Differential studies of inclusive J/ψ and ψ(2S) production at forward rapidity in Pb-Pb collisions at √sNN=2.76 TeV

JHEP05(2016)179 Published for SISSA by Springer Received:February 5, 2016 Accepted:May 9, 2016 Published:May 31, 2016 Differential studies of inclusive J/ψand ψ(2S) production at forward rapidity in Pb-Pb collisions at √sNN = 2.76 TeV The ALICE collaboration E-mail: [email protected] Abstract: The production of J/ψand ψ(2S) was studied with the ALICE detector in Pb-Pb collisions at the LHC. The measurement was performed at forward rapidity (2.5< y < 4) down to zero transverse momentum (pt) in the dimuon decay channel. Inclusive J/ψyields were extracted in different centrality classes and the centrality dependence of the average ptis presented. The J/ψsuppression, quantified with the nuclear modification factor (RAA), was measured as a function of centrality, transverse momentum and rapidity. Comparisons with similar measurements at lower collision energy and theoretical models indicate that the J/ψproduction is the result of an interplay between color screening and recombination mechanisms in a deconfined partonic medium, or at its hadronization. Results on the ψ(2S) suppression are provided via the ratio of ψ(2S) over J/ψmeasured in pp and Pb-Pb collisions. Keywords: Heavy Ion Experiments, Quark gluon plasma ArXiv ePrint: 1506.08804 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP05(2016)179 JHEP05(2016)179 Contents 1 Introduction 2 2 The ALICE detector 3 3 Data sample 4 4 Definition of observables 5 5 Signal extraction 7 5.1 Muon reconstruction 7 5.2 J/ψsignal 9 5.3 ψ(2S) signal 11 6 Acceptance and efficiency correction 12 7 Systematic uncertainties 14 7.1 Signal extraction 14 7.2 Monte Carlo input parametrization 15 7.3 Centrality dependence of the [ψ(2S)/J/ψ]A×ε15 7.4 Tracking efficiency 15 7.5 Trigger efficiency 16 7.6 Matching efficiency 17 7.7 pp reference 17 7.8 Normalization 17 7.9 Others 17 7.10 Summary 18 8 Inclusive J/ψ mean transverse momentum 18 9 Nuclear modification factor 21 9.1 Centrality dependence of RAA 21 9.2 Transverse momentum dependence of RAA 24 9.3 Rapidity dependence of RAA 26 10 [ψ(2S)/J/ψ] ratio 27 11 Conclusions 29 A Data tables 32 The ALICE collaboration 42 – 1 – JHEP05(2016)179 1 Introduction At high temperature, lattice quantum chromodynamics predicts the existence of a deconfined phase of quarks and gluons where chiral symmetry is restored [1]. This state of matter is known as the Quark Gluon Plasma (QGP) [2], and its characterization is the goal of ultra-relativistic heavy-ion collision studies. Among the probes used to investigate the QGP and quantify its properties, quarkonium states are one of the most prominent and have generated a large amount of results both theoretical and experimental. According to the color-screening model [3,4], measurement of the in-medium dissociation probability of the different quarkonium states could provide an estimate of the system temperature. Dissociation is expected to take place when the medium reaches or exceeds the critical temperature for the phase transition (Tc), depending on the binding energy of the quarkonium state. In the charmonium (c¯c) family, the strongly bound J/ψ could survive significantly above Tc(1.5–2 Tc) whereas χcand ψ(2S) melting should occur near Tc(1.1–1.2 Tc) [5,6]. The determination of the in-medium quarkonium properties remains a challenging theoretical task. Intense and persistent investigations on the theory side are ongoing [7]. Shortly after quarkonium suppression was suggested as a strong evidence of QGP formation, the first ideas of charmonium enhancement via recombination of c and ¯c appeared [8,9]. Since then, the J/ψ enhancement mechanism has been more formalized and quantitative predictions [10–14] were made. Since the charm quark density produced in hadronic collisions increases with energy [15], recombination mechanisms are predicted to give rise to a sizable J/ψ production at LHC energies, which is likely to partially compensate or exceed the J/ψ suppression due to color-screening in the QGP. The observation of J/ψ enhancement in nucleus-nucleus collisions via recombination would constitute an evidence for deconfinement and hence for QGP formation. In addition, information for the characterization of the QGP can come from the study of the ψ(2S) meson, a state which is less strongly bound and not affected by higher mass charmonium decays with respect to the J/ψ. In the pure melting scenario, the relative production of ψ(2S) with respect to J/ψ is expected to be very small at the LHC [4], which is not the case if recombination occurs [16,17]. J/ψ suppression was observed experimentally in the most central heavy-nucleus collisions at the SPS [18,19], RHIC [20–23] and LHC [24–28], ranging from a center-of-mass energy per nucleon pair (√sNN ) of about 17 GeV to 2.76 TeV. The ψ(2S) suppression was measured at the SPS [29] and the LHC [30]. The interpretation of these results is not straightforward as they are also subject to other effects, not all related to the presence of a QGP. A fraction of J/ψ originates from the strong and electromagnetic feed-down of the χcand ψ(2S). Therefore, a melting of these higher mass states before they can decay into the J/ψ will lead to an effective suppression of the J/ψ yield already for a medium that does not reach the J/ψ dissociation temperature. Assuming charmonium states are initially produced with the same relative abundancies in Pb–Pb collisions as in pp collisions, the χcand ψ(2S) melting would result in a reduction of the J/ψ yield of about 40% [31]. In addition, a non-prompt J/ψ and ψ(2S) component from the weak decay of beauty hadrons also contributes to the inclusive measurements. Since the beauty hadrons – 2 – JHEP05(2016)179 decay outside the QGP volume, this contribution is not sensitive to the color-screening of charmonia. Finally, a fraction of the J/ψ and ψ(2S) suppression can be ascribed to cold nuclear matter (CNM) effects, also present in proton-nucleus collisions [32,33]. The CNM effects group together the nuclear absorption of the charmonia, the modification of the parton distribution functions (PDF) in the nuclei that leads to a reduction (shadowing) or an enhancement (anti-shadowing) of the c¯c pair production, and the energy loss of charm quarks in the nucleus. Numerous studies of J/ψ production in different collision systems at different energies are now available. Comparisons between experiments and to theoretical models can be made over wide kinematic ranges in rapidity and transverse momentum. We already published the centrality, transverse momentum (pt) and rapidity (y) dependence of the J/ψ nuclear modification factor in Pb–Pb collisions at √sNN = 2.76 TeV [26,27]. In this paper, those results are extensively compared to available theoretical models and lower energy data. New results on the J/ψ hptiand hp2 tiversus centrality, and on the centrality (pt) dependence of the J/ψ suppression for various pt(centrality) ranges are also presented. Furthermore, we show results on ψ(2S) in Pb–Pb collisions, measured via the [ψ(2S)/J/ψ] ratio, as a function of centrality. The remainder of this paper is organized as follows: the experimental apparatus and the data sample are presented in sections 2 and 3. Section 4 gives the definition of the observables used in the analysis. The analysis procedure is then described in sections 5 and 6. Systematic uncertainties are discussed in section 7. The J/ψ results are given in sections 8 and 9 while section 10 is dedicated to the ψ(2S) results. Finally, section 11 presents our conclusions. 2 The ALICE detector The ALICE detector is described in detail in [34]. At forward rapidity (2.5< y < 4) the production of quarkonium states is studied in the muon spectrometer via their µ+µ−decay channels down to zero pt. In the ALICE reference frame, the positive zdirection is along the counter-clockwise beam direction. The muon spectrometer covers a negative pseudorapidity (η) range and consequently a negative yrange. However, due to the symmetry of the Pb–Pb system, the results are presented with a positive ynotation, while keeping the negative sign for η. The muon spectrometer consists of a ten-interaction-lengths (4.1 m) thick absorber, which filters the muons, in front of five tracking stations comprising two planes of cathode pad chambers each. The third station is located inside a dipole magnet with a 3 Tm field integral. The tracking apparatus is completed by a Muon Trigger system (MTR) composed of four planes of resistive plate chambers downstream from a seven-interaction- lengths (1.2 m) thick iron wall, which absorbs secondary hadrons escaping from the front absorber and low-momentum muons coming mainly from charged pion and kaon decays. A small-angle conical absorber protects the tracking and trigger chambers against secondary particles produced by the interaction of large rapidity primary particles with the beam pipe. – 3 – JHEP05(2016)179 Finally, a rear absorber protects the trigger chambers from the background generated by beam-gas interactions downstream from the spectrometer. In addition, the Silicon Pixel Detector (SPD), scintillator arrays (V0) and Zero Degree Calorimeters (ZDC) were used in this analysis. The SPD consists of two cylindrical layers covering |η|<2.0 and |η|<1.4 for the inner and outer ones, respectively, and provides the coordinates of the primary vertex of the collision. The V0 counters, two arrays of 32 scintillator tiles each, are located on both sides of the nominal interaction point and cover 2.8< η < 5.1 (V0-A) and −3.7< η < −1.7 (V0-C). The ZDC are located on either side of the interaction point at z≈ ±114 m and detect spectator nucleons at zero degree with respect to the LHC beam axis. The V0 and ZDC detectors provide triggering information and event characterization. 3 Data sample The data sample analysed in this paper corresponds to Pb–Pb collisions at √sNN = 2.76 TeV. These collisions were delivered by the LHC during 190 hours of stable beam operations spread over three weeks in November and December 2011. The Level-0 (L0) minimum bias (MB) trigger was defined as the coincidence of signals in V0-A and V0-C detectors synchronized with the passage of two crossing lead bunches. This choice for the MB condition provides a high triggering efficiency (>95%) for hadronic interactions. To improve the trigger purity, a threshold on the energy deposited in the neutron ZDC rejects the contribution from electromagnetic dissociation processes at the Level-1 (L1) trigger level. Beam induced background is further reduced at the offline level by timing cuts on the signals from the V0 and the ZDC. The charmonium analysis was carried out on a data sample, where in addition to the MB prerequisite, a trigger condition of at least one or two reconstructed muon candidate tracks in the MTR (trigger tracks) was required in each event. The MTR logic allows for programming several L0 trigger decisions based on (i) the detection of one or two muon trigger tracks, (ii) the presence of opposite-sign or like-sign trigger track pairs and (iii) a lower threshold on the approximate transverse momentum (ptrig t) of the muon candidates. The latter selection is performed by applying a cut on the maximum deviation of the trigger track from an infinite momentum track originating at the nominal interaction point. Due to the finite spatial resolution of the trigger chambers, this does not lead to a sharp cut in pt, and the corresponding ptrig tthreshold is defined in simulation as the ptvalue for which the muon trigger probability is 50%. The following muon-specific L0 triggers were used: •Single muon low pt(ptrig t= 1 GeV/c): MSL •Opposite-sign dimuon low pt(ptrig t= 1 GeV/c on each muon): MUL •Like-sign dimuon low pt(ptrig t= 1 GeV/c on each muon): MLL A data sample of 17.3·106Pb–Pb collisions was collected with the µµ-MB trigger, defined as the coincidence of the MB and MUL conditions. A scaling factor Fnorm is computed for each run — corresponding to a few hours maximum of continuous data taking — in order – 4 – JHEP05(2016)179 to normalize the number of µµ-MB triggers to the number of equivalent MB triggers. It is defined as the ratio, in a MB data sample, between the total number of events and the number of events fulfilling the µµ-MB trigger condition. It should be noted that the MB sample used in this calculation was recorded in parallel to the µµ-MB triggers. The Fnorm value, 30.56±0.01(stat.)±1.10(syst.), is given by the average over all runs weighted by the statistical uncertainties. A small fraction of opposite-sign dimuons were misidentified by the trigger algorithm as like-sign pairs. Although for the J/ψ it amounts to less than 1% when considering the full sample, it increases up to 4% at high ptin peripheral collisions. In this analysis, the missing fraction of opposite-sign dimuons was recovered by extracting the number of produced J/ψ and ψ(2S) from the union of the MUL and MLL data sample (MUL∪MLL). This is different from the selection applied in the former paper [27], where only the MUL data sample was used. On the other hand, the efficiency of the trigger algorithm to determine the sign of the muon pairs does not impact the normalization of the collected data sample to the number of equivalent MB events described above. This was cross-checked by computing the normalization factor of the MUL∪MLL data sample, resulting in less than 1% difference in the extracted number of equivalent MB events. The integrated luminosity corresponding to the analysed data sample is Lint =Nµµ-MB· Fnorm/σPb–Pb = 68.8±0.9(stat.)±2.5(syst. Fnorm)+5.5 −4.5(syst. σPb–Pb)µb−1using an inelastic Pb–Pb cross section σPb–Pb = 7.7±0.1+0.6 −0.5b [35]. 4 Definition of observables The centrality determination is based on a fit of the V0 signal amplitude distribution as described in [36]. Variables characterizing the collision such as the average number of participant nucleons (hNparti) and the average nuclear overlap function (hTAAi) for each centrality class are given in table 1. In this analysis a cut corresponding to the most central 90% of the inelastic nuclear cross section was applied as for these events the MB trigger is fully efficient and the residual contamination from electromagnetic processes is negligible. For each centrality class i, the measured number of J/ψ (Ni J/ψ) is normalized to the equivalent number of minimum bias events (Ni events). To obtain Ni events, one simply multiplies the number of µµ-MB triggered events by the Fnorm factor scaled by the width of the centrality class. Corrections for the branching ratio of the dimuon decay channel (BRJ/ψ→µ+µ−= 5.93 ±0.06%) and for the acceptance times efficiency (A×i) of the detector are then applied. The J/ψ yield (Yi J/ψ) in a centrality class iis given by d2Yi J/ψ dptdy=d2Ni J/ψ/dptdy BRJ/ψ→µ+µ−·Ni events ·A×i(pt, y).(4.1) It is then combined with the inclusive J/ψ cross section measured in pp collisions at the same energy to form the nuclear modification factor RAA defined as Ri AA(pt, y) = d2Yi J/ψ/dptdy hTAAii·d2σpp J/ψ/dptdy.(4.2) – 5 – JHEP05(2016)179 Centrality hNparti hTAAi(mb−1) Centrality hNparti hTAAi(mb−1) 0–10% 356.0±3.6 23.44±0.76 0–20% 308.1±3.7 18.91±0.61 10–20% 260.1±3.8 14.39±0.45 0–40% 232.6±3.4 12.88±0.42 20–30% 185.8±3.3 8.70±0.27 0–90% 124.4±2.2 6.27±0.21 30–40% 128.5±2.9 5.00±0.18 20–40% 157.2±3.1 6.85±0.23 40–50% 84.7±2.4 2.68±0.12 20–60% 112.8±2.6 4.42±0.16 50–60% 52.4±1.6 1.317±0.071 40–60% 68.6±2.0 1.996±0.097 60–70% 29.77±0.98 0.591±0.036 40–90% 37.9±1.2 0.985±0.051 70–80% 15.27±0.55 0.243±0.016 50–90% 26.23±0.84 0.563±0.033 80–90% 7.49±0.22 0.0983±0.0076 60–90% 17.51±0.59 0.311±0.020 Table 1. The average number of participant nucleons hNpartiand the average value of the nuclear overlap function hTAAiwith their associated systematic uncertainties for the centrality classes, expressed in percentages of the nuclear cross section [36], used in these analyses. The ptand yintegrated J/ψ cross section is σpp J/ψ(pt<8 GeV/c, 2.5< y < 4) = 3.34 ± 0.13(stat.)±0.24(syst.)±0.12(luminosity)+0.53 −1.07(polarization)µb [37]. The ALICE measurements reported here refer to inclusive J/ψ yields, i.e. include prompt J/ψ (direct J/ψ and feed-down from ψ(2S) and χc) and non-prompt J/ψ (decay of B-mesons). Contrary to prompt J/ψ, J/ψ from B-meson decays do not directly probe the hot and dense medium created in the Pb–Pb collisions. Beauty hadron decays occur outside the QGP, so the non-prompt J/ψ RAA is instead related to the energy loss of the beauty quarks in the medium. Although the prompt J/ψ RAA cannot be directly measured with the ALICE muon spectrometer, it can be evaluated via Rprompt AA =RAA −FB·Rnon-prompt AA 1−FB (4.3) where FBis the fraction of non-prompt to inclusive J/ψ measured in pp collisions, and Rnon-prompt AA is the nuclear modification factor of J/ψ from B-meson decays in Pb–Pb collisions. The non-prompt and prompt J/ψ differential cross sections as a function of pt and ywere measured by LHCb in pp collisions at √s= 2.76 and 7 TeV [38,39] in a kinematic range overlapping with that of the ALICE muon spectrometer. Therefore, one can extract the ptand ydependence of FBfrom these data and use it in eq. (4.3). A reliable determination of Rnon-prompt AA presents further complications. We have thus chosen two extreme hypotheses, independent of centrality, corresponding to the absence of medium effects on beauty hadrons (Rnon-prompt AA = 1) or to a complete suppression (Rnon-prompt AA = 0), to evaluate conservative limits on Rprompt AA . An excess of J/ψ compared to the yield expected assuming a smooth evolution of the J/ψ hadro-production and nuclear modification factor was observed in peripheral Pb–Pb collisions at very low pt[40]. This excess might originate from the photo-production of J/ψ. This contribution is negligible in pp collisions — from LHCb measurement at – 6 – JHEP05(2016)179 √s= 7 TeV [41], it is O(10−3)% — but it is enhanced by a factor O(104) in Pb–Pb collisions, thus reaching the order of magnitude of the observed excess. The J/ψ coherent photo-production has been measured in ultra-peripheral Pb–Pb collisions [42]. It is centered at very low pt, with ∼98% of these J/ψ below 0.3 GeV/c. An incoherent photoproduction component is also observed in ultra-peripheral Pb–Pb collisions. About 30% of this contribution has a pt<0.3 GeV/c, the rest being mainly located in the ptrange 0.3–1 GeV/c. The influence of possible photo-production mechanisms on the inclusive J/ψ RAA presented in this paper has been evaluated by repeating the analysis placing a low ptthreshold on the J/ψ of 0.3 GeV/c. Assuming that the observed excess in peripheral Pb–Pb collisions is indeed due to the photo-production of J/ψ, and that the relative contribution of the incoherent over coherent components is the same as the one estimated in ultra-peripheral collisions, then this selection would remove about 75% of the full photoproduction contribution. Numerical values of RAA with the low ptthreshold at 0.3 GeV/c are given in the appendix A. All the figures and values presented in the paper refer to the inclusive J/ψ RAA but estimates of the difference between the inclusive and hadronic (without J/ψ photo-production) J/ψ RAA, are indicated where appropriate. The results for the ψ(2S) analysis are given in terms of the ratio of their production cross sections (or, equivalently, of their production yields), expressed as ψ(2S)/J/ψ =Ni ψ(2S) Ni J/ψ ·(A×εi)J/ψ (A×εi)ψ(2S) .(4.4) When forming such a ratio the normalization factor Ni events cancels out, as do most of the systematic uncertainties on A×εcorrections. The double ratio [ψ(2S)/J/ψ]Pb–Pb /[ψ(2S)/J/ψ]pp is used in order to directly compare the relative abundances of ψ(2S) and J/ψ in nucleus-nucleus and pp collisions. 5 Signal extraction After a description of the muon selection procedure, we present here the two methods used to extract the J/ψ and ψ(2S) signals. The first one is directly based on fits of the µ+µ− invariant mass distribution while the second one makes use of the event mixing technique to subtract the combinatorial background. 5.1 Muon reconstruction The muon reconstruction starts with the exclusion of parts of the detector that show problems during data taking such as high voltage trips, large electronic noise, pedestal determination issues. This selection is performed on a run-by-run basis to account for the time evolution of the apparatus. After pedestal subtraction, the adjacent well-functioning pads of both cathodes of each tracking chamber having collected a charge are grouped to form pre-clusters. These pre-clusters might be the superposition of several clusters of charges deposited by several particles crossing the detector close to each others. The number of clusters of charges contributing to the pre-cluster and their approximate location are – 7 – JHEP05(2016)179 determined with a Maximum Likelihood - Expectation Maximization (MLEM) algorithm. It assumes that the charge distribution of each single cluster follows a two-dimensional integral of the Mathieson function [43]. If the estimated number of clusters is larger than 3, the pre-cluster is split into several groups of 1, 2 or 3 clusters selected with the minimum total coupling to all the other clusters into the pre-cluster. Each group of clusters is then fitted using a sum of Mathieson functions, taking the MLEM results as a seed, to extract the precise location of where the particles crossed the detector. The overall spatial resolution is around 200 (550) µm in average in the (non-)bending direction. The track reconstruction starts from the most downstream stations, where the multiplicity of secondary particles is smallest, by forming pairs of clusters in the two chambers of station 5(4), and deriving the parameters and associated errors of the resulting muon track candidates. The candidates are then extrapolated to the station 4(5), validated if at least one compatible cluster is found in the station and duplicate tracks are removed. The procedure continues extrapolating the tracks to stations 3, 2 and 1, validating them by the inclusion of at least one cluster per station. The selection of compatible clusters is based on a 5σcut on a χ2computed from the cluster and track local positions and errors. If several compatible clusters are found in the same chamber, the track is duplicated to consider all the possibilities and for each of them the track parameters and associated errors are recomputed using a Kalman filter. At each of the tracking steps, the track candidates, whose parameters indicate that they will exit the geometrical acceptance of the spectrometer in the next steps are removed. At the end of the procedure, the quality of the track is improved by adding/removing clusters based on a 4σcut on the local χ2and fake tracks sharing clusters with others in the three outermost stations with respect to the interaction point are removed. The choice of the χ2cuts is a compromise between maximizing the tracking efficiency (<1–2% muon rejection) and minimizing the amount of fake tracks (negligible background for this analysis). Finally, muon track candidates are extrapolated to the interaction vertex measured by the SPD taking into account the energy loss and the multiple Coulomb scattering in the front absorber. An accurate measurement of the tracking chamber alignment is essential to reconstruct the tracks with enough precision to identify resonances in the µ+µ−invariant mass spectrum, especially the ψ(2S) for which the signal-to-background ratio is low. The absolute position of the chambers was first measured using photogrammetry before the data taking. Their relative position was then precisely determined using a modified version of the MILLEPEDE package [44], combining several samples of tracks taken with and without magnetic field. The small displacement of the chambers when switching on the dipole was measured by the Geometry Monitoring System (an array of optical sensors fixed on the chambers) and taken into account. The resulting alignment precision is ∼100 µm, leading to a reconstructed J/ψ invariant mass resolution of about 70 MeV/c2, and about 10% higher for the ψ(2S). The resolution is dominated by the energy loss fluctuation and multiple Coulomb scattering of the muons in the front absorber. More details on the muon spectrometer performances are given in [45]. In this analysis, the muon track candidates also have to fulfill the following requirements. First, the reconstructed track must match a trigger track with a ptrig tabove the – 8 – JHEP05(2016)179 systematic uncertainty on the signal extraction varies from 1% to 4%. Concerning the ψ(2S) analysis, in the intervals where the signal was extracted, the systematic uncertainty is 14%, 45% and 24% for centrality ranges 60–90%, 40–60% and 20–40% for pt<3 GeV/c. 7.2 Monte Carlo input parametrization The estimation of A×εfactors depends on the charmonium ptand yshapes used as input distributions in the MC simulation. In order to evaluate the sensitivity of the results on this choice, several MC simulations were performed, each one including modified pt and ydistributions. For the J/ψ, the modification of the shapes was done in order to take into account the possible correlation between ptand y(as observed by LHCb in pp collisions [39]) and the correlation between pt(y) and the centrality of the collision (as reported in this paper). A systematic uncertainty of 3% is found for A×εintegrated over ptand yand is taken as correlated as a function of the centrality. The pt(y) dependence of this uncertainty varies in the range 0–1% (3–8%). The larger effect seen in the ydependence occurs at the low and high limits, where the acceptance falls steeply. The same procedure was followed for the ψ(2S), assuming that the correlations between ptand yand with the centrality are of the same magnitude as those observed for the J/ψ. A systematic uncertainty of 2% is evaluated for the [ψ(2S)/J/ψ] ratio in the pt< 3 GeV/c interval. 7.3 Centrality dependence of the [ψ(2S)/J/ψ]A×ε The embedding technique was not used for the ψ(2S) MC simulations as we have assumed the same A×εdependence as a function of the centrality for the ψ(2S) and the J/ψ. In order to evaluate the systematic uncertainty introduced by this assumption, a conservative ±30% variation of the A×εloss as a function of centrality was applied to the ψ(2S). This corresponds to the maximum variation of the A×εloss between peripheral and central collisions observed for the J/ψ in different ptand yintervals. The effect on the (A×ε)J/ψ /(A×ε)ψ(2S) ratio is 1% or lower in all the centrality classes considered. Since this effect is much smaller than the systematic uncertainty on the signal extraction, it is neglected. 7.4 Tracking efficiency The tracking algorithm, as described in section 5.1, does not require all the chambers to have fired to reconstruct a track. This redundancy of the tracking chambers can be used to measure their individual efficiencies from data, and since such efficiencies are independent from each other, we can combine them to assess the overall tracking efficiency. This evaluation of the tracking efficiency is not precise enough to be used to directly correct the data, because only the mean efficiency per chamber can be computed with the statistics available in each run. However, by comparing the result obtained from data with the same measurement performed in simulations, we can control the accuracy of these simulations and assess the corresponding systematic uncertainty on the A×εcorrections. – 15 – JHEP05(2016)179 A 9% relative systematic uncertainty is obtained for the J/ψ by comparing the measured tracking efficiency in simulations and in peripheral Pb–Pb collisions. This uncertainty is constant and fully correlated as a function of centrality. From low to high pt(y), the systematic uncertainty varies from 9% to 7% (7% to 6% with a maximum of 12% at y≃3.25). On top of that, a small difference was observed in the centrality dependence of this measurement between data and embedding simulations. This results in an additional 1% systematic uncertainty in the 0–10% centrality class and 0.5% in 10–20%. Another systematic uncertainty can arise from correlated dead areas located in front of each other in the same station, which cannot be detected with the method detailed above. A dedicated study has shown that this effect introduces a 2% systematic uncertainty, fully correlated as a function of centrality and predominantly uncorrelated as a function of ptand y. In the [ψ(2S)/J/ψ] ratio the systematic uncertainty on the tracking efficiency largely cancels out because the ψ(2S) and J/ψ decay muons have similar ptand ydistributions and, therefore, cross about the same regions of the detector. Since the possible remaining systematic uncertainty is much smaller than that on the signal extraction, it is neglected in this analysis. 7.5 Trigger efficiency The systematic uncertainty on the J/ψ A×εcorrections related to the trigger efficiency has two origins: the intrinsic efficiency of the trigger chambers and the response of the trigger algorithm. The first part was determined from the uncertainties on the trigger chamber efficiencies measured from data and applied to simulations. Propagating these efficiencies in J/ψ simulations results in a 2% systematic uncertainty on the A×εcorrections, fully correlated as a function of centrality and mainly uncorrelated as a function of ptand y. The effect of the systematic uncertainty on the shape of the trigger response as a function of the muon ptwas determined by weighting MC J/ψ decay muons with different trigger response functions obtained in data and simulations. These functions were defined as the fraction, versus pt, of the single muons passing a 0.5 GeV/c ptrig tthreshold that also satisfy the 1 GeV/c ptrig tthreshold used in this analysis. The resulting systematic uncertainty on the J/ψ A ×εcorrection integrated over ptand yis 1%. As a function of pt, it amounts to 3% for pt<1 GeV/c and 1% elsewhere. As a function of y, a 1% uncorrelated systematic uncertainty was obtained. These uncertainties are fully correlated as a function of centrality. The systematic uncertainty on the modification of the trigger response as a function of centrality, i.e. for increasing multiplicity, was assessed by changing the detector response (space size of the deposited charge) to the passage of particles in embedding simulations. The corresponding uncertainties on the J/ψ A ×εcorrections are 1% in the 0–10% and 10–20% centrality classes, and 0.5% in 20–30% and 30–40%. As for the case of tracking efficiency, this source of systematic uncertainty largely cancels out in the [ψ(2S)/J/ψ] ratio and is neglected. – 16 – JHEP05(2016)179 7.6 Matching efficiency The systematic uncertainty on the matching efficiency between the tracking and the trigger tracks is 1%. It is given by the differences observed between data and simulations when applying different χ2cuts on the matching between the track reconstructed in the tracking chambers and the one reconstructed in the trigger chambers. This uncertainty is fully correlated as a function of the centrality and largely uncorrelated as a function of ptand y. Also in this case, the effect on the [ψ(2S)/J/ψ] ratio is negligible. 7.7 pp reference The statistical and systematic uncertainties on the measurement of the J/ψ differential cross section in pp collisions at √s= 2.76 TeV are available in [37]. The statistical uncertainty is combined with that of the Pb–Pb measurement when calculating the RAA as a function of ptand y, but is considered as a fully correlated systematic uncertainty as a function of the centrality. The correlated and uncorrelated part of the systematic uncertainty on the pp reference as a function of ptand yare both fully correlated as a function of the centrality. The ψ(2S) statistics in the √s= 2.76 TeV pp data sample are too low to be used for the normalization of the [ψ(2S)/J/ψ]Pb–Pb ratio. For this reason, pp results obtained at higher energy (√s= 7 TeV) [50] were used, thus introducing an additional source of systematic uncertainty. An interpolation procedure, as the one described in [33], was applied in order to extract the [ψ(2S)/J/ψ]pp ratio at √s= 2.76 TeV. The discrepancy between the result of this interpolation in the kinematic range pt<3 GeV/c 2.5< y < 4 and the value obtained at √s= 7 TeV is 10%: this relative difference is included in the systematic uncertainty on the pp reference. 7.8 Normalization The systematic uncertainty on the normalization is the one attached to the scaling factor Fnorm and amounts to 4%. This value corresponds to one standard deviation of the distribution of the Fnorm computed for each run used in the analysis. This systematic uncertainty is fully correlated as a function of the centrality, ptand y. 7.9 Others Systematic uncertainties on the nuclear overlap function hTAAiare available in table 1. Another systematic uncertainty on the definition of the centrality classes arises from the V0 amplitude cut, which corresponds to 90% of the hadronic cross section [36]. A maximum uncertainty of 5% is obtained in the centrality class (80–90%) vanishing with increasing centrality or in wider centrality classes. Systematic uncertainties due to the unknown polarization of the J/ψ are not propagated and we assume that J/ψ production is unpolarized both in pp and in Pb–Pb collisions. In pp collisions at √s= 7 TeV, J/ψ polarization measurements at mid-rapidity (pt>10 GeV/c) and forward-rapidity (pt>2 GeV/c) are compatible with zero [51–53]. In Pb–Pb collisions, J/ψ mesons produced from initial parton-parton hard scattering are expected to have the same polarization as in pp collisions and those produced from charm quarks recombination in the medium are expected to be unpolarized. – 17 – JHEP05(2016)179 Sources Centrality pty[27] pt<8 GeV/c [27]ptbins 0–90% [27] centrality bins Signal extraction 1–3 1–4 1–4 1–5 1–4 MC parametrization 3∗1–3∗0–1 0–1 3–8 Tracking efficiency 0–1 and 11∗0–1 and 9–11∗9–11 and 1∗9–11 and 0–1∗8–14 and 1∗ Trigger efficiency 0–1 and 2∗0–1 and 2∗2–4 and 1∗2–4 and 0–1∗2 and 1∗ Matching efficiency 1∗1∗1 1 1 σpp J/ψ stat. 4∗5–12∗6–21 6–21 7–11 syst. 8∗7∗5–6 and 6∗5–6 and 6∗5–6 and 6∗ Fnorm 4∗4∗4∗4∗4∗ hTAAi3–8 3–6 3∗3–5∗3∗ Centrality limits 0–5 0–3 0 0–2∗0 B.R. n/a n/a n/a 1∗n/a Table 2. Summary of the systematic uncertainties (in %) entering the J/ψ yield and/or RAA calculation as a function of centrality, ptand y. Numbers with an asterisk correspond to the systematic uncertainties fully correlated as a function of the given variable. 7.10 Summary The systematic uncertainties related to the J/ψ analysis are summarized in table 2. Concerning the ψ(2S) analysis, most of the systematic uncertainties cancel out in the [ψ(2S)/J/ψ] ratio and the main contributors are the signal extraction (14–45%) and the pp reference (10%). 8 Inclusive J/ψ mean transverse momentum The ptdependence of the J/ψ yields per MB collision, defined by eq. (4.1), was studied for three centrality classes (0–20%, 20–40% and 40–90%) and is displayed in figure 5. The statistical uncertainties appear as vertical lines. The systematic uncertainties uncorrelated as a function of ptare shown as open boxes, while the ones fully correlated as a function of ptbut uncorrelated as a function of centrality are shown as shaded areas (mostly hidden by the points). The global systematic uncertainty, fully correlated as a function of centrality and pt, is quoted directly in the figure. Numerical values for the J/ψ yields can be found in appendix A. The inclusive J/ψ mean transverse momentum was computed by fitting the ptdistribution of inclusive J/ψ yields with the function f(pt) = C×pt (1 + (pt/p0)2)n,(8.1) where C,p0and nare free parameters. This function is commonly used to reproduce the J/ψ ptdistribution in hadronic collisions, see for instance [54–56]. Fit results for the three centrality classes are displayed as full lines in the figure. An excess over this function is revealed in the lowest ptinterval (corresponding to 0 < pt<500 MeV/c) for peripheral Pb–Pb collisions. It could be caused by a residual contribution from J/ψ coherent photo-production, which was measured in ultra-peripheral collisions [42]. A quantitative measurement of this contribution in hadronic collisions is reported in [40]. Thus, in the most peripheral centrality class (40–90%) the fit was performed for pt>500 MeV/c and – 18 – JHEP05(2016)179 )c (GeV/ T p 0 1 2 3 4 5 6 7 8 -1 )c (GeV/ T pdy/dY 2 d 5− 10 4− 10 3− 10 2− 10 = 2.76 TeV NN sALICE Pb-Pb <4y, 2.5< - µ + µ → ψInclusive J/ 4%±global syst. = 0-20% 20-40% 40-90% Figure 5. Differential yields of inclusive J/ψ in Pb–Pb collisions at √sNN = 2.76 TeV as a function of ptfor three centrality classes. Solid lines correspond to the results from the fit described in the text. extrapolated down to zero (dotted line). In the 0–20% and 20–40% centrality classes, no J/ψ excess was observed and fits were performed down to zero pt. As a cross-check, the same procedure as for the peripheral centrality class was tested and the obtained results are fully compatible within uncertainties. Values of the mean transverse momentum (hpti) and mean squared transverse momentum (hp2 ti) obtained from the fits are given in table 3as a function of centrality. The statistical (systematic) uncertainty is extracted by fitting the ptdistribution considering only the statistical (pt-uncorrelated systematic) uncertainty of the measurement. For comparisons, the hptiand hp2 tiresults from PHENIX were recomputed with the function defined by eq. (8.1), adjusted in the measured ptrange and extrapolated to pt= 8 GeV/c to match our ptrange. These results are also given in table 3along with the measurement in pp collisions at √s= 2.76 TeV with updated uncertainties [57]. The hptiof inclusive J/ψ measured in pp and Pb–Pb collisions at √sNN = 2.76 TeV is shown in figure 6(left side) as a function of hNparti. The error bars (open boxes) represent the statistical (systematic) uncertainties. A clear downward trend in hptiis observed when going from pp to the most central Pb–Pb collisions. The hptidecrease from peripheral (40–90%) to central (0–20%) collisions is significant, the two values being separated by more than 5σ. These results are compared to the ones obtained by PHENIX in pp, Cu–Cu and Au–Au collisions at √sNN = 0.2 TeV. There is no evidence for a decreasing trend, contrary to what is observed in the ALICE measurement. In order to compare the evolution of hp2 tiA–Aat different energies, one can form the variable rAA defined as rAA =hp2 tiA–A hp2 tipp .(8.2) This variable was measured over the wide range of energies and colliding systems covered by NA50 and PHENIX experiments. The comparison with the ALICE results is done in – 19 – JHEP05(2016)179 ptrange yrange Centrality hpti ± stat. ±syst. hp2 ti ± stat. ±syst. ( GeV/c) ( GeV/c) ( GeV2/c2) Pb–Pb √sNN = 2.76 TeV 0–8 2.5–4 0–20% 1.92 ±0.02 ±0.03 5.17 ±0.12 ±0.16 0–8 2.5–4 20–40% 2.04 ±0.02 ±0.04 5.83 ±0.11 ±0.17 0.5–8 2.5–4 40–90% 2.22 ±0.03 ±0.04 6.72 ±0.14 ±0.20 pp √s= 2.76 TeV [57] 0–8 2.5–4 n/a 2.28 ±0.04 ±0.03 7.06 ±0.26 ±0.13 pp √s= 0.2 TeV [55] 0–7 1.2–2.2 n/a 1.61 ±0.01 ±0.012 3.60 ±0.06 ±0.07 Au–Au √sNN = 0.2 TeV [21] 0–5 1.2–2.2 0–20% 1.94 ±0.18 5.79 ±1.33 0–6 1.2–2.2 20–40% 1.87 ±0.07 4.78 ±0.34 0–6 1.2–2.2 40–60% 1.74 ±0.04 4.19 ±0.27 0–6 1.2–2.2 60–92% 1.61 ±0.05 3.87 ±0.27 Cu–Cu √sNN = 0.2 TeV [58] 0–5 1.2–2.2 0–20% 1.68 ±0.04 ±0.02 3.79 ±0.25 ±0.11 0–5 1.2–2.2 20–40% 1.69 ±0.04 ±0.02 3.71 ±0.18 ±0.08 0–5 1.2–2.2 40–60% 1.68 ±0.05 ±0.02 3.91 ±0.30 ±0.11 0–5 1.2–2.2 60–94% 1.66 ±0.10 ±0.04 4.13 ±0.64 ±0.24 Table 3. Values of hptiand hp2 tiat various energies and colliding systems. The statistical and systematic uncertainties are quoted separately, except for PHENIX measurements in Au–Au collisions where the quadratic sum is given. If the measurement is not available or not used in the range 0 < pt<8 GeV/c, the fit function is extrapolated down to 0 and up to 8 GeV/c to compute hptiand hp2 ti. 〉 part N〈 1 10 2 10 (GeV/c)〉 T p〈 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 <4y, 2.5< - µ + µ → ψALICE inclusive J/ = 2.76 TeV NN s Pb-Pb = 2.76 TeVspp |<2.2y, 1.2<| - µ + µ → ψPHENIX inclusive J/ = 0.2 TeV NN s Au-Au = 0.2 TeV NN sCu-Cu = 0.2 TeVspp 〉 part N〈 1 10 2 10 AA r 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 <4y, 2.5< - µ + µ → ψALICE inclusive J/ = 2.76 TeV, global syst. = 4% NN sPb-Pb |<2.2y, 1.2<| - µ + µ → ψPHENIX inclusive J/ = 0.2 TeV, global syst. = 3% NN s Au-Au and Cu-Cu <1y, 0< - µ + µ → ψNA50 inclusive J/ = 0.017 TeV, global syst. = 3% NN s Pb-Pb Transport model calculations TM1 ALICE TM2 ALICE RHIC SPS Figure 6. Mean transverse momentum hptimeasured by ALICE [37,57] and PHENIX [21,55,58] as a function of the number of participant nucleons (left). rAA measured by NA50 [59], PHENIX and ALICE and compared to model calculations [13,60], as a function of the number of participant nucleons (right). – 20 – JHEP05(2016)179 figure 6(right side). A very different hNpartidependence is seen, especially when comparing Pb–Pb collisions at the SPS and the LHC. At the SPS energy of √sNN = 0.017 TeV [59], the increase of the J/ψ hp2 tiwith the centrality of the collision was attributed to the Cronin effect [61], interpreted as an extra ptkick due to multiple scatterings of the initial partons producing the J/ψ. At the LHC, a clear decrease of rAA is observed as a function of hNparti. This behavior could be related to the onset of recombination phenomena and to the thermalization of charm quarks. Theoretical calculations [13,60], based on transport models (described in the next section) are able to reproduce the rAA at SPS, RHIC and LHC energies. They correlate the specific dependence of rAA on collision centrality with the increased importance of recombination effects in the J/ψ production mechanism at the LHC. 9 Nuclear modification factor Some of the RAA results presented here were already published in [27] and are shown again in this section, where they are compared with model calculations and with results from previous experiments. They include the centrality dependence of RAA (figure 7), the pt dependence of RAA for the full centrality range 0–90% and for the centrality class 0–20% (figure 9top row) and the rapidity dependence of RAA (figure 10). The new results shown in this section include the centrality dependence of RAA for three ptintervals (figure 8) and the ptdependence of RAA for the centrality classes 20–40% and 40–90% (figure 9 bottom row). These new results were obtained using a slightly different trigger selection, as explained in section 3. The consistency of the results obtained with the two selections was verified. 9.1 Centrality dependence of RAA Our measurement of the inclusive J/ψ RAA at √sNN = 2.76 TeV in the range 2.5< y < 4 and pt<8 GeV/c is shown in figure 7as a function of hNparti. Statistical (uncorrelated systematic) uncertainties are represented by vertical error bars (open boxes). A global correlated systematic uncertainty affecting all the values by the same amount is quoted in the legend. The same convention is applied in the following figures, unless otherwise specified. The J/ψ RAA in the centrality class 0–90% (corresponding to hNparti ∼ 124, see table 1) is R0–90% AA = 0.58 ±0.01(stat.)±0.09(syst.), indicating a clear J/ψ suppression. This suppression is significantly less pronounced than that observed at lower energy in PHENIX in a similar kinematic range, as previously discussed in [26,27]. For hNparti larger than 70, corresponding to the 50% most central Pb–Pb collisions, the J/ψ RAA is consistent with being constant, within uncertainties. Such behavior was not observed in heavy ion collisions at lower energies (SPS, RHIC), where RAA is continuously decreasing as a function of centrality. The impact of non-prompt J/ψ on the inclusive RAA analysis was studied. The RAA of prompt J/ψ is estimated (see eq. (4.3)) to be about 7% larger than the inclusive J/ψ RAA if the beauty component is fully suppressed. In the other extreme case, where the B-meson production is not affected by the medium and scales with the number of binary – 21 – JHEP05(2016)179 〉 part N〈 0 50 100 150 200 250 300 350 AA R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 =0.2 TeV NN s=2.76 TeV, Au-Au NN s, Pb-Pb - µ + µ → ψInclusive J/ 15%± global syst.= c<8 GeV/ T p<4, yALICE, 2.5< 9.2%± global syst.= c>0 GeV/ T p|<2.2, yPHENIX, 1.2<| 〉 part N〈 0 50 100 150 200 250 300 350 AA R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 = 2.76 TeV NN s, Pb-Pb - µ + µ → ψInclusive J/ 15%± global syst.= c<8 GeV/ T p<4, yALICE, 2.5< SHM TM1 TM2 CIM Figure 7. Inclusive J/ψ RAA as a function of the number of participant nucleons measured in Pb–Pb collisions at √sNN = 2.76 TeV [27], compared to the PHENIX measurement in Au–Au collisions at √sNN = 0.2 TeV [21] (left) and to theoretical models [13,60,62,63], which all include a J/ψ regeneration component (right). The brackets shown in the three most peripheral centrality classes on the right figure quantify the possible range of variation of the hadronic J/ψ RAA for two extreme hypotheses on the photo-production contamination in the inclusive measurement, see text for details. collisions, i.e. Rnon-prompt AA = 1, the RAA of prompt J/ψ would be about 6% smaller in central collisions and about 1% smaller in peripheral collisions. The excess in the inclusive J/ψ yield observed at very low pt[40] also influences the RAA in the most peripheral collisions. A large fraction of this contribution (about 75% as explained in section 4) can be removed by selecting J/ψ with a pthigher than 0.3 GeV/c. Assuming that the hadronic J/ψ RAA in the ranges 0 < pt<0.3 GeV/c and 0.3< pt<8 GeV/c are the same, it becomes possible to estimate the impact of the J/ψ photo-production on the inclusive RAA. In the centrality classes 60–70%, 70–80% and 80–90%, the hadronic J/ψ RAA would be about 5%, 11% and 25% lower, respectively. Extreme hypotheses were made to define upper and lower limits, represented with brackets on the figures 7,8and 9. The upper limit calculation assumes no J/ψ from photo-production thus the inclusive measurement only contains hadronic production. The lower limit assumes that i) all J/ψ produced with aptsmaller than 0.3 GeV/c originate from photo-production and ii) the efficiency of the 0.3 GeV/c ptselection is reduced from 75% to 60% (corresponding to an increase by a factor two of the J/ψ photo-production above 0.3 GeV/c). The comparison with theoretical models, shown on the right-hand side of figure 7, helps in the interpretation of the large difference observed between the PHENIX and the ALICE results. The Statistical Hadronization Model (SHM) [62] assumes deconfinement and thermal equilibration of the bulk of the c¯c pairs. Charmonium production occurs at the phase boundary via the statistical hadronization of charm quarks. The prediction is given for two values of the charm cross section dσc¯c/dy= 0.15 and 0.25 mb at forward rapidity. These values are derived from the measured charm cross section in pp collisions at √s= 2.76 and 7 TeV [15] bracketing the expectation for gluon shadowing in the Pb-nucleus between 0.6 and 1.0. Production of non-prompt J/ψ from decays of B-mesons is not considered. – 22 – JHEP05(2016)179 The two transport models from Zhao (TM1) [13] and Zhou (TM2) [60] mainly differ in the rate equation controlling the J/ψ dissociation and regeneration. In TM1, shadowing is implemented via a simple parametrization, leading to a 30% suppression in the most central Pb–Pb collisions. The charm cross section is assumed to be dσc¯c/dy≈0.5 mb at forward rapidity, the fraction of J/ψ from beauty hadrons to be 10% and no b-quenching is introduced in the calculation. This model is presented as a band connecting the results obtained with (lower limit) and without (upper limit) shadowing and is interpreted by the authors as the uncertainty of the prediction. In TM2, the shadowing is given by the EKS98 parametrization [64]. The charm cross section is taken in the range dσc¯c/dy≈0.4–0.5 mb at forward rapidity; the calculations for these two values provide the lower and upper limits of the band displayed in the figure. The fraction of J/ψ from beauty hadrons is assumed to be 10% with a b-quenching of 0.8, increased to 0.4 for ptabove 5 GeV/c. The Comover Interaction Model (CIM) [63] implements shadowing, interaction with a co-moving dense partonic medium and recombination effects. The shadowing is calculated within the Glauber-Gribov theory making use of the generalized Schwimmer model of multiple scattering. The J/ψ dissociation cross section due to comover interaction is taken as σco = 0.65 mb from low-energy data. Recombination effects are included by adding a gain term proportional to σco and to the number of c and ¯c quarks, thus no additional parameter is added to the model. The charm cross section dσc¯c/dyat forward rapidity is taken in the range 0.4 to 0.6 mb, which gives respectively the lower and upper limits of the calculation. Production of non-prompt J/ψ is not considered. To match our J/ψ RAA results, all models above need to include in their calculation a sizeable J/ψ production from deconfined c and ¯c quarks. A different test of these models was carried out by studying the J/ψ RAA centrality dependence in ptintervals. Figure 8displays the measurement of the inclusive J/ψ RAA as a function of the number of participant nucleons measured in Pb–Pb collisions at √sNN = 2.76 TeV for the three ptranges 0–2, 2–5 and 5–8 GeV/c. The uncorrelated systematic uncertainties shown at each point were separated into uncorrelated as a function of centrality (open boxes) and fully correlated as a function of centrality but uncorrelated as a function of pt(shaded areas). For hNparti&150, the low ptJ/ψ RAA is significantly larger than the mid and high ptones. In the most central bin, the RAA values corresponding to the lowest and the highest ptare separated by 3.9σ. For hNparti.150, the centrality dependence exhibits similar trends for the 2–5 and 5–8 GeV/c ranges, while the most peripheral (hNparti ∼ 20) RAA measurement in the low pt(0–2 GeV/c) range appears to deviate from the others. However, the J/ψ yield excess observed at very low ptmay have a sizable effect in the 0–2 GeV/c interval. In the centrality classes 40–50%, 50–60% and 60–90%, based on the same assumptions made for the 0 < pt<8 GeV/c case, the hadronic J/ψ RAA would be about 5%, 6% and 18% lower, respectively. Due to the increase of the non-prompt J/ψ component at large pt, the difference between the measured inclusive J/ψ RAA and the prompt J/ψ RAA increases with pt. If the beauty contribution is fully (not) suppressed, RAA of prompt J/ψ is estimated to be 6%, 8% and 11% larger (0–3%, 3–10% and 7–30% smaller, depending on centrality) for the ptranges 0–2, 2–5 and 5–8 GeV/c, respectively. – 23 – JHEP05(2016)179 〉 part N〈 0 50 100 150 200 250 300 350 AA R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 = 2.76 TeV NN s<4, ALICE Pb-Pb y, 2.5< - µ + µ → ψInclusive J/ c < 2 GeV/ T p c < 5 GeV/ T p2 < c < 8 GeV/ T p5 < 7%±global syst.= 〉 part N〈 0 50 100 150 200 250 300 350 AA R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 = 2.76 TeV NN s<4, ALICE Pb-Pb y, 2.5< - µ + µ → ψInclusive J/ c < 2 GeV/ T p c < 8 GeV/ T p5 < 7%±global syst.= CIM c < 2 GeV/ T pc < 8 GeV/ T p5 < 〉 part N〈 0 50 100 150 200 250 300 350 AA R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 = 2.76 TeV NN s<4, ALICE Pb-Pb y, 2.5< - µ + µ → ψInclusive J/ c < 2 GeV/ T p c < 8 GeV/ T p5 < 7%±global syst.= TM1 c < 2 GeV/ T pc < 8 GeV/ T p5 < 〉 part N〈 0 50 100 150 200 250 300 350 AA R 0 0.2 0.4 0.6 0.8 1 1.2 1.4 = 2.76 TeV NN s<4, ALICE Pb-Pb y, 2.5< - µ + µ → ψInclusive J/ c < 2 GeV/ T p c < 8 GeV/ T p5 < 7%±global syst.= TM2 c < 2 GeV/ T p c < 8 GeV/ T p5 < Figure 8. Inclusive J/ψ RAA as a function of the number of participant nucleons measured in Pb–Pb collisions at √sNN = 2.76 TeV for three ptranges (0–2, 2–5 and 5–8 GeV/c) and comparisons of the lowest and highest ptrange to the transport and to the comover interaction models [13,60,63]. The brackets quantify the possible range of variation of the hadronic J/ψ RAA for two extreme hypotheses on the photo-production contamination in the inclusive measurement. Calculations from the transport models and the comover interaction model are plotted on top of the results shown in figure 8. For the most peripheral collisions (hNparti.100), the models cannot correctly reproduce the RAA centrality dependence for both the low and high ptranges. For the most central collisions (hNparti&100), the RAA centrality dependence for high ptJ/ψ is reasonably reproduced by all models. Concerning the low ptrange in the most central events, the measurement is compatible with the upper side of the theoretical uncertainty band from the CIM and TM2 models. For these models, it corresponds to the highest value for dσc¯c/dy, 0.6 and 0.5 mb respectively. 9.2 Transverse momentum dependence of RAA The ptdependence of the inclusive J/ψ RAA in the rapidity range 2.5< y < 4 is shown in figure 9for the full centrality range 0–90% and for three centrality classes 0–20% [27], 20–40% and 40–90%. In figure 9top left corner, the inclusive J/ψ RAA in the centrality class 0–90% shows a decrease of about 50% from low to high pt. At low pt, the measurement is close to 0.8 showing very little suppression. At high pt, our RAA value is similar to that of CMS [25]. They measured, in the different rapidity range 1.6<|y|<2.4, an inclusive – 24 – JHEP05(2016)179 nology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ and CEA, France; German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian Orszagos Tudomanyos Kutatasi Alappgrammok (OTKA) and National Office for Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi — Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion General de Asuntos del Personal Academico (DGAPA), M´exico, Amerique Latine Formation academique — European Commission (ALFA-EC) and the EPLANET Program (European Particle Physics Latin American Network); Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); National Science Centre, Poland; Ministry of National Education/Institute for Atomic Physics and Consiliul Nat¸ional al Cercet˘arii S¸tiint¸ifice — Executive Agency for Higher Education Research Development and Innovation Funding (CNCS-UEFISCDI) — Romania; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure shared between Europe and Latin America (EELA), Ministerio de Econom´ıa y Competitividad (MINECO) of Spain, Xunta de Galicia (Conseller´ıa de Educaci´on), Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Cubaenerg´ıa, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio; Ministry of Science, Education and Sports of Croatia and Unity through Knowledge Fund, Croatia; Council of Scientific and Industrial Research (CSIR), New Delhi, India. – 31 – JHEP05(2016)179 A Data tables This appendix provides all the numerical values obtained in this analysis. The inclusive J/ψ differential ptyields in Pb–Pb in centrality classes are given in table 4. Tables 5to 8present the inclusive J/ψ RAA and associated Pb–Pb yields as a function of centrality for 2.5< y < 4.0 and four ptranges, pt<8 GeV/c,pt≤2 GeV/c, 2< pt<5 GeV/c and 5 < pt<8 GeV/c. Tables 9to 13 show the ptdependence of the inclusive J/ψ RAA and associated Pb–Pb yields for the centrality classes 0–20%, 20–40%, 0–40%, 40–90% and 0–90%. Table 14 shows the ydependence of the inclusive J/ψ RAA and associated Pb–Pb yields for the centrality class 0–90% in the ptrange pt<8 GeV/c. Then, the inclusive J/ψ RAA results with a low ptcut at 0.3 GeV/c are presented. The reference pp cross section needed to build the RAA was extracted with the method described in [40]. The inclusive J/ψ RAA centrality dependence for 2.5< y < 4 in the ptranges 0.3< pt<8 GeV/c and 0.3< pt<2 GeV/c is shown in table 15. The inclusive J/ψ RAA in the ptrange 0.3< pt<1 GeV/c for 2.5< y < 4 in four centrality classes 0–90%, 0–20%, 20–40% and 40–90% is given in table 16. Finally, table 17 presents the inclusive [ψ(2S)/J/ψ]Pb–Pb and [ψ(2S)/J/ψ]Pb–Pb /[ψ(2S)/J/ψ]pp ratios as a function of centrality for the ptintervals pt<3 GeV/c and 3 < pt<8 GeV/c. d2YJ/ψ/dydpt( GeV/c)−1×103 pt( GeV/c) 0–20% 20–40% 40–90% 0.0–0.5 3.253 ±0.386 ±0.446 1.366 ±0.081 ±0.165 0.257 ±0.017 ±0.031 0.5–1.0 8.012 ±0.487 ±1.087 2.571 ±0.199 ±0.310 0.346 ±0.024 ±0.042 1.0–1.5 9.909 ±0.603 ±1.149 3.494 ±0.255 ±0.388 0.533 ±0.030 ±0.061 1.5–2.0 8.193 ±0.505 ±0.907 2.907 ±0.194 ±0.320 0.493 ±0.037 ±0.053 2.0–2.5 6.342 ±0.401 ±0.701 2.371 ±0.164 ±0.260 0.441 ±0.034 ±0.049 2.5–3.0 4.759 ±0.316 ±0.542 1.997 ±0.134 ±0.227 0.270 ±0.020 ±0.029 3.0–3.5 2.735 ±0.183 ±0.290 1.313 ±0.087 ±0.151 0.222 ±0.016 ±0.023 3.5–4.0 1.876 ±0.134 ±0.201 0.874 ±0.068 ±0.092 0.174 ±0.013 ±0.018 4.0–4.5 1.075 ±0.098 ±0.109 0.483 ±0.037 ±0.048 0.108 ±0.009 ±0.011 4.5–5.0 0.731 ±0.069 ±0.073 0.339 ±0.030 ±0.033 0.076 ±0.007 ±0.007 5.0–5.5 0.453 ±0.047 ±0.045 0.263 ±0.023 ±0.026 0.042 ±0.005 ±0.004 5.5–6.0 0.345 ±0.039 ±0.046 0.132 ±0.016 ±0.014 0.028 ±0.004 ±0.003 6.0–8.0 0.099 ±0.009 ±0.010 0.068 ±0.005 ±0.007 0.012 ±0.001 ±0.001 Table 4. Inclusive J/ψ yields (as defined by eq. (4.1)) in ptintervals for the 0–20%, 20–40% and 40–90% most central Pb–Pb collisions. The rapidity range is 2.5< y < 4. Statistical and systematic uncertainties are also reported as d2YJ/ψ/dydpt±statistical uncertainty±systematic uncertainty. A global systematic uncertainty of 4% affects all the values. A 2%, 1% and 2% systematic uncertainty, independent of pt, affects the centrality classes 0–20%, 20–40% and 40–90%, respectively. – 32 – JHEP05(2016)179 Centrality RAA ±(stat.)±(syst.) [27]YJ/ψ ±(stat.)±(syst.)×103 0–10% 0.557 ±0.019 ±0.024 43.095 ±1.454 ±1.049 10–20% 0.573 ±0.020 ±0.022 27.212 ±0.979 ±0.501 20–30% 0.598 ±0.022 ±0.020 17.409 ±0.638 ±0.188 30–40% 0.577 ±0.024 ±0.025 9.671 ±0.406 ±0.211 40–50% 0.609 ±0.028 ±0.030 5.413 ±0.247 ±0.041 50–60% 0.725 ±0.036 ±0.043 3.246 ±0.160 ±0.050 60–70% 0.839 ±0.041 ±0.058 1.677 ±0.083 ±0.024 70–80% 0.849 ±0.063 ±0.068 0.701 ±0.051 ±0.014 80–90% 1.094 ±0.106 ±0.104 0.362 ±0.033 ±0.008 Table 5. Inclusive J/ψ RAA and Pb–Pb yields as a function of centrality, for pt<8 GeV/c and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 15% (12%) affects all the RAA (yields) values. Centrality RAA ±(stat.)±(syst.)YJ/ψ ±(stat.)±(syst.)×103 0–10% 0.732 ±0.034 ±0.041 27.932 ±1.302 ±1.282 10–20% 0.733 ±0.035 ±0.028 17.159 ±0.824 ±0.383 20–30% 0.715 ±0.038 ±0.024 10.113 ±0.541 ±0.115 30–40% 0.678 ±0.040 ±0.033 5.516 ±0.322 ±0.182 40–50% 0.641 ±0.044 ±0.032 2.789 ±0.190 ±0.064 50–60% 0.839 ±0.048 ±0.056 1.799 ±0.103 ±0.070 60–90% 1.104 ±0.064 ±0.078 0.559 ±0.032 ±0.016 Table 6. Inclusive J/ψ RAA and Pb–Pb yields as a function of centrality, for pt<2 GeV/c and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 15% (12%) affects all the RAA (yields) values. Centrality RAA ±(stat.)±(syst.)YJ/ψ ±(stat.)±(syst.)×103 0–10% 0.425 ±0.019 ±0.017 15.540 ±0.681 ±0.379 10–20% 0.461 ±0.019 ±0.016 10.336 ±0.431 ±0.168 20–30% 0.529 ±0.022 ±0.018 7.164 ±0.293 ±0.106 30–40% 0.498 ±0.025 ±0.027 3.879 ±0.194 ±0.153 40–50% 0.595 ±0.030 ±0.029 2.481 ±0.126 ±0.049 50–60% 0.675 ±0.042 ±0.041 1.386 ±0.085 ±0.037 60–90% 0.722 ±0.044 ±0.050 0.350 ±0.021 ±0.009 Table 7. Inclusive J/ψ RAA and Pb–Pb yields as a function of centrality, for 2 < pt<5 GeV/c and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 14% (11%) affects all the RAA (yields) values. – 33 – JHEP05(2016)179 Centrality RAA ±(stat.)±(syst.)YJ/ψ ±(stat.)±(syst.)×103 0–10% 0.280 ±0.021 ±0.011 1.093 ±0.081 ±0.027 10–20% 0.282 ±0.027 ±0.011 0.677 ±0.064 ±0.016 20–30% 0.410 ±0.029 ±0.013 0.594 ±0.042 ±0.006 30–40% 0.540 ±0.039 ±0.024 0.449 ±0.033 ±0.012 40–50% 0.529 ±0.053 ±0.031 0.236 ±0.024 ±0.009 50–60% 0.587 ±0.073 ±0.036 0.129 ±0.016 ±0.004 60–90% 0.644 ±0.083 ±0.047 0.033 ±0.004 ±0.001 Table 8. Inclusive J/ψ RAA and Pb–Pb yields as a function of centrality, for 5 < pt<8 GeV/c and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 18% (10%) affects all the RAA (yields) values. pt( GeV/c)RAA ±(stat.)±(syst.) [27] d2YJ/ψ/dydpt±(stat.)±(syst.)( GeV/c)−1×103 0–1 0.803 ±0.084 ±0.113 5.771 ±0.345 ±0.748 1–2 0.690 ±0.052 ±0.084 9.134 ±0.411 ±0.987 2–3 0.505 ±0.042 ±0.062 5.539 ±0.284 ±0.604 3–4 0.381 ±0.037 ±0.046 2.305 ±0.116 ±0.247 4–5 0.355 ±0.052 ±0.041 0.905 ±0.068 ±0.090 5–6 0.282 ±0.048 ±0.032 0.388 ±0.030 ±0.038 6–8 0.279 ±0.064 ±0.032 0.100 ±0.009 ±0.010 Table 9. Inclusive J/ψ RAA and Pb–Pb yields as a function of ptfor the 0–20% centrality class and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 8% (4%) affects all the RAA (yields) values. pt( GeV/c)RAA ±(stat.)±(syst.) d2YJ/ψ/dydpt±(stat.)±(syst.)( GeV/c)−1×103 0–1 0.733 ±0.080 ±0.097 1.909 ±0.128 ±0.229 1–2 0.660 ±0.051 ±0.080 3.189 ±0.154 ±0.344 2–3 0.543 ±0.044 ±0.067 2.167 ±0.106 ±0.238 3–4 0.493 ±0.048 ±0.060 1.084 ±0.055 ±0.117 4–5 0.444 ±0.063 ±0.051 0.411 ±0.027 ±0.040 5–6 0.399 ±0.067 ±0.045 0.200 ±0.014 ±0.020 6–8 0.523 ±0.116 ±0.059 0.068 ±0.005 ±0.007 Table 10. Inclusive J/ψ RAA and Pb–Pb yields as a function of ptfor the 20–40% centrality class and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 8% (4%) affects all the RAA (yields) values. – 34 – JHEP05(2016)179 pt( GeV/c)RAA ±(stat.)±(syst.) d2YJ/ψ/dydpt±(stat.)±(syst.)( GeV/c)−1×103 0–1 0.767 ±0.074 ±0.105 3.754 ±0.163 ±0.472 1–2 0.672 ±0.046 ±0.082 6.103 ±0.212 ±0.662 2–3 0.515 ±0.038 ±0.064 3.865 ±0.134 ±0.428 3–4 0.411 ±0.038 ±0.049 1.698 ±0.063 ±0.178 4–5 0.376 ±0.051 ±0.043 0.655 ±0.033 ±0.064 5–6 0.315 ±0.050 ±0.036 0.296 ±0.016 ±0.029 6–8 0.340 ±0.075 ±0.038 0.083 ±0.005 ±0.008 Table 11. Inclusive J/ψ RAA and Pb–Pb yields as a function of ptfor the 0–40% centrality class and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 8% (4%) affects all the RAA (yields) values. pt( GeV/c)RAA ±(stat.)±(syst.) d2YJ/ψ/dydpt±(stat.)±(syst.)( GeV/c)−1×103 0–1 0.815 ±0.081 ±0.107 0.305 ±0.015 ±0.036 1–2 0.732 ±0.059 ±0.090 0.508 ±0.028 ±0.055 2–3 0.617 ±0.053 ±0.076 0.354 ±0.020 ±0.038 3–4 0.627 ±0.062 ±0.074 0.198 ±0.010 ±0.020 4–5 0.693 ±0.097 ±0.079 0.092 ±0.006 ±0.009 5–6 0.489 ±0.087 ±0.055 0.035 ±0.003 ±0.003 6–8 0.646 ±0.150 ±0.072 0.012 ±0.001 ±0.001 Table 12. Inclusive J/ψ RAA and Pb–Pb yields as a function of ptfor the 40–90% centrality class and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 9% (4%) affects all the RAA (yields) values. pt( GeV/c)RAA ±(stat.)±(syst.) [27] d2YJ/ψ/dydpt±(stat.)±(syst.)( GeV/c)−1×103 0–1 0.779 ±0.076 ±0.106 1.857 ±0.081 ±0.230 1–2 0.677 ±0.047 ±0.083 2.993 ±0.104 ±0.323 2–3 0.519 ±0.038 ±0.064 1.896 ±0.064 ±0.206 3–4 0.425 ±0.039 ±0.051 0.855 ±0.029 ±0.089 4–5 0.405 ±0.054 ±0.047 0.343 ±0.015 ±0.033 5–6 0.322 ±0.052 ±0.036 0.147 ±0.007 ±0.015 6–8 0.364 ±0.079 ±0.041 0.043 ±0.002 ±0.004 Table 13. Inclusive J/ψ RAA and Pb–Pb yields as a function of ptfor the 0–90% centrality class and 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 8% (4%) affects all the RAA (yields) values. – 35 – JHEP05(2016)179 y RAA ±(stat.)±(syst.) [27] d2YJ/ψ/dydpt±(stat.)±(syst.)( GeV/c)−1×103 2.50–2.75 0.631 ±0.087 ±0.088 1.509 ±0.114 ±0.191 2.75–3.00 0.747 ±0.068 ±0.097 1.387 ±0.058 ±0.162 3.00–3.25 0.632 ±0.048 ±0.094 1.120 ±0.039 ±0.154 3.25–3.50 0.566 ±0.044 ±0.088 0.891 ±0.032 ±0.130 3.50–3.75 0.467 ±0.041 ±0.070 0.733 ±0.025 ±0.101 3.75–4.00 0.395 ±0.050 ±0.050 0.528 ±0.029 ±0.058 Table 14. Inclusive J/ψ RAA and Pb–Pb yields as a function of yfor the 0–90% centrality class and pt<8 GeV/c. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 8% (4%) affects all the RAA (yields) values. RAA ±(stat.)±(syst.) Centrality 0.3< pt<8 GeV/c 0.3< pt<2 GeV/c 0–10% 0.545 ±0.017 ±0.026 0.745 ±0.041 ±0.042 10–20% 0.560 ±0.018 ±0.021 0.736 ±0.036 ±0.028 20–30% 0.594 ±0.020 ±0.020 0.716 ±0.038 ±0.025 30–40% 0.570 ±0.021 ±0.025 0.671 ±0.040 ±0.032 40–50% 0.592 ±0.025 ±0.029 0.619 ±0.045 ±0.032 50–60% 0.715 ±0.033 ±0.044 0.801 ±0.049 ±0.054 60–70% 0.805 ±0.043 ±0.057 )0.959 ±0.057 ±0.067 70–80% 0.778 ±0.062 ±0.064 80–90% 0.887 ±0.097 ±0.088 Table 15. Inclusive J/ψ RAA as a function of centrality, for 0.3< pt<8 GeV/c and 0.3< pt< 2 GeV/c in the rapidity range 2.5< y < 4.0. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 15% affects all the RAA values. Centrality RAA ±(stat.)±(syst.) for 0.3< pt<1 GeV/c 0–90% 0.775 ±0.057 ±0.113 0–20% 0.803 ±0.066 ±0.123 20–40% 0.733 ±0.067 ±0.103 40–90% 0.688 ±0.057 ±0.098 Table 16. Inclusive J/ψ RAA for 2.5< y < 4.0 in the centrality classes 0–90%, 0–20%, 20–40% and 40–90% for the lowest ptrange when the 0.3 GeV/c ptcut is applied. Statistical and systematic uncertainties are also reported. A global systematic uncertainty of 8%, 8%, 8% and 9% affect the RAA values, respectively. – 36 – JHEP05(2016)179 pt( GeV/c) Centrality [ψ(2S)/J/ψ]Pb–Pb [ψ(2S)/J/ψ]Pb–Pb /[ψ(2S)/J/ψ]pp 0–3 0–20% <0.012 (95% CL) <0.65 (95% CL) 0–3 20–40% 0.017 ±0.010 ±0.004 0.86 ±0.51 ±0.23 0–3 40–60% 0.013 ±0.012 ±0.006 0.65 ±0.65 ±0.30 0–3 60–90% 0.029 ±0.012 ±0.004 1.49 ±0.62 ±0.27 3–8 0–20% <0.046 (95% CL) <1.71 (95% CL) 3–8 20–60% <0.033 (95% CL) <1.24 (95% CL) Table 17. Inclusive [ψ(2S)/J/ψ]Pb–Pb and [ψ(2S)/J/ψ]Pb–Pb /[ψ(2S)/J/ψ]pp ratios as a function of centrality for two ptintervals. Statistical and systematic uncertainties are reported when the value is not given as an upper limit. Open Access. 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Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia 132 Variable Energy Cyclotron Centre, Kolkata, India 133 Vinˇca Institute of Nuclear Sciences, Belgrade, Serbia 134 Warsaw University of Technology, Warsaw, Poland 135 Wayne State University, Detroit, Michigan, United States 136 Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary 137 Yale University, New Haven, Connecticut, United States 138 Yonsei University, Seoul, South Korea 139 Zentrum f¨ur Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany – 48 –