E. Karpechev62, U. Kebschull74, R. Keidel47, M. Keil34, B. Ketzer43, Z. Khabanova90, A.M. Khan6, S. Khan16, S.A. Khan142, A. Khanzadeev98, Y. Kharlov91, A. Khatun16, A. Khuntia119, B. Kileng36, B. Kim61, B. Kim134, D. Kim148, D.J. Kim127, E.J. Kim73, H. Kim17,148, J. Kim148, J.S. Kim41, J. Kim104, J. Kim148, J. Kim73, M. Kim104, S. Kim18, T. Kim148, T. Kim148, S. Kirsch39,68, I. Kisel39, S. Kiselev92, A. Kisiel143, J.L. Klay5, C. Klein68, J. Klein34,59, S. Klein80, C. Klein-B¨osing145, M. Kleiner68, A. Kluge34, M.L. Knichel34, A.G. Knospe126, C. Kobdaj116, M.K. K¨ohler104, T. Kollegger107, A. Kondratyev75, N. Kondratyeva93, E. Kondratyuk91, J. Konig68, P.J. Konopka34, L. Koska117, O. Kovalenko85, V. Kovalenko113, M. Kowalski119, I. Kr´alik64, A. Kravˇc´akov´a38, L. Kreis107, M. Krivda64,111, F. Krizek95, K. Krizkova Gajdosova37, M. Kr¨uger68, E. Kryshen98, M. Krzewicki39, A.M. Kubera97, V. Kuˇcera34,61, C. Kuhn137, P.G. Kuijer90, L. Kumar100, S. Kundu86, P. Kurashvili85, A. Kurepin62, A.B. Kurepin62, A. Kuryakin109, S. Kushpil95, J. Kvapil111, M.J. Kweon61, J.Y. Kwon61, Y. Kwon148, S.L. La Pointe39, P. La Rocca27, Y.S. Lai80, R. Langoy130, K. Lapidus34, A. Lardeux20, P. Larionov52, E. Laudi34, R. Lavicka37, T. Lazareva113, R. Lea24, L. Leardini104, J. Lee134, S. Lee148, F. Lehas90, S. Lehner114, J. Lehrbach39, R.C. Lemmon94, I. Le´on Monz´on121, E.D. Lesser19, M. Lettrich34, P. L´evai146, X. Li12, X.L. Li6, J. Lien130, R. Lietava111, B. Lim17, V. Lindenstruth39, A. Lindner48, S.W. Lindsay128, C. Lippmann107, M.A. Lisa97, A. Liu19, J. Liu128, S. Liu97, W.J. Llope144, I.M. Lofnes21, V. Loginov93, C. Loizides96, P. Loncar35, J.A. Lopez104, X. Lopez135, E. L´opez Torres8, J.R. Luhder145, M. Lunardon28, G. Luparello60, Y.G. Ma40, A. Maevskaya62, M. Mager34, S.M. Mahmood20, T. Mahmoud43, A. Maire137, R.D. Majka147,i, M. Malaev98, Q.W. Malik20, L. Malinina75,iv, D. Mal’Kevich92, P. Malzacher107, G. Mandaglio32,56, V. Manko88, F. Manso135, V. Manzari53, Y. Mao6, M. Marchisone136, J. Mareˇs66, G.V. Margagliotti24, A. Margotti54, J. Margutti63, A. Mar´ın107, C. Markert120, M. Marquard68, C.D. Martin24, N.A. Martin104, P. Martinengo34, J.L. Martinez126, M.I. Mart´ınez45, G. Mart´ınez – 22 – JHEP06(2020)035 Garc´ıa115, S. Masciocchi107, M. Masera25, A. Masoni55, L. Massacrier78, E. Masson115, A. Mastroserio53,139, A.M. Mathis105,118, O. Matonoha81, P.F.T. Matuoka122, A. Matyja119, C. Mayer119, F. Mazzaschi25, M. Mazzilli53, M.A. Mazzoni58, A.F. Mechler68, F. Meddi22, Y. Melikyan62,93, A. Menchaca-Rocha71, C. Mengke6, E. Meninno29,114, M. Meres13, S. Mhlanga125, Y. Miake134, L. Micheletti25, D.L. Mihaylov105, K. Mikhaylov75,92, A.N. Mishra69, D. Mi´skowiec107, A. Modak3, N. Mohammadi34, A.P. Mohanty63, B. Mohanty86, M. Mohisin Khan16,v, Z. Moravcova89, C. Mordasini105, D.A. Moreira De Godoy145, L.A.P. Moreno45, I. Morozov62, A. Morsch34, T. Mrnjavac34, V. Muccifora52, E. Mudnic35, D. M¨uhlheim145, S. Muhuri142, J.D. Mulligan80, M.G. Munhoz122, R.H. Munzer68, H. Murakami133, S. Murray125, L. Musa34, J. Musinsky64, C.J. Myers126, J.W. Myrcha143, B. Naik49, R. Nair85, B.K. Nandi49, R. Nania10,54, E. Nappi53, M.U. Naru14, A.F. Nassirpour81, C. Nattrass131, R. Nayak49, T.K. Nayak86, S. Nazarenko109, A. Neagu20, R.A. Negrao De Oliveira68, L. Nellen69, S.V. Nesbo36, G. Neskovic39, D. Nesterov113, L.T. Neumann143, B.S. Nielsen89, S. Nikolaev88, S. Nikulin88, V. Nikulin98, F. Noferini10,54, P. Nomokonov75, J. Norman79,128, N. Novitzky134, P. Nowakowski143, A. Nyanin88, J. Nystrand21, M. Ogino82, A. Ohlson81,104, J. Oleniacz143, A.C. Oliveira Da Silva131, M.H. Oliver147, C. Oppedisano59, A. Ortiz Velasquez69, A. Oskarsson81, J. Otwinowski119, K. Oyama82, Y. Pachmayer104, V. Pacik89, D. Pagano141, G. Pai´c69, J. Pan144, S. Panebianco138, P. Pareek50,142, J. Park61, J.E. Parkkila127, S. Parmar100, S.P. Pathak126, B. Paul23, H. Pei6, T. Peitzmann63, X. Peng6, L.G. Pereira70, H. Pereira Da Costa138, D. Peresunko88, G.M. Perez8, Y. Pestov4, V. Petr´aˇcek37, M. Petrovici48, R.P. Pezzi70, S. Piano60, M. Pikna13, P. Pillot115, O. Pinazza34,54, L. Pinsky126, C. Pinto27, S. Pisano10,52, D. Pistone56, M. P losko´n80, M. Planinic99, F. Pliquett68, S. Pochybova146,i, M.G. Poghosyan96, B. Polichtchouk91, N. Poljak99, A. Pop48, S. Porteboeuf-Houssais135, V. Pozdniakov75, S.K. Prasad3, R. Preghenella54, F. Prino59, C.A. Pruneau144, I. Pshenichnov62, M. Puccio34, J. Putschke144, L. Quaglia25, R.E. Quishpe126, S. Ragoni111, S. Raha3, S. Rajput101, J. Rak127, A. Rakotozafindrabe138, L. Ramello31, F. Rami137, S.A.R. Ramirez45, R. Raniwala102, S. Raniwala102, S.S. R¨as¨anen44, R. Rath50, V. Ratza43, I. Ravasenga90, K.F. Read96,131, A.R. Redelbach39, K. Redlich85,vi, A. Rehman21, P. Reichelt68, F. Reidt34, X. Ren6, R. Renfordt68, Z. Rescakova38, K. Reygers104, V. Riabov98, T. Richert81,89, M. Richter20, P. Riedler34, W. Riegler34, F. Riggi27, C. Ristea67, S.P. Rode50, M. Rodr´ıguez Cahuantzi45, K. Røed20, R. Rogalev91, E. Rogochaya75, D. Rohr34, D. R¨ohrich21, P.S. Rokita143, F. Ronchetti52, A. Rosano56, E.D. Rosas69, K. Roslon143, A. Rossi28,57, A. Rotondi140, A. Roy50, P. Roy110, O.V. Rueda81, R. Rui24, B. Rumyantsev75, A. Rustamov87, E. Ryabinkin88, Y. Ryabov98, A. Rybicki119, H. Rytkonen127, O.A.M. Saarimaki44, S. Sadhu142, S. Sadovsky91, K. ˇ Safaˇr´ık37, S.K. Saha142, B. Sahoo49, P. Sahoo49, R. Sahoo50, S. Sahoo65, P.K. Sahu65, J. Saini142, S. Sakai134, S. Sambyal101, V. Samsonov93,98, D. Sarkar144, N. Sarkar142, P. Sarma42, V.M. Sarti105, M.H.P. Sas63, E. Scapparone54, J. Schambach120, H.S. Scheid68, C. Schiaua48, R. Schicker104, A. Schmah104, C. Schmidt107, H.R. Schmidt103, M.O. Schmidt104, M. Schmidt103, N.V. Schmidt68,96, A.R. Schmier131, J. Schukraft89, Y. Schutz34,137, K. Schwarz107, K. Schweda107, G. Scioli26, E. Scomparin59, M. ˇ Sefˇc´ık38, J.E. Seger15, Y. Sekiguchi133, D. Sekihata133, I. Selyuzhenkov93,107, S. Senyukov137, D. Serebryakov62, A. Sevcenco67, A. Shabanov62, A. Shabetai115, R. Shahoyan34, W. Shaikh110, A. Shangaraev91, A. Sharma100, A. Sharma101, H. Sharma119, M. Sharma101, N. Sharma100, S. Sharma101, A.I. Sheikh142, K. Shigaki46, M. Shimomura83, S. Shirinkin92, Q. Shou40, Y. Sibiriak88, S. Siddhanta55, T. Siemiarczuk85, D. Silvermyr81, G. Simatovic90, G. Simonetti34, B. Singh105, R. Singh86, R. Singh101, R. Singh50, V.K. Singh142, V. Singhal142, T. Sinha110, B. Sitar13, M. Sitta31, T.B. Skaali20, M. Slupecki127, N. Smirnov147, R.J.M. Snellings63, C. Soncco112, J. Song126, A. Songmoolnak116, F. Soramel28, S. Sorensen131, I. Sputowska119, J. Stachel104, I. Stan67, – 23 –