Centrality dependence of high-pT D meson suppression 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. Centrality dependence of high-pT D meson suppression in Pb-Pb collisions at √sNN = 2.76 TeV ALICE Collaboration ALICE Collaboration. (2015). Centrality dependence of high-pT D meson suppression in Pb-Pb collisions at √sNN = 2.76 TeV. Journal of High Energy Physics, 2015(11), Article 205. https://doi.org/10.1007/JHEP11(2015)205 2015 JHEP11(2015)205 Published for SISSA by Springer Received:August 7, 2015 Accepted:October 28, 2015 Published:November 30, 2015 Centrality dependence of high-pTD meson suppression in Pb-Pb collisions at √sNN = 2.76 TeV The ALICE collaboration E-mail: [email protected] Abstract: The nuclear modification factor, RAA, of the prompt charmed mesons D0, D+ and D∗+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy √sNN = 2.76 TeV in two transverse momentum intervals, 5 < pT<8 GeV/c and 8 < pT<16 GeV/c, and in six collision centrality classes. The RAA shows a maximum suppression of a factor of 5–6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the RAA of non-prompt J/ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions. Keywords: Charm physics, Heavy Ions, Heavy-ion collision ArXiv ePrint: 1506.06604 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP11(2015)205 JHEP11(2015)205 Contents 1 Introduction 1 2 Experimental apparatus and data sample 2 3 Data analysis 3 4 Results and discussion 7 5 Summary 11 The ALICE collaboration 17 1 Introduction When heavy nuclei collide at high energy, a state of strongly-interacting matter with high energy density is expected to form. According to Quantum Chromodynamics (QCD) calculations on the lattice, this state of matter, the so-called Quark-Gluon Plasma (QGP) is characterised by the deconfinement of the colour charge (see e.g. [1–4]). High-momentum partons, produced at the early stage of the nuclear collision, lose energy as they interact with the QGP constituents. This energy loss is expected to proceed via both inelastic (gluon radiation) [5,6] and elastic (collisional) processes [7–9]. The nuclear modification factor RAA is used to characterise parton energy loss by comparing particle production yields in nucleus-nucleus collisions to a scaled proton-proton (pp) reference, that corresponds to a superposition of independent nucleon-nucleon collisions. RAA is defined as RAA =1 hTAAi·dNAA/dpT dσpp/dpT ,(1.1) where dσpp/dpTand dNAA/dpTare the transverse momentum (pT) differential cross section and yield in proton-proton and nucleus-nucleus (AA) collisions, respectively. hTAAiis the average nuclear overlap function, estimated within the Glauber model of the nucleusnucleus collision geometry, and proportional to the average number of nucleon-nucleon (binary) collisions [10,11]. Energy loss shifts the momentum of quarks and gluons, and thus hadrons, towards lower values, leading to a suppression of hadron yields with respect to binary scaling at pTlarger than few GeV/c (RAA <1). Energy loss is expected to be smaller for quarks than for gluons because the colour charge factor of quarks is smaller than that of gluons [5,6]. In the energy regime of the Large Hadron Collider (LHC), light-flavour hadrons with pTranging from 5 to 20 GeV/c originate predominantly from gluon fragmentation (see e.g. [12]). At variance, charmed mesons provide an experimental tag for a quark parent. Because of their large mass mc,b – 1 – JHEP11(2015)205 (mc≈1.3 GeV/c2,mb≈4.5 GeV/c2[13]), heavy quarks are produced at the initial stage of heavy-ion collisions in hard scattering processes that are characterised by a timescale ∆t < 1/(2 mc,b)∼0.1 (0.01) fm/c for c (b) quarks. This time is shorter than the formation time of the QGP medium (a recent estimate for the LHC energy is about 0.3 fm/c [14]). As discussed in ref. [15], this should be the case also for charm and beauty quarks produced in gluon splitting processes, if their transverse momentum is lower than about 50 GeV/c. Therefore, the comparison of the heavy-flavour hadron RAA with that of pions allows the colour-charge dependence of parton energy loss to be tested. The softer fragmentation of gluons than that of charm quarks, and the observed increase of the charged hadron RAA towards high pT[16], tend to counterbalance the effect of the larger energy loss of gluons on the RAA. The model predictions range from a rather moderate effect Rπ AA < RD AA [17–20] to an overall compensation Rπ AA ≈RD AA (as recently shown in [12]) in the pTinterval from 5 to about 15 GeV/c. Several mass-dependent effects are expected to influence the energy loss for quarks (see [15] for a recent review). The dead-cone effect should reduce small-angle gluon radiation for quarks that have moderate energy-over-mass values, i.e. for c and b quarks with momenta up to about 10 and 30 GeV/c, respectively [18,21–24]. Likewise, collisional energy loss is expected to be reduced for heavier quarks, because the spatial diffusion coefficient that regulates the momentum exchange with the medium is expected to scale as the inverse of the quark mass [25]. In the pTinterval up to about 20 GeV/c, where the masses of heavy quarks are not negligible with respect to their momenta, essentially all models predict RD AA < RB AA [17–20,26–35], which stems directly from the mass dependence of the quark-medium interaction and is only moderately affected by the different production and fragmentation kinematics of c and b quarks (see e.g. [36]). A first comparison of light-flavour, charm and beauty hadron nuclear modification factors based on measurements by the ALICE and CMS Collaborations [16,37,38] from the 2010 LHC Pb-Pb data at a centre-of-mass energy √sNN = 2.76 TeV was presented in [37]. In this paper we present the centrality dependence of the D meson RAA in Pb-Pb collisions at the same energy, measured with the ALICE detector [39] using data from both 2010 and 2011 periods (integrated luminosities of about 2.2 and 21 µb−1, respectively). The focus here is on the study of the parton energy loss; therefore, the data are presented for the high-pTinterval 5–16 GeV/c, where the largest suppression relative to binary scaling was observed [37]. The results are compared with charged pions, measured by the ALICE Collaboration [40], with non-prompt J/ψmesons, measured by the CMS Collaboration [38], and with model predictions. 2 Experimental apparatus and data sample The Pb-Pb collisions were recorded using a minimum-bias interaction trigger, based on the information of the signal coincidence of the V0 scintillator detectors that cover the full azimuth in the pseudo-rapidity intervals −3.7< η < −1.7 and 2.8< η < 5.1 [41]. The measurement of the summed signal amplitudes from the V0 detectors was used to sort the events in classes of collision centrality, defined in terms of percentiles of the Pb-Pb – 2 – JHEP11(2015)205 hadronic cross section [42]. The trigger efficiency is 100% for the events considered in this analysis, which correspond to the most central 80% of the Pb-Pb hadronic cross section. An online selection based on the information of the V0 detectors was applied to increase the statistics of central collisions for the 2011 data sample. An offline selection using the V0 and the neutron Zero-Degree Calorimeters (ZDC) was applied to remove background from interactions of the beams with residual atoms in the vacuum tube. Events with a reconstructed primary vertex outside the interval ±10 cm from the interaction point along the beam direction (zcoordinate) were removed. The event sample used in the analysis corresponds to an integrated luminosity Lint = (21.3±0.7) µb−1in the 0–10% centrality class (16.4×106events) and (5.8±0.2) µb−1in each of the 10–20%, 20–30%, 30–40%, 40–50% classes (4.5×106events per class). In the 50–80% class, where 2010 data were used, the analyzed event sample corresponds to (2.2±0.1) µb−1(5.1×106events). The decays D0→K−π+, D+→K−π+π+and D∗+→D0π+, and their charge conjugates, were reconstructed as described in [37] using the central barrel detectors, which are located in a solenoid that generates a 0.5 T magnetic field parallel to the beam direction. Charged particle tracks were reconstructed with the Time Projection Chamber (TPC) [43] and the Inner Tracking System (ITS), which consists of six cylindrical layers of silicon detectors [44]. Both detectors provide full azimuthal coverage in the interval |η|<0.9. D0and D+candidates were formed from pairs and triplets of tracks with |η|<0.8, pT>0.4 GeV/c, at least 70 associated space points in the TPC, and at least two hits in the ITS, out of which one had to be in either of the two innermost layers. D∗+candidates were formed by combining D0candidates with tracks with |η|<0.8, pT>0.1 GeV/c, and at least three associated hits in the ITS for the 10% most central collisions (two in the other centrality classes). The decay tracks of the candidate D mesons were identified on the basis of their specific ionization energy deposition dE/dxin the TPC and of their flight times to the Time Of Flight (TOF) detector, which has the same ηacceptance as the TPC. Particles were identified as pions (kaons) by requiring the measured signal to be within three times the resolution (±3σ) around the expected mean values of dE/dxand time-of-flight for pions (kaons). Only D meson candidates with rapidity |y|<0.8 were considered, because the acceptance decreases rapidly outside this interval. 3 Data analysis The selection of the D meson decay topology is mainly based on the displacement of the decay tracks from the primary vertex, and on the pointing of the reconstructed D meson momentum to the primary vertex [37]. The raw yields were determined in each centrality and pTinterval using fits to the distributions of invariant mass M(K−π+) and M(K−π+π+), in the case of D0and D+mesons, and of the difference M(K−π+π+)− M(K−π+) for D∗+mesons. The fit function is the sum of a Gaussian, for the signal, and either an exponential function (D0and D+) or a power-law multiplied with an exponential function (D∗+) to describe the background distribution [37]. For D0mesons, an additional term was included in the fit function to account for the socalled ‘reflections’, i.e. signal candidates that are present in the invariant mass distribution – 3 – JHEP11(2015)205 5< pT<8 GeV/c 8< pT<16 GeV/c D0D+D∗+D0D+D∗+ Pb-Pb yields: Yield Extraction 6 8 6 7 8 7 Tracking efficiency 10 15 15 10 15 15 PID identification 5 5 5 5 5 5 Cut efficiency 5 10 5 5 10 5 DpTdistribution in sim. 2 2 2 2 2 2 Feed-down subtraction +12 −13 +10 −10 +6 −8 +12 −12 +10 −10 + 7 −10 hTAAi[42] 4 4 pp reference 16 20 17 16 19 17 Reference scaling in √s+ 6 −12 +5 −6 Centrality limits <0.1 Table 1. Systematic uncertainties (%) on RAA of prompt D mesons with 5 < pT<8 GeV/c and 8< pT<16 GeV/c in the 0–10% centrality class. also when the (K, π) mass hypothesis for the decay tracks is swapped. A large fraction (about 70%) of these reflections is rejected by the particle identification selection. The residual contribution was studied with Monte Carlo simulations (described later in this section). It was found that the reflections have a broad invariant mass distribution, which is well described by a sum of two Gaussians, and its integral amounts to about 30% of the yield of the signal in the pTinterval used in the analysis presented in this article. In order to account for the contribution of reflections in the data, a template consisting of two Gaussians was included in the fit. The centroids and widths, as well as the ratios of the integrals of these Gaussians to the signal integral, were fixed to the values obtained in the simulation (see [45] for more details). In the most central centrality class (0–10%), the statistical significance of the invariant mass signal peaks varies from 8 to 18 depending on the D meson species and pT, while the signal-over-background ratio ranges from 0.1 to 0.4. In the most peripheral centrality class (50–80%), the statistical significance varies from 4 to 11, while the signal-over-background ranges from 0.4 to 1.5. In figure 1the invariant mass distributions of the three meson species are shown in the 0–10% centrality class and in the transverse momentum intervals 5< pT<8 GeV/c and 8 < pT<16 GeV/c. The correction for acceptance and efficiency was determined using Monte Carlo simulations. Pb-Pb events were simulated using the HIJING generator [46] and D meson signals were added with the PYTHIA 6 generator [47]. The pTdistribution of the D mesons was weighted in order to match the shape measured for D0mesons in central Pb-Pb collisions [37]. A detailed description of the detector response, based on the GEANT3 transport package [48], was included. The contribution of feed-down from B →D+Xto the inclusive D meson raw yield depends on pTand on the geometrical selection criteria, because the secondary vertices of D mesons from B-hadron decays are typically more displaced from – 4 – JHEP11(2015)205 fhi__1__1__1__1__1__1 Entries 68199 Mean 1.871 RMS 0.09082 ) 2 c) (GeV/π(KM 1.75 1.8 1.85 1.9 1.95 2 2.05 ) 2 cEntries / (10 MeV/ 200 400 600 800 1000 1200 1400 1600 fhi__1__1__1__1__1__1__1 Entries 68199 Mean 1.871 RMS 0.09082 and charge conj. + π - K→ 0 D c < 8 GeV/ T p 5 < = 2.76 TeV NN s ALICE, 0-10% Pb-Pb, |y| < 0.8 2 c 0.001) GeV/± = (1.866 µ 2 c 0.001) GeV/± = (0.018 σ 106±) = 2034 σS(3 ) = 0.24σS/B(3 fhi4__2__2__2__2__2__2 Entries 19834 Mean 1.878 RMS 0.1068 ) 2 c) (GeV/π(KM 1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 ) 2 cEntries / (10 MeV/ 100 200 300 400 500 600 fhi4__2__2__2__2__2__2__2 Entries 19834 Mean 1.878 RMS 0.1068 c < 16 GeV/ T p 8 < 2 c 0.002) GeV/± = (1.866 µ 2 c 0.002) GeV/± = (0.024 σ 68±) = 1127 σS(3 ) = 0.36σS/B(3 ) 2 c) (GeV/ππ(KM 1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 ) 2 cEntries / (9 MeV/ 200 400 600 800 1000 1200 and charge conj. + π + π - K→ + D c < 8 GeV/ T p5 < 2 c 0.002) GeV/± = (1.868 µ 2 c 0.002) GeV/± = (0.014 σ 71±) = 597 σS(3 ) = 0.14σS/B(3 ) 2 c) (GeV/ππ(KM 1.7 1.75 1.8 1.85 1.9 1.95 2 2.05 ) 2 cEntries / (9 MeV/ 100 200 300 400 500 600 700 c < 16 GeV/ T p 8 < 2 c 0.002) GeV/± = (1.866 µ 2 c 0.002) GeV/± = (0.019 σ 74±) = 702 σS(3 ) = 0.21σS/B(3 ) 2 c) (GeV/π(KM)-ππ(KM 0.14 0.145 0.15 0.155 ) 2 cEntries / (0.5 MeV/ 200 400 600 800 1000 and charge conj. + π 0 D→ *+ D c < 8 GeV/ T p5 < 2 c 0.06) MeV/± = (145.44 µ 2 c 60) keV/± = (460 σ 67±) = 553 σS(3 ) = 0.17σS/B(3 fhi2__6__6__6__6__6__6 Entries 175 Mean 0.1486 RMS 0.004073 ) 2 c) (GeV/π(KM)-ππ(KM 0.14 0.145 0.15 0.155 ) 2 cEntries / (0.5 MeV/ 50 100 150 200 250 fhi2__6__6__6__6__6__6__6 Entries 175 Mean 0.1486 RMS 0.004073 c < 16 GeV/ T p 8< 2 c 0.07) MeV/± = (145.45 µ 2 c 90) keV/± = (620 σ 41±) = 335 σS(3 ) = 0.39σS/B(3 Figure 1. Distributions of the Kπinvariant mass for D0candidates (upper panels) and Kππ invariant mass for D+candidates (central panels) and of the invariant mass difference M(Kππ)−M(Kπ) for D∗+candidates (lower panels) and the corresponding charge conjugates in two pTintervals (left and right panels) for 16.4×106Pb-Pb collisions in the 0–10% centrality class. The curves show the fit functions described in the text. The red short-dashed line represents the background fit function. For the D0meson, the gray dashed line represents the background without the inclusion of the template for the contribution of reflections, i.e. signal candidates with swapped (K, π) mass hypothesis. The template is defined as the sum of two Gaussians with parameters fixed to the values obtained in simulation. the primary vertex than those of prompt D mesons. This contribution was subtracted using the beauty-hadron production cross section in pp collisions from FONLL calculations [49], convoluted with the decay kinematics as implemented in the EvtGen decay package [50] and multiplied by the efficiency for feed-down D mesons from the simulation, the average nuclear overlap function hTAAiin each centrality class, and an assumed value for RAA of feed-down D-mesons [37]. On the basis of the comparison shown in this paper, this assumption was taken as Rfeed-down D AA = 2 Rprompt D AA and a systematic uncertainty was estimated by varying it in the interval 1 < Rfeed-down D AA /Rprompt D AA <3. The feed-down contribution is about 20–25%, depending on the D meson species and on the pTinterval. – 5 – JHEP11(2015)205 The pT-differential cross section of prompt D mesons with |y|<0.5 in pp collisions at √s= 2.76 TeV, used as reference for RAA, was obtained by scaling the measurement at √s= 7 TeV [51]. The pT-dependent scaling factor and its uncertainty were determined with FONLL calculations [52]. The result of the scaling was validated by comparison with the measurement obtained from a smaller sample of pp collisions at √s= 2.76 TeV [53]. This measurement covers a reduced pTinterval 1–12 GeV/c with a statistical uncertainty of 20–25 % and was, therefore, not used as a pp reference in the present analysis. The yields in Pb–Pb collisions were normalized to the same rapidity interval as the reference (|y|<0.5) by dividing them by ∆y= 1.6. The systematic uncertainties were estimated as a function of pTand centrality using the procedure described in [37,45] and briefly outlined in the following. The sources of systematic uncertainty on the nuclear modification factor are listed in table 1, along with their values for the two pTintervals in the most central collisions (0–10%). The uncertainties are approximately independent of centrality. The systematic uncertainty on the yield extraction was estimated by varying the fit conditions (fit interval and functional form used to describe the background) or by considering, as an alternative method, the bin counting of the invariant mass distribution obtained after subtracting the background estimated from a fit in the side-bands of the signal peak. The uncertainty amounts to about 6–8%. This includes in the case of the D0 a contribution of about 5% obtained by varying the ratio of the integral of the reflections to the integral of the signal by ±50%. The systematic uncertainty on the tracking efficiency correction was evaluated by varying the track selection criteria and amounts to 5% per track, thus 10% for the D0(two-track final state) and 15% for the D+and D∗+mesons (three-track final states). The correction for the particle identification (PID) efficiency introduces a systematic uncertainty of 5%, which was estimated by repeating the analysis without this selection and comparing the corrected yields. A systematic uncertainty of 5–10% associated with the selection efficiency correction was estimated by varying the D meson selection cuts. The D meson pTdistribution used in the simulation to calculate the acceptance and efficiency was varied between the measured distribution and the prediction of a theoretical calculation including parton energy loss [32,54,55]. The resulting variation of 2% of the efficiencies was assigned as a systematic uncertainty. The systematic uncertainty on the correction for feed-down from B-hadron decays was estimated, as described in [45], by varying the parameters of the FONLL calculation and the hypothesis on the RAA of the feed-down D mesons in the range 1 < Rfeed-down D AA /Rprompt D AA <3. This variation yields the main contribution to the uncertainty, which amounts to 6–13%, depending on the D meson species and pTinterval. The contribution to the systematic uncertainty due to the 1.1% relative uncertainty on the fraction of hadronic cross section used in the Glauber fit to determine the centrality classes was obtained as in [37] and estimated to be <0.1% in the central centrality class (0–10%) and 3% in the most peripheral centrality class (50–80%). The systematic uncertainties on the denominator of the nuclear modification factor include the uncertainty on hTAAi, which ranges from 4% in the 0–10% centrality class to – 6 – JHEP11(2015)205 7.5% in the 50–80% centrality class [42], and the uncertainty on the pp reference. The latter has a contribution of about 16–20% from the pp measurement at √s= 7 TeV and a contribution of +12 −6% from the energy scaling down to √s= 2.76 TeV. 4 Results and discussion Figure 2shows the RAA as a function of centrality for D0, D+and D∗+in the intervals 5< pT<8 GeV/c (left) and 8 < pT<16 GeV/c (right). Centrality is quantified in terms of the average number of nucleons participating in the collision in each multiplicity class, hNparti, evaluated with a Monte Carlo Glauber calculation [42]. The bars represent the statistical uncertainties. The filled and empty boxes represent the quadratic sum of the systematic uncertainties that are, respectively, correlated between centrality intervals (pp reference, B-hadron cross section used for feed-down correction, particle identification, track reconstruction efficiency, hTAAi) and uncorrelated (yield extraction, selection efficiency corrections, value of feed-down D meson RAA). The latter category also includes the systematic uncertainties that are partially correlated between adjacent centrality classes. The measurements for the three D meson species share part of the systematic uncertainties and are consistent within statistical uncertainties. The suppression increases with centrality and reaches a factor of 5–6 in the most central collisions for both pTintervals. A weighted average of the RAA of the three D meson species was computed using the inverse of the relative statistical uncertainties as weights. The systematic uncertainties of the weighted average were calculated considering the contributions from the tracking efficiency, the feed-down correction, and the reference energy scaling factor from 7 to 2.76 TeV as fully correlated among the three D meson species. Figure 3shows the average of the D0, D+and D∗+nuclear modification factors as a function of centrality, for the intervals 5 < pT<8 GeV/c (left) and 8 < pT<16 GeV/c (right), compared with the RAA of charged pions with |y|<0.8 for the same pTintervals1, and of non-prompt J/ψ mesons measured by the CMS Collaboration for 6.5< pT< 30 GeV/c in |y|<2.4 [38]. Care has to be taken when comparing with the non-central CMS data point as it is plotted at the Npart mean value of the broad 20–100% centrality interval. The pTinterval 8–16 GeV/cfor D mesons was chosen in order to obtain a significant overlap with the pTdistribution of B mesons decaying to J/ψ particles with 6.5< pT< 30 GeV/c. Using a simulation based on the FONLL calculation [49] and the EvtGen particle decay package [50], it was estimated that about 70% of these parent B mesons have 8< pT<16 GeV/c, with a median of the pTdistribution of about 11.3 GeV/c. A median value of (9.5±0.5) GeV/c was estimated for D mesons with 8 < pT<16 GeV/c in the 0–10% centrality class. The estimate was based on the pTdistribution of D0mesons in pT intervals with a width of 1 GeV/c. The effect of the different width of the rapidity interval for D and non-prompt J/ψ mesons (|y|<0.5 and |y|<2.4, respectively) is expected to be mild because the intervals are partially overlapping and a preliminary measurement by the CMS Collaboration does not indicate a significant ydependence of the RAA of non-prompt J/ψ mesons in |y|<2.4 [56]. 1The charged pion results were obtained with the analysis method described in [40]. – 7 – JHEP11(2015)205 [17] N. Armesto, A. Dainese, C.A. Salgado and U.A. Wiedemann, Testing the color charge and mass dependence of parton energy loss with heavy-to-light ratios at RHIC and CERN LHC, Phys. Rev. D 71 (2005) 054027 [hep-ph/0501225] [INSPIRE]. [18] S. Wicks, W. Horowitz, M. Djordjevic and M. 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Rodr´ıguez Cahuantzi2, A. Rodriguez Manso81 , K. Røed22 , E. Rogochaya66 , D. Rohr43 , D. R¨ohrich18 , R. Romita124 , F. Ronchetti72 , L. Ronflette113 , P. Rosnet70 , A. Rossi30 ,36 , F. Roukoutakis88 , A. Roy49 , C. Roy55 , P. Roy101 , A.J. Rubio Montero10 , R. Rui26 , R. Russo27 , E. Ryabinkin100 , Y. Ryabov85 , A. Rybicki117 , S. Sadovsky112 , K. ˇ Safaˇr´ık36 , B. Sahlmuller53 , P. Sahoo49 , R. Sahoo49 , S. Sahoo61 , P.K. Sahu61 , J. Saini132 , S. Sakai72 , M.A. Saleh135 , C.A. Salgado17 , J. Salzwedel20 , S. Sambyal90 , V. Samsonov85 , X. Sanchez Castro55 , L. ˇ S´andor59 , A. Sandoval64 , M. Sano128 , D. Sarkar132 , E. Scapparone105 , F. Scarlassara30 , R.P. Scharenberg95 , C. Schiaua78 , R. Schicker93 , C. Schmidt97 , H.R. Schmidt35 , S. Schuchmann53 , J. Schukraft36 , M. Schulc40 , T. Schuster137 , Y. Schutz113 ,36 , K. Schwarz97 , K. Schweda97 , G. Scioli28 , E. Scomparin111 , R. Scott125 , K.S. Seeder120 , J.E. Seger86 , Y. Sekiguchi127 , D. Sekihata47 , I. Selyuzhenkov97 , K. Senosi65 , J. Seo96 ,67 , E. Serradilla64 ,10 , A. Sevcenco62 , A. Shabanov56 , A. Shabetai113 , O. Shadura3, R. Shahoyan36 , A. Shangaraev112 , A. Sharma90 , M. Sharma90 , M. Sharma90 , N. Sharma125 ,61 , K. Shigaki47 , K. Shtejer9,27 , Y. Sibiriak100 , S. Siddhanta106 , K.M. Sielewicz36 , T. Siemiarczuk77 , D. Silvermyr84 ,34 , C. Silvestre71 , G. Simatovic129 , G. Simonetti36 , R. Singaraju132 , R. Singh79 , S. Singha132 ,79 , V. Singhal132 , B.C. Sinha132 , T. Sinha101 , B. Sitar39 , M. Sitta32 , T.B. Skaali22 , M. Slupecki123 , N. Smirnov137 , – 19 – JHEP11(2015)205 R.J.M. Snellings57 , T.W. Snellman123 , C. Søgaard34 , R. Soltz75 , J. Song96 , M. Song138 , Z. Song7, F. Soramel30 , S. Sorensen125 , M. Spacek40 , E. Spiriti72 , I. Sputowska117 , M. Spyropoulou-Stassinaki88 , B.K. Srivastava95 , J. Stachel93 , I. Stan62 , G. Stefanek77 , M. Steinpreis20 , E. Stenlund34 , G. Steyn65 , J.H. Stiller93 , D. Stocco113 , P. Strmen39 , A.A.P. Suaide120 , T. Sugitate47 , C. Suire51 , M. Suleymanov16 , R. Sultanov58 , M. ˇ Sumbera83 , T.J.M. Symons74 , A. Szabo39 , A. Szanto de Toledo120 ,i, I. Szarka39 , A. Szczepankiewicz36 , M. Szymanski134 , J. Takahashi121 , N. Tanaka128 , M.A. Tangaro33 , J.D. Tapia Takaki,iii,51 , A. Tarantola Peloni53 , M. Tarhini51 , M. Tariq19 , M.G. Tarzila78 , A. Tauro36 , G. Tejeda Mu˜noz2, A. Telesca36 , K. Terasaki127 , C. Terrevoli30 ,25 , B. Teyssier130 , J. Th¨ader74 ,97 , D. Thomas118 , R. Tieulent130 , A.R. Timmins122 , A. Toia53 , S. Trogolo111 , V. Trubnikov3, W.H. Trzaska123 , T. Tsuji127 , A. Tumkin99 , R. Turrisi108 , T.S. Tveter22 , K. Ullaland18 , A. Uras130 , G.L. Usai25 , A. Utrobicic129 , M. Vajzer83 , M. Vala59 , L. Valencia Palomo70 , S. Vallero27 , J. Van Der Maarel57 , J.W. Van Hoorne36 , M. van Leeuwen57 , T. Vanat83 , P. Vande Vyvre36 , D. Varga136 , A. Vargas2, M. Vargyas123 , R. Varma48 , M. Vasileiou88 , A. Vasiliev100 , A. Vauthier71 , V. Vechernin131 , A.M. Veen57 , M. Veldhoen57 , A. Velure18 , M. Venaruzzo73 , E. Vercellin27 , S. Vergara Lim´on2, R. Vernet8, M. Verweij135 ,36 , L. Vickovic116 , G. Viesti30 ,i, J. Viinikainen123 , Z. Vilakazi126 , O. Villalobos Baillie102 , A. Vinogradov100 , L. Vinogradov131 , Y. Vinogradov99 ,i, T. Virgili31 , V. Vislavicius34 , Y.P. Viyogi132 , A. Vodopyanov66 , M.A. V¨olkl93 , K. Voloshin58 , S.A. Voloshin135 , G. Volpe136 ,36 , B. von Haller36 , I. Vorobyev37 ,92 , D. Vranic36 ,97 , J. Vrl´akov´a41 , B. Vulpescu70 , A. Vyushin99 , B. Wagner18 , J. Wagner97 , H. Wang57 , M. Wang7,113 , Y. Wang93 , D. Watanabe128 , Y. Watanabe127 , M. Weber36 , S.G. Weber97 , J.P. Wessels54 , U. Westerhoff54 , J. Wiechula35 , J. Wikne22 , M. Wilde54 , G. Wilk77 , J. Wilkinson93 , M.C.S. Williams105 , B. Windelband93 , M. Winn93 , C.G. Yaldo135 , H. Yang57 , P. Yang7, S. Yano47 , Z. Yin7, H. Yokoyama128 , I.-K. Yoo96 , V. Yurchenko3, I. Yushmanov100 , A. Zaborowska134 , V. Zaccolo80 , A. Zaman16 , C. Zampolli105 , H.J.C. Zanoli120 , S. Zaporozhets66 , N. Zardoshti102 , A. Zarochentsev131 , P. Z´avada60 , N. Zaviyalov99 , H. Zbroszczyk134 , I.S. Zgura62 , M. Zhalov85 , H. Zhang18 ,7, X. Zhang74 , Y. Zhang7, C. Zhao22 , N. Zhigareva58 , D. Zhou7, Y. Zhou80 ,57 , Z. Zhou18 , H. Zhu18 ,7, J. Zhu113 ,7, X. Zhu7, A. Zichichi12 ,28 , A. Zimmermann93 , M.B. Zimmermann54 ,36 , G. Zinovjev3, M. Zyzak43 iDeceased ii Also at: M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear, Physics, Moscow, Russia iii Also at: University of Kansas, Lawrence, Kansas, United States 1A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation, Yerevan, Armenia 2Benem´erita Universidad Aut´onoma de Puebla, Puebla, Mexico 3Bogolyubov Institute for Theoretical Physics, Kiev, Ukraine 4Bose Institute, Department of Physics and Centre for Astroparticle Physics and Space Science (CAPSS), Kolkata, India 5Budker Institute for Nuclear Physics, Novosibirsk, Russia 6California Polytechnic State University, San Luis Obispo, California, United States 7Central China Normal University, Wuhan, China 8Centre de Calcul de l’IN2P3, Villeurbanne, France 9Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Havana, Cuba – 20 – JHEP11(2015)205 10 Centro de Investigaciones Energ´eticas Medioambientales y Tecnol´ogicas (CIEMAT), Madrid, Spain 11 Centro de Investigaci´on y de Estudios Avanzados (CINVESTAV), Mexico City and M´erida, Mexico 12 Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Rome, Italy 13 Chicago State University, Chicago, Illinois, USA 14 China Institute of Atomic Energy, Beijing, China 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 – 21 – JHEP11(2015)205 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 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, Nationaal instituut voor subatomaire fysica, 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 – 22 – JHEP11(2015)205 106 Sezione INFN, Cagliari, Italy 107 Sezione INFN, Catania, Italy 108 Sezione INFN, Padova, Italy 109 Sezione INFN, Rome, Italy 110 Sezione INFN, Trieste, Italy 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 – 23 –