Inclusive, prompt and non-prompt J/ψ production at mid-rapidity in Pb-Pb collisions at √sNN =2.76 TeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Inclusive, prompt and non-prompt J/ψ production at mid-rapidity in Pb-Pb collisions at √sNN =2.76 TeV ALICE Collaboration ALICE Collaboration. (2015). Inclusive, prompt and non-prompt J/ψ production at mid-rapidity in Pb-Pb collisions at √sNN =2.76 TeV. Journal of High Energy Physics, 2015(7), Article 51. https://doi.org/10.1007/JHEP07(2015)051 2015 JHEP07(2015)051 Published for SISSA by Springer Received:April 28, 2015 Accepted:June 17, 2015 Published:July 10, 2015 Inclusive, prompt and non-prompt J/ψ production at mid-rapidity in Pb-Pb collisions at √sNN = 2.76 TeV The ALICE collaboration E-mail: [email protected] Abstract: The transverse momentum (pT) dependence of the nuclear modification factor RAA and the centrality dependence of the average transverse momentum hpTifor inclusive J/ψ have been measured with ALICE for Pb-Pb collisions at √sNN = 2.76 TeV in the e+e−decay channel at mid-rapidity (|y|<0.8). The hpTiis significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of RAA is observed towards low pT. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction of non-prompt J/ψ from beauty hadron decays, fB, has been determined in the region 1.5< pT<10 GeV/c in three centrality intervals. No significant centrality dependence of fBis observed. Finally, the RAA of non-prompt J/ψ is discussed and compared with model predictions. The nuclear modification in the region 4.5< pT<10 GeV/c is found to be stronger than predicted by most models. Keywords: Hadron-Hadron Scattering ArXiv ePrint: 1504.07151 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP07(2015)051 JHEP07(2015)051 Contents 1 Introduction 1 2 Data analysis 2 2.1 Inclusive J/ψ 3 2.2 Non-prompt J/ψ 8 3 Results 13 4 Conclusions 19 The ALICE collaboration 26 1 Introduction Heavy-ion collisions at high energies allow the study of strongly interacting matter under extreme conditions. Calculations based on Quantum-Chromo-Dynamics (QCD) on the lattice indicate that the hot and dense medium created in these collisions behaves like a strongly coupled Quark-Gluon Plasma (QGP) [1–4]. Heavy quarks are an important probe for the properties of this state of matter, since they are produced via hard partonic collisions at a very early stage and thus experience the complete evolution of the system. Quarkonium states, i.e. bound states of a heavy quark and anti-quark such as the J/ψ meson (c¯c state) are of particular interest. It was predicted that the J/ψ formation is suppressed in a QGP due to the screening of the c¯c potential in the presence of free colour charges [5]. Experimentally, a suppression of the inclusive J/ψ yield in heavy-ion collisions relative to the corresponding yield in pp, scaled by the number of binary nucleon-nucleon collisions, has been observed at the Super Proton Synchrotron (SPS) [6–8] and the Relativistic Heavy Ion Collider (RHIC) [9,10]. The level of suppression was found to be similar at SPS and RHIC, despite the significantly different collision energy. More recently, the nuclear modification of J/ψ was also measured for Pb-Pb collisions at the LHC [11–13]. While at high transverse momentum (pT>4 GeV/c) the suppression factor is at the same level as the one observed at RHIC in the low pTregion, a significant reduction of the suppression is measured towards lower pT. This has been interpreted as the effect of an additional contribution to J/ψ production at low pT, due to the combination of correlated or uncorrelated c and ¯c quarks [14,15]. This contribution becomes sizable at LHC energies, since the number of c¯c pairs is much higher than at lower energies. Assuming that a deconfined phase is produced and that all the J/ψ are dissociated, this process happens at the chemical freezeout stage of the fireball evolution. This is the approach followed within the statistical hadronization models described in refs. [16,17]. Alternatively, J/ψ could be generated via coalescence throughout the full evolution of the QGP phase, if their survival probability in – 1 – JHEP07(2015)051 this environment is large enough. This scenario has been implemented in several partonic transport models [18,19]. It was found that both approaches can provide a description of the measured nuclear modification factors [12] and of the elliptic flow of inclusive J/ψ [20]. The production of open beauty hadrons is expected to be sensitive to the density of the medium created in heavy-ion collisions due to the energy loss experienced by the parent parton (a beauty quark) which hadronizes into the beauty hadron. This energy loss is expected to occur via medium-induced gluon radiation [21,22] and elastic collisional energy loss processes [23–25] and it depends on the QCD Casimir coupling factor of the parton (larger for gluons than for quarks) and on the parton mass [26–29]. Other mechanisms, such as in-medium hadron formation and dissociation, can be envisaged as particularly relevant for heavy-flavour hadrons due to their small formation times [30–32]. Inclusive J/ψ production is the sum of several contributions. In addition to the directly produced J/ψ, the decays of heavier charmonium states, such as the χcand ψ(2S), also contribute to the inclusive J/ψ yield. These two sources (direct and charmonium decays) are defined as prompt J/ψ, where the contribution from charmonium decays is about 35% as measured in pp collisions [33]. Since heavier charmonia are less strongly bound than the J/ψ they should be more easily dissolved in a deconfined medium [34]. The J/ψ suppression measured at the SPS is indeed compatible with the assumption that only the excited states are dissolved and not the directly produced J/ψ [6,7]. On top of the prompt J/ψ production, there is an additional non-prompt contribution to the inclusive J/ψ at high centre-of-mass energies, coming from the decay of beauty hadrons. Since these decays proceed via weak interactions, the resulting J/ψ will originate from a decay vertex that is displaced from the main interaction vertex. Their measurement provides a direct determination of the nuclear modification of beauty hadrons. By subtracting the non-prompt contribution from the inclusive J/ψ yield one can also provide an unbiased information on medium modification of prompt charmonia. The non-prompt J/ψ contribution at midrapidity has already been measured in pp collisions at √s= 7 TeV by ATLAS [35], CMS [36] and ALICE [37]. For Pb-Pb collisions at √sNN = 2.76 TeV CMS has also published prompt and non-prompt J/ψ production results at mid-rapidity for pT>6.5 GeV/c [13]. In this paper we present a differential measurement of the inclusive J/ψ production at mid-rapidity in Pb-Pb collisions at √sNN = 2.76 TeV. The pTdependence of the nuclear modification factor and the centrality dependence of the average transverse momentum of J/ψ have been obtained, extending the set of results presented in [12]. A measurement of the prompt and non-prompt contributions to the inclusive J/ψ production is also presented. The nuclear modification factor of non-prompt J/ψ is determined down to pT= 1.5 GeV/c and compared to model predictions. 2 Data analysis A detailed description of the ALICE detector can be found in [38]. For the analysis presented here the detectors of the central barrel have been used, in particular the Inner Tracking System (ITS) and the Time Projection Chamber (TPC). These detectors are located inside a large solenoidal magnet with a field strength of 0.5 T. They allow the – 2 – JHEP07(2015)051 measurement of J/ψ mesons via the dielectron decay channel in the central rapidity region down to zero pT. The ITS [39] consists of six layers of silicon detectors surrounding the beam pipe at radial positions between 3.9 cm and 43.0 cm. Its two innermost layers are composed of Silicon Pixel Detectors (SPD), which provide the spatial resolution to separate on a statistical basis the non-prompt J/ψ. The active volume of the TPC [40] covers the range along the beam direction −250 < z < 250 cm relative to the Interaction Point (IP) and extends in radial direction from 85 cm to 247 cm. It is the main tracking device in the central barrel and is in addition used for particle identification via the measurement of the specific ionization (dE/dx) in the detector gas. Triggering and event characterization is performed via forward detectors, the V0 [41] and two Zero Degree Calorimeters (ZDC) [42]. The V0 detectors consist of two scintillator arrays positioned at z=−90 cm and z= +340 cm and cover the pseudo-rapidity ranges −3.7≤η≤ −1.7 and 2.8≤η≤5.1. The ZDCs, each one consisting of two quartz fiber sampling calorimeters, are placed at a distance of 114 m relative to the IP in both directions along the beam axis and are used to detect spectator nucleons. The results presented in this article are based on data samples collected during the Pb-Pb data taking periods of the LHC in the years 2010 and 2011. In the case of the 2011 data sample the Minimum Bias (MB) Level-0 (L0) trigger condition was defined by the coincidence of signals in both V0 detectors along with a valid bunch crossing trigger. For the 2010 data sample, in addition, the detection of at least two hits in the ITS was required. Both MB trigger definitions lead to trigger efficiencies larger than 95% for inelastic Pb-Pb collisions. Electromagnetic interactions were rejected by the Level-1 (L1) trigger, which required a minimum energy deposition in the ZDC by spectator neutrons. The beam-induced background was further reduced during the offline analysis by selecting events according to the relative timing of signals in V0 and ZDC. The offline centrality selection is done using the sum of the two V0 signal amplitudes. By fitting the corresponding distribution with the results of Glauber model simulations, the average number of participants hNpartiand the average nuclear overlap function hTAAi=hNcolli/σinel NN for a given centrality class can be determined as described in [43]. Here, hNcolliis the average number of binary nucleon-nucleon collisions and σinel NN the inelastic nucleon-nucleon cross section. The numerical values for hNparti,hNcolli, and hTAAiare tabulated in [12]. The 2010 data sample consists of 1.5×107events, taken with the corresponding MB trigger. The 2011 event sample was enriched with central and semi-central Pb-Pb collisions by using thresholds on the V0 multiplicity at the L0 trigger. From the latter data set we analyzed 1.9×107central (0–10% of the centrality distribution) and 1.7×107semi-central (10–50%) events. The summed 2010 and 2011 data samples correspond to an integrated luminosity of Lint = 26.4±0.3(stat.)+2.1 −1.7(syst.) µb−1[12]. 2.1 Inclusive J/ψ J/ψ candidates are reconstructed by combining opposite-sign (OS) pairs of electron/positron candidates and calculating their invariant mass mee. These candidates are selected from tracks reconstructed in the ITS and the TPC by employing the set of quality criteria described in [12,44]. In order to reject the background from photon conversions – 3 – JHEP07(2015)051 in the detector material, tracks are required to have a hit in one of the SPD layers. In addition, at least 70 out of a maximum of 159 space points reconstructed in the TPC must be assigned to a given track, which also needs to fulfill a quality criterion of the track fit (χ2/ndf < 4). The tracks are required to be in the range |η|<0.8, where the tracking and particle identification performance of the TPC is optimal, and to have pT>0.85 GeV/c to improve the signal-to-background ratio in the J/ψ mass region. Electron candidates are selected by requiring that the dE/dxmeasurement in the TPC lies within a band [−1σ, +3σ] around the momentum-dependent parameterization of the expected signal, where σis the phase space dependent dE/dxresolution (details can be found in [45]). The selection is asymmetric in order to minimize the contribution from pions. To further suppress the hadron contamination, tracks that are compatible within ±4σwith the proton expectation are rejected. A side effect of this cut is that tracks below pT= 1 GeV/c are effectively removed. Measurement of the inclusive J/ψ yield. The J/ψ signal counts NJ/ψ are obtained from the number of entries in the background subtracted invariant mass distributions in the range 2.92 < mee <3.16 GeV/c2. The uncorrelated background is evaluated with a mixed event (ME) technique. In order to achieve a good description of the background only electrons and positrons from events with similar properties in terms of centrality, primary vertex position, and event plane angle are combined. The ME distributions are scaled to the same event (SE) distributions in the mass ranges 1.5< mee <2.5 GeV/c2 and 3.2< mee <4.2 GeV/c2, so that the J/ψ signal region is excluded. The normalization area contains the ψ(2S) signal, but its contribution is negligible and can therefore be safely ignored. Also, contributions from the tail of the J/ψ signal shape to this mass interval are below the percent level and will thus not significantly affect the normalization. Figure 1shows a comparison of the SE and ME invariant mass distributions for the 0–40% most central Pb-Pb collisions for electron-positron pairs at mid-rapidity (|y|<0.8) in two pTintervals: 0–2.5 GeV/c and 2.5–6 GeV/c. The agreement between the SE and ME distributions outside the signal region is very good and allows signal extraction with significances larger than eight. The J/ψ yield per MB event in a given pTinterval, YJ/ψ, is obtained as YJ/ψ(pT) = NJ/ψ(pT) BRee Nevts hA×i(pT).(2.1) Here BRee is the branching ratio for the decay J/ψ →e+e−,Nevts the number of events, and hA×ithe phase space dependent product of acceptance Aand reconstruction efficiency . The latter is calculated from Monte Carlo (MC) simulations as the ratio between the number of reconstructed and generated MC J/ψ, which are assumed to be unpolarized. In pp collisions at √s= 7 TeV the J/ψ polarization has been measured and was found to be compatible with zero at mid-rapidity (pT>10 GeV/c) and forward rapidity (pT>2 GeV/c) [46–48]. In heavy-ion collisions no measurement exists, but J/ψ mesons produced from the recombination of charm quarks in the medium are expected to be un- – 4 – JHEP07(2015)051 Figure 1. The invariant mass distributions of inclusive J/ψ at mid-rapidity (|y|<0.8) for Pb-Pb collisions (0–40% most central) at √sNN = 2.76 TeV. The left panels show the interval 0< pT<2.5 GeV/c and the right ones 2.5< pT<6 GeV/c. The upper panels display the opposite sign distributions together with the result of the mixed event procedure. In the lower panels the background subtracted distributions are shown and compared to the simulated line shape. Also, the signal-to-background ratio S/B and the significance of the signal are given. polarized.1The MC events used for the calculation of hA×iare constructed by adding to background events, generated with the HIJING model [49], J/ψ mesons decaying into e+e−pairs, whose phase space distribution is obtained from extrapolations of other measurements [50], taking into account shadowing effects as parameterized in EKS98 [51]. The dielectron decay is simulated with the EvtGen [52] package, using the PHOTOS model [53] to describe the influence of final state radiation. This choice, together with the simulation of bremsstrahlung in the detector material, is mandatory for a proper description of the low mass tail in the measured J/ψ mass distribution and ensures that the fraction of the signal outside of the mee integration window is properly accounted for in the correction hA×i. The propagation of the simulated particles is done by GEANT3 [54] and a full simulation of the detector response is performed. The same reconstruction procedure and cuts are applied to MC events and to real data. The quality of the simulation is illustrated by the good agreement of the background-subtracted invariant mass distributions with the 1The impact of the polarization on the acceptance was studied for extreme polarization scenarios in [44]. – 5 – JHEP07(2015)051 MC simulation of the J/ψ signal shape, after normalizing it to the same integral as the measured signal (see figure 1). The analysis has been performed in two slightly different centrality intervals (0–40% and 0–50%), where the larger one is used for the extraction of non-prompt J/ψ which requires a higher statistics than the inclusive measurement. Also, the pTintervals have been optimized for the different analyses. It was checked that the results for inclusive J/ψ obtained with the two centrality binnings are in good agreement. Determination of the pp reference for RAA.From the corrected J/ψ yield YJ/ψ(pT) the nuclear modification factor RAA(pT) is calculated as RAA(pT) = YJ/ψ(pT) hTAAiσpp J/ψ(pT).(2.2) Since no differential J/ψ measurement at mid-rapidity at low pTis available for pp collisions at √s= 2.76 TeV [55], the reference needed for the construction of RAA is based on an interpolation of the mid-rapidity measurements by PHENIX at √s= 0.2 TeV [56], CDF at √s= 1.96 TeV [57], and ALICE at √s= 7 TeV [55]. The interpolated pTdistribution is obtained by fitting the following parameterization to the available data sets [50] 1 dσ/dy d2σ dzTdy=czT (1 + a2z2 T)n.(2.3) Here, zTis defined as pT/hpTi,a= Γ(3/2) Γ(n−3/2)/Γ(n−1), and c= 2(n−1) a, where nis the only free fit parameter. The value for hpTi(calculated in the pTrange 0–10 GeV/c) at √s= 2.76 TeV, which is needed to translate this parameterization into dσ/dpT, is determined by interpolating between the existing hpTimeasurements for pp and p¯p collisions [55–57]. This interpolation is done using various functional forms for the √sdependence to determine the systematic uncertainty. For the absolute normalization of the parametrized spectrum, the same interpolated value dσ/dy= 4.25 ±0.28(stat.) ± 0.43(syst.) µb as in [12] is used. The main sources of systematic uncertainties for the pTdependent RAA of inclusive J/ψ are the signal reconstruction procedure, the MC input kinematics, the uncertainties on the interpolated pp reference and on the nuclear overlap function. The corresponding values are summarized in table 1. While the first two components are uncorrelated between the pTintervals (type II), the uncertainty due to the nuclear overlap function is fully correlated (type I). The pp reference on the other hand introduces both uncorrelated and correlated contributions. To determine the uncertainty related to the signal reconstruction, the normalization range of the ME background and the size and positions of the mee bins have been varied. All track and electron selection criteria, such as the electron inclusion cut and the SPD hit requirement, have been relaxed and/or tightened in order to test the stability of the result, as was performed in [12]. The value of the systematic uncertainty is determined as the standard deviation of the distribution of all results obtained with the listed variations. The evaluation of the uncertainties associated with the MC input kinematics is also described in [12], while the uncertainty of the pp reference is estimated – 6 – JHEP07(2015)051 Figure 2. The average transverse momentum hpTiof e+e−pairs, measured for the pTrange 0– 10 GeV/c, as a function of the invariant mass mee in centrality selected Pb-Pb collisions at √sNN = 2.76 TeV. The shown uncertainties are statistical only. The background hpTidistributions and the total fit results are also shown superimposed to the data points. from the differences between the cross-section values obtained with the fitting procedure based on eq. (2.3) and the measured values used for the fit at the various energies. Determination of hpTiand hp2 Ti.Since the collected Pb-Pb statistics would allow the extraction of the J/ψ yield in a few pTintervals only, the average transverse momentum hpTiis determined by a fit to the distribution of the hpTiof e+e−pairs as a function of mee. When building such a distribution, the individual e+e−pairs are weighted by the inverse of their acceptance times efficiency (A×)−1, assuming that they come from the decay of a J/ψ. The resulting hpTidistributions are fitted by the expression hpTimeas =1 S(mee) + B(mee)S(mee)hpTiJ/ψ +B(mee)hpTiBkg.(2.4) Both factors Sand Bdepend on mee and correspond to the distribution of the J/ψ signal and of the background. For Sthe same background subtracted signal distribution S(mee) is used as for the extraction of the yield (see lower panels of figure 1), while the background Bis generated from the ME sample, as B(mee) = cBBME(mee). The normalization factor is determined by fitting cBBME(mee) to the corresponding mee distribution of e+e−pairs in the regions 1.5< mee <2.5 GeV/c2and 3.2< mee <4.2 GeV/c2, thus excluding the signal region. For the sum S(mee)+B(mee) in the denominator of eq. (2.4), the measured OS pair mee distribution is used. The hpTiBkg, defined as the hpTiof the combinatorial background pairs, is also calculated from the ME sample. This analysis is performed in three different centrality intervals: 0–10%, 10–40%, and 40–90%. Figure 2shows the measured hpTiof the e+e−pairs in the pTrange 0–10 GeV/c together with the results of the fit procedure. In addition, with an equivalent method, the mean square transverse momentum hp2 Tiis also calculated for the same centrality intervals. The systematic uncertainties of the hpTimeasurement for inclusive J/ψ are mainly determined by the signal extraction, the stability of track and electron selection criteria and the fit procedure (see table 2). While the first two components are not correlated between the different centrality intervals (type II), the systematic uncertainty intrinsic to the fit procedure can affect the data points in a correlated way (type I). The uncertainties – 7 – JHEP07(2015)051 Figure 4. The average transverse momentum hpTiof inclusive J/ψ measured at mid-rapidity (|y|<0.8) in centrality selected Pb-Pb collisions (filled circles) and pp collisions (open circles) at √sNN = 2.76 TeV as a function of the number of participants hNparti. The uncorrelated systematic uncertainties (type II) are depicted by the open boxes. Left panel: a comparison to results obtained by the PHENIX collaboration for Au-Au and Cu-Cu collisions at √sNN = 0.2 TeV [9,72] (open and filled diamonds) and by the NA50 collaboration for Pb-Pb collisions at √sNN = 17.3 GeV [73] (crosses). The hpTivalues are calculated for NA50 and PHENIX in the pTinterval 0–5 GeV/c, while for ALICE the pTinterval is 0–10 GeV/c. Right panel: hpTiis compared to theory predictions by Zhou et al. [74] and Zhao et al. [75,76] for the pTinterval 0–10 GeV/c. Centrality hpTi(GeV/c)hp2 Ti(GeV 2/c2) 0–10% 2.23 ±0.10 ±0.08 5.50 ±0.58 ±0.25 10–40% 2.01 ±0.12 ±0.08 4.97 ±0.65 ±0.34 40–90% 2.02 ±0.19 ±0.29 5.15 ±1.05 ±1.23 pp 2.54 ±0.02 ±0.01 9.07 ±0.15 ±0.07 Table 4. The numerical values of hpTiand hp2 Ticalculated in the range 0 < pT<10 GeV/c for the three analyzed centrality intervals in Pb-Pb collisions (the first uncertainty is the statistical and the second is the uncorrelated systematic (type II), the correlated uncertainty has a value of 2%, see table 2). The values for pp collisions obtained by the interpolation procedure are given as a reference. J/ψ spectral functions, throughout the evolution of a thermally expanding fireball. It also incorporates nuclear shadowing by reducing the input charm cross section by a factor of up to 1/3, with a centrality dependence as estimated in [78]. There is a fair agreement between our hpTiresults and the model calculation, while the rAA is not described by this prediction. Our hpTiand rAA results are also compared with the calculations by Zhou et al. [74]. These calculations are also based on a transport approach and incorporate dissociation and regeneration of J/ψ and heavier charmonia, as well as nuclear shadowing – 14 – JHEP07(2015)051 Figure 5. The ratio rAA =hp2 TiAA/hp2 Tipp in the pTinterval 0–10 GeV/c for inclusive J/ψ measured at mid-rapidity (|y|<0.8) in centrality selected Pb-Pb collisions (filled circles) at √sNN = 2.76 TeV as a function of the number of participants hNparti. The uncorrelated systematic uncertainties (type II) are depicted by the open boxes, while correlated uncertainty (type I) is shown as the filled box at unity. Left panel: a comparison to results obtained by the PHENIX collaboration for Au-Au and Cu-Cu collisions at √sNN = 0.2 TeV [9,72] (filled diamonds) and by the NA50 collaboration for Pb-Pb collisions at √sNN = 17.3 GeV [73] (crosses). The PHENIX and NA50 rAA values are calculated in the pTinterval 0–5 GeV/c. Right panel: rAA is compared to theory predictions by Zhou et al. [74] and Zhao et al. [75,76] for the pTinterval 0–10 GeV/c. according to EKS98 [51]. While the most central data point is matched by the prediction, it does not describe the evolution of rAA towards peripheral collisions. It must be noted that our results from Pb-Pb collisions at forward rapidity [79] exhibit a continuous decrease of hpTiand rAA from peripheral towards central events and are thus closer to the theory predictions, while the behaviour of mid-rapidity Pb-Pb results is more compatible with a flat hNpartidependence. The RAA of inclusive J/ψ in three pTintervals is shown in figure 6along with the results by the CMS collaboration for the interval 6.5< pT<30 GeV/c [13], both in 0–40% most central Pb-Pb collisions. The corresponding numerical values are 0.82 ±0.11(stat.) ± 0.10(syst.) for the interval 0 < pT<2.5 GeV/c and 0.58 ±0.06(stat.) ±0.08(syst.) for 2.5< pT<6 GeV/c, where the systematic uncertainties quoted here are the uncorrelated (type II) ones, as listed in table 1. The data point for 4.5< pT<10 GeV/c corresponds to the RAA value given in table 5(centrality range 0–50%). Table 5also contains the RAA values for prompt J/ψ, which are numerically identical to the ones for inclusive J/ψ. The inclusive RAA values below pT= 6 GeV/c are significantly higher than those measured at higher pT, corresponding to a decrease of RAA with increasing pT, while the high pTdata point is close to the CMS measurement. This pTdependence is similar to the one observed at forward rapidity [12], and is in clear contrast to the pTdependence measured at lower – 15 – JHEP07(2015)051 Figure 6. The nuclear modification factor RAA of inclusive J/ψ, measured at mid-rapidity (|y|<0.8) in Pb-Pb collisions (0–40% most central) at √sNN = 2.76 TeV, as a function of transverse momentum pT. The filled symbols are placed at the measured pTfor the given interval. Since for the data point in 4.5< pT<10 GeV/c (open symbol, 0–50% most central) hpTiis not available due to the limited statistics, it is plotted at the centre of the pTinterval. The uncorrelated systematic uncertainties (type II) are depicted by the open boxes, while the correlated uncertainties (type I) are shown as the filled boxes at unity. The data are compared to corresponding results by PHENIX for Au-Au collisions (0–40% most central) at √sNN = 0.2 TeV [9], by CMS for Pb-Pb collisions (0–40% most central) at √sNN = 2.76 TeV [13], and to predictions by the model of Zhou et al. [74] and Zhao et al. [75,76]. centre-of-mass energies by the PHENIX collaboration for √sNN = 0.2 TeV [9]. Figure 6 also shows the model predictions by Zhou et al. [74]. The value of the predicted RAA is systematically below the measurement and exhibits a pTdependence similar to the one in the data. The prediction by Zhao et al. [75,76] is close to our result. In both models, the rise of RAA towards pT= 0 is due to the dominant contribution from J/ψ regeneration via coalescence. The fraction of non-prompt J/ψ in the pTrange 1.5–10 GeV/c is shown as a function of the number of participants for the centrality intervals 40–90% (hNparti= 38), 10–40% (hNparti= 192), and 0–10% (hNparti= 356) in the left panel of figure 7. Within uncertainties, no centrality dependence is observed. The pTdependence of fB(centrality: 0–50%) is shown in the right panel of figure 7and compared with the measurements by CMS in the centrality interval 0–100% and pT>6.5 GeV/c (for the numerical values see table 5). Our results at low transverse momenta extend the CMS measurements in Pb-Pb collisions towards lower pT. Also shown are results at mid-rapidity in pp at √s= 7 TeV (ALICE [37], ATLAS [35] and CMS [70]) and in p¯p collisions at √s= 1.96 TeV (CDF [57]). Considering the ALICE and CMS results in Pb-Pb collisions together, a similar pTdependence as in pp – 16 – JHEP07(2015)051 Figure 7. The fraction of J/ψ from beauty hadron decays fBat mid-rapidity measured in the pTinterval 1.5< pT<10 GeV/c for centrality selected Pb-Pb collisions at √sNN = 2.76 TeV (left). The pTdependence of fBat mid-rapidity for Pb-Pb (√sNN = 2.76 TeV, |yJ/ψ|<0.8) and pp (√s= 7 TeV, |yJ/ψ|<0.9) [37] collisions is compared with measurements by CDF (|yJ/ψ|<0.6) [57], ATLAS (|yJ/ψ|<0.75) [35], and CMS (|yJ/ψ|<0.9) [13,70] (right). pT(GeV/c)fB(%) RAA(inclusive J/ψ)RAA(prompt J/ψ)RAA(non-prompt J/ψ) 0.0–1.5 – 0.89±0.20±0.21 – – 1.5–4.5 10.7±4.8±2.5 0.76±0.09±0.08 0.76±0.10±0.08 0.73±0.34±0.20 4.5–10.0 17.0±6.1±2.2 0.38±0.07±0.06 0.38±0.07±0.06 0.37±0.15±0.09 Table 5. The numerical values on the fraction of J/ψ from beauty hadron decays fBat midrapidity and the nuclear modification factors RAA of inclusive, prompt and non-prompt J/ψ for Pb-Pb collisions at √s= 2.76 TeV. These results correspond to the centrality interval 0–50%. The first uncertainty is statistical and the second uncorrelated systematic (type II). is observed. However, this similarity could be coincidental, being due to a compensation of the medium effects on the prompt component (J/ψ dissociation and recombination) and on the non-prompt part (b-quark energy loss). In figure 8the nuclear modification factor for non-prompt J/ψ for 1.5< pT< 4.5 GeV/c and 4.5< pT<10 GeV/c is shown together with the result by CMS for 6.5< pT<30 GeV/c [13] and with theoretical model predictions [30,31,59–61,80–85]. One should note that the centrality ranges are not the same for ALICE (0–50%) and CMS (0–20% and 20–100%). However, the results obtained by CMS for these two centrality bins are compatible with each other, and also compatible with our measurement in the high pTinterval (4.5< pT<10 GeV/c). The model by Uphoff et al. [61] follows a partonic transport approach based on the Boltzmann equation, which allows interactions among all partons. It does not include radiative processes for heavy quarks. The calculation has been – 17 – JHEP07(2015)051 Figure 8. The nuclear modification factor RAA at mid-rapidity (|y|<0.8) for non-prompt J/ψ in Pb-Pb collisions at √sNN = 2.76 TeV as a function of transverse momentum pT. The ALICE measurement corresponds to the 0–50% centrality range and to the pTintervals 1.5< pT<4.5 GeV/c and 4.5< pT<10 GeV/c. The uncorrelated systematic uncertainties (type II) are depicted by the open boxes, while the correlated uncertainties (type I) are shown as filled boxes at unity. Results by CMS for higher pTin the centrality range 0–20% and 20–100% [13] are also shown (the two points have been slightly displaced horizontally for better visibility). The data are compared to theoretical predictions at mid-rapidity (see text for details). In the right panel, the ALICE result in the pTinterval 4.5< pT<10 GeV/c is compared to theoretical predictions integrated over the same pTrange. performed for a fixed impact parameter b= 5 fm. In the model of Alberico et al. [59,60] the propagation of the heavy quarks in the medium is described by the relativistic Langevin equation. The predicted pTdependence of RAA is strongly influenced by the choice of transport coefficients. Two values are considered, either as provided by a perturbative calculation (hard thermal loop approach) or extracted from lattice-QCD simulations. The calculations have been provided for the centrality range 0–50%. A transport approach, which is based on a strong-coupling scheme, is employed in the model of He et al. [80]. The transport is implemented using non-perturbative interactions for heavy quarks and mesons through the QGP, hadronization and hadronic phases of a nuclear collision. In particular, the elastic heavy-quark scattering in the QGP is evaluated within a thermodynamic T-matrix approach, by generating resonances close to the critical temperature that can in turn recombine into B mesons, followed by hadronic diffusion using effective hadronic scattering amplitudes. The hydrodynamic evolution of the system is quantitatively constrained by the measured transverse momentum distributions and elliptic flow of light hadrons. Radiative processes, which should improve the description at high pT, are not included in this approach. The calculations have been performed in the centrality range 0–50%. The model of Vitev et al. [30,31] assumes the existence of open heavy flavour bound-state solutions in the QGP in the vicinity of the critical temperature. A description of beauty quark quenching is combined with B meson inelastic breakup processes. Furthermore, modified – 18 – JHEP07(2015)051 beauty parton distribution functions and beauty fragmentation functions in a co-moving plasma are implemented in this calculation. The prediction is shown for a fixed centrality, corresponding to hNparti= 200, a value very close to the average number of participants in the centrality range 0–50%. In the model, a sizable fraction of the suppression is ascribed to the inelastic break-up processes (collisional dissociation), as can be deduced from figure 8 by comparing the full model prediction with and without the contribution of this specific process. The model of Djordjevic [81], shown in figure 8for the centrality range 0–50%, uses a formalism that takes into account finite size dynamical QCD medium with finite magnetic mass effects and running coupling. In the WHDG model [82] (centrality range 0–50%) the energy loss is computed using perturbative QCD and considering both elastic and inelastic partonic collisions and path length fluctuations. The approach of Aichelin et al. [83,84] includes a contribution of radiative gluon emission in the interaction of heavy quarks with light quarks, which are considered as dynamical scattering centers. In this model the relative contribution to the energy loss by radiative processes, as compared to collisional ones, is influenced by introducing a finite gluon mass. The results of the model shown in figure 8, which are obtained for the centrality range 0–50%, correspond to either a pure collisional scenario or a combination of collisional and radiative energy loss. Finally, in the model of Horowitz and Gyulassy [85], also applied to the centrality interval 0–50%, the string inspired AdS/CFT gravity-gauge theory correspondence [86,87] is applied to the case of heavy quark energy loss. In the right hand inset of figure 8, the ALICE RAA value, integrated over the range 4.5< pT<10 GeV/c, is compared to theoretical predictions computed in the same pTrange. Most of the models predict a larger value of RAA than observed in the measurement. However, more precise data are needed to discriminate among the different models. The next LHC run will provide increased statistics for this measurement. 4 Conclusions A study of J/ψ production at mid-rapidity in Pb-Pb collisions at √sNN = 2.76 TeV has been presented. A reduction of the inclusive J/ψ hpTiis observed in Pb-Pb collisions in comparison to pp. The ratio rAA =hp2 TiAA/hp2 Tipp is found to be significantly below unity, corresponding to a medium-induced change in the shape of the pTspectra. The nuclear modification factor RAA depends on pT. It is around 0.8 for pT<2.5 GeV/c and reaches, at higher pT, almost the same level of suppression as observed at RHIC energies at low pT. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production at low pT. Transport models including this additional component are able to qualitatively describe the features seen in the data. The fraction of J/ψ from beauty hadron decays is determined as a function of centrality and pT. No significant centrality dependence is observed. By combining this measurement with the inclusive J/ψ results the RAA of non-prompt J/ψ is obtained in the region 1.5< pT<10 GeV/c, thus extending the coverage of CMS to the low pTregion. The nuclear modification in the region 4.5< pT<10 GeV/c is found to be stronger than predicted by most of the models. – 19 – JHEP07(2015)051 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: State Committee of Science, World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia, 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); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology 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 National al Cercetˇarii S¸tiintifice – 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 – 20 – JHEP07(2015)051 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. 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Bossu et al., Phenomenological interpolation of the inclusive J/ψ cross section to proton-proton collisions at 2.76 TeV and 5.5 TeV,arXiv:1103.2394 [INSPIRE]. – 23 – JHEP07(2015)051 15 Commissariat `a l’Energie Atomique, IRFU, Saclay, France 16 COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan 17 Departamento de F´ısica de Part´ıculas and IGFAE, Universidad de Santiago de Compostela, Santiago de Compostela, Spain 18 Department of Physics and Technology, University of Bergen, Bergen, Norway 19 Department of Physics, Aligarh Muslim University, Aligarh, India 20 Department of Physics, Ohio State University, Columbus, Ohio, United States 21 Department of Physics, Sejong University, Seoul, South Korea 22 Department of Physics, University of Oslo, Oslo, Norway 23 Dipartimento di Elettrotecnica ed Elettronica del Politecnico, Bari, Italy 24 Dipartimento di Fisica dell’Universit`a ’La Sapienza’ and Sezione INFN Rome, Italy 25 Dipartimento di Fisica dell’Universit`a and Sezione INFN, Cagliari, Italy 26 Dipartimento di Fisica dell’Universit`a and Sezione INFN, Trieste, Italy 27 Dipartimento di Fisica dell’Universit`a and Sezione INFN, Turin, Italy 28 Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Bologna, Italy 29 Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Catania, Italy 30 Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Padova, Italy 31 Dipartimento di Fisica ‘E.R. Caianiello’ dell’Universit`a and Gruppo Collegato INFN, Salerno, Italy 32 Dipartimento di Scienze e Innovazione Tecnologica dell’Universit`a del Piemonte Orientale and Gruppo Collegato INFN, Alessandria, Italy 33 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 34 Division of Experimental High Energy Physics, University of Lund, Lund, Sweden 35 Eberhard Karls Universit¨at T¨ubingen, T¨ubingen, Germany 36 European Organization for Nuclear Research (CERN), Geneva, Switzerland 37 Excellence Cluster Universe, Technische Universit¨at M¨unchen, Munich, Germany 38 Faculty of Engineering, Bergen University College, Bergen, Norway 39 Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia 40 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 41 Faculty of Science, P.J. ˇ Saf´arik University, Koˇsice, Slovakia 42 Faculty of Technology, Buskerud and Vestfold University College, Vestfold, Norway 43 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany 44 Gangneung-Wonju National University, Gangneung, South Korea 45 Gauhati University, Department of Physics, Guwahati, India 46 Helsinki Institute of Physics (HIP), Helsinki, Finland 47 Hiroshima University, Hiroshima, Japan 48 Indian Institute of Technology Bombay (IIT), Mumbai, India 49 Indian Institute of Technology Indore, Indore (IITI), India 50 Inha University, Incheon, South Korea 51 Institut de Physique Nucl´eaire d’Orsay (IPNO), Universit´e Paris-Sud, CNRS-IN2P3, Orsay, France 52 Institut f¨ur Informatik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany 53 Institut f¨ur Kernphysik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany 54 Institut f¨ur Kernphysik, Westf¨alische Wilhelms-Universit¨at M¨unster, M¨unster, Germany 55 Institut Pluridisciplinaire Hubert Curien (IPHC), Universit´e de Strasbourg, CNRS-IN2P3, Strasbourg, France 56 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 57 Institute for Subatomic Physics of Utrecht University, Utrecht, Netherlands 58 Institute for Theoretical and Experimental Physics, Moscow, Russia 59 Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovakia 60 Institute of Physics, Academy of Sciences of the Czech Republic, Prague, Czech Republic 61 Institute of Physics, Bhubaneswar, India – 30 – JHEP07(2015)051 62 Institute of Space Science (ISS), Bucharest, Romania 63 Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico 64 Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico 65 iThemba LABS, National Research Foundation, Somerset West, South Africa 66 Joint Institute for Nuclear Research (JINR), Dubna, Russia 67 Konkuk University, Seoul, South Korea 68 Korea Institute of Science and Technology Information, Daejeon, South Korea 69 KTO Karatay University, Konya, Turkey 70 Laboratoire de Physique Corpusculaire (LPC), Clermont Universit´e, Universit´e Blaise Pascal, CNRS–IN2P3, Clermont-Ferrand, France 71 Laboratoire de Physique Subatomique et de Cosmologie, Universit´e Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 72 Laboratori Nazionali di Frascati, INFN, Frascati, Italy 73 Laboratori Nazionali di Legnaro, INFN, Legnaro, Italy 74 Lawrence Berkeley National Laboratory, Berkeley, California, United States 75 Lawrence Livermore National Laboratory, Livermore, California, United States 76 Moscow Engineering Physics Institute, Moscow, Russia 77 National Centre for Nuclear Studies, Warsaw, Poland 78 National Institute for Physics and Nuclear Engineering, Bucharest, Romania 79 National Institute of Science Education and Research, Bhubaneswar, India 80 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 81 Nikhef, National Institute for Subatomic Physics, Amsterdam, Netherlands 82 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 83 Nuclear Physics Institute, Academy of Sciences of the Czech Republic, ˇ Reˇz u Prahy, Czech Republic 84 Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 85 Petersburg Nuclear Physics Institute, Gatchina, Russia 86 Physics Department, Creighton University, Omaha, Nebraska, United States 87 Physics Department, Panjab University, Chandigarh, India 88 Physics Department, University of Athens, Athens, Greece 89 Physics Department, University of Cape Town, Cape Town, South Africa 90 Physics Department, University of Jammu, Jammu, India 91 Physics Department, University of Rajasthan, Jaipur, India 92 Physik Department, Technische Universit¨at M¨unchen, Munich, Germany 93 Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 94 Politecnico di Torino, Turin, Italy 95 Purdue University, West Lafayette, Indiana, United States 96 Pusan National University, Pusan, South Korea 97 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum f¨ur Schwerionenforschung, Darmstadt, Germany 98 Rudjer Boˇskovi´c Institute, Zagreb, Croatia 99 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia 100 Russian Research Centre Kurchatov Institute, Moscow, Russia 101 Saha Institute of Nuclear Physics, Kolkata, India 102 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 103 Secci´on F´ısica, Departamento de Ciencias, Pontificia Universidad Cat´olica del Per´u, Lima, Peru 104 Sezione INFN, Bari, Italy 105 Sezione INFN, Bologna, Italy 106 Sezione INFN, Cagliari, Italy 107 Sezione INFN, Catania, Italy 108 Sezione INFN, Padova, Italy 109 Sezione INFN, Rome, Italy 110 Sezione INFN, Trieste, Italy – 31 – JHEP07(2015)051 111 Sezione INFN, Turin, Italy 112 SSC IHEP of NRC Kurchatov institute, Protvino, Russia 113 SUBATECH, Ecole des Mines de Nantes, Universit´e de Nantes, CNRS-IN2P3, Nantes, France 114 Suranaree University of Technology, Nakhon Ratchasima, Thailand 115 Technical University of Koˇsice, Koˇsice, Slovakia 116 Technical University of Split FESB, Split, Croatia 117 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 118 The University of Texas at Austin, Physics Department, Austin, Texas, USA 119 Universidad Aut´onoma de Sinaloa, Culiac´an, Mexico 120 Universidade de S˜ao Paulo (USP), S˜ao Paulo, Brazil 121 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 122 University of Houston, Houston, Texas, United States 123 University of Jyv¨askyl¨a, Jyv¨askyl¨a, Finland 124 University of Liverpool, Liverpool, United Kingdom 125 University of Tennessee, Knoxville, Tennessee, United States 126 University of the Witwatersrand, Johannesburg, South Africa 127 University of Tokyo, Tokyo, Japan 128 University of Tsukuba, Tsukuba, Japan 129 University of Zagreb, Zagreb, Croatia 130 Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, France 131 V. 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 – 32 –