Measurement of D0, D+, D*+ and Ds+ production in Pb-Pb collisions at √sNN = 5.02 TeV

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Measurement of D0, D+, D*+ and Ds+ production in Pb-Pb collisions at √sNN = 5.02 TeV © CERN, for the benefit of the ALICE Collaboration, 2018 Published version ALICE Collaboration ALICE Collaboration. (2018). Measurement of D0, D+, D*+ and Ds+ production in Pb-Pb collisions at √sNN = 5.02 TeV. Journal of High Energy Physics, 2018(10), Article 174. https://doi.org/10.1007/jhep10(2018)174 2018 JHEP10(2018)174 Published for SISSA by Springer Received:May 3, 2018 Accepted:September 24, 2018 Published:October 29, 2018 Measurement of D0, D+, D∗+and D+ sproduction in Pb–Pb collisions at √sNN =5.02 TeV The ALICE collaboration E-mail: [email protected] Abstract: We report measurements of the production of prompt D0, D+, D∗+and D+ smesons in Pb–Pb collisions at the centre-of-mass energy per nucleon-nucleon pair √sNN = 5.02 TeV, in the centrality classes 0–10%, 30–50% and 60–80%. The D-meson production yields are measured at mid-rapidity (|y|<0.5) as a function of transverse momentum (pT). The pTintervals covered in central collisions are: 1 < pT<50 GeV/c for D0, 2 < pT<50 GeV/c for D+, 3 < pT<50 GeV/c for D∗+, and 4 < pT<16 GeV/c for D+ smesons. The nuclear modification factors (RAA) for non-strange D mesons (D0, D+, D∗+) show minimum values of about 0.2 for pT= 6–10 GeV/c in the most central collisions and are compatible within uncertainties with those measured at √sNN = 2.76 TeV. For D+ s mesons, the values of RAA are larger than those of non-strange D mesons, but compatible within uncertainties. In central collisions the average RAA of non-strange D mesons is compatible with that of charged particles for pT>8 GeV/c, while it is larger at lower pT. The nuclear modification factors for strange and non-strange D mesons are also compared to theoretical models with different implementations of in-medium energy loss. Keywords: Heavy Ion Experiments ArXiv ePrint: 1804.09083 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. https://doi.org/10.1007/JHEP10(2018)174 JHEP10(2018)174 Contents 1 Introduction 1 2 Experimental apparatus and data sample 2 3 Data analysis 3 4 Proton-proton reference for RAA 9 5 Systematic uncertainties 9 6 Results 12 7 Summary 20 The ALICE collaboration 28 1 Introduction Ultra-relativistic collisions of heavy nuclei produce a state of strongly-interacting matter characterised by high energy density and temperature. According to Quantum Chromodynamics (QCD) on the lattice, in these extreme conditions matter undergoes a phase transition to a Quark-Gluon Plasma (QGP) state in which quarks and gluons are deconfined and chiral symmetry is partially restored [1–4]. Heavy quarks (such as charm and beauty) are predominantly produced in the early stage of the collision in hard scattering processes between partons of the incoming nuclei. Because of their large masses, their production time (∼0.1 and 0.02 fm/cfor charm and beauty, respectively [5]) is shorter than the formation time of the QGP, which is about 0.3–1.5 fm/cat Large Hadron Collider (LHC) energies [6]. In contrast, the thermal production and annihilation rates are negligible [7]. Heavy quarks therefore experience the full evolution of the hot and dense QCD medium. During their propagation through the medium, heavy quarks are exposed to interactions with the medium constituents and lose part of their energy via inelastic (gluon radiation) [8,9] or elastic scatterings (collisional processes) [10–12]. The colour-charge dependence of the strong interaction and parton-mass-dependent effects are predicted to influence the amount of energy loss (see [5,13] for recent reviews). Low-momentum heavy quarks can participate in the collective expansion of the system as a consequence of multiple interactions with the medium [14,15]. It was also suggested that low-momentum heavy quarks could hadronise not only via fragmentation in the vacuum, but also via the mechanism of recombination with other quarks in the medium [15,16]. In this scenario, – 1 – JHEP10(2018)174 the large abundance of strange quarks in nucleus-nucleus collisions with respect to protonproton collisions is expected to lead to an increased production of D+ smesons relative to non-strange D mesons [17]. The effects of energy loss and the dynamics of heavy-quark hadronisation can be studied using the nuclear modification factor RAA, which compares the transverse-momentum (pT) differential production yields in nucleus-nucleus collisions (dNAA/dpT) with the cross section in proton-proton collisions (dσpp/dpT) scaled by the average nuclear overlap function hTAAi: RAA(pT) = 1 hTAAi·dNAA/dpT dσpp/dpT .(1.1) The average nuclear overlap function hTAAiis defined as the average number of nucleonnucleon collisions hNcolli, which can be estimated via Glauber model calculations [18–21], divided by the inelastic nucleon-nucleon cross section. Measurements of prompt D-meson production by the ALICE collaboration in Pb–Pb collisions at √sNN = 2.76 TeV [22–25] showed a strong suppression of the D-meson yields by a factor of 5–6 for 8 < pT<12 GeV/c in the 10% most central collisions. Recent results from the CMS collaboration on D0production in the pTrange 2–100 GeV/c show a similar suppression for 6 < pT<10 GeV/c in the 10% most central Pb–Pb collisions at √sNN = 5.02 TeV, decreasing with increasing pT[26]. In contrast, the D-meson nuclear modification factor in p–Pb collisions at √sNN = 5.02 TeV, where an extended QGP phase is not expected to be formed, was found to be consistent with unity within uncertainties for 0 < pT<24 GeV/c [27]. These results indicate that the strong suppression is due to substantial final-state interactions of charm quarks with the QGP formed in Pb–Pb collisions. In this article, we present the measurement of pT-differential yields and the nuclear modification factor for prompt D0, D+, D∗+and D+ smesons (including their antiparticles), in Pb–Pb collisions at √sNN = 5.02 TeV collected with the ALICE detector during the LHC Run 2 in 2015. Prompt D mesons are defined as those produced by the hadronisation of charm quarks or from the decay of excited open charm and charmonium states, hence excluding the decays of beauty hadrons. The experimental apparatus is briefly presented in section 2, together with the data sample used for the analysis. The reconstruction of D-meson hadronic decays and all corrections applied to the raw yields are presented in section 3. The procedure used to obtain the proton-proton reference cross section at √s= 5.02 TeV and the estimation of the systematic uncertainties are described in section 4 and section 5, respectively. The results for the central (0–10%), semi-central (30–50%) and peripheral (60–80%) collisions are presented in section 6. A comparison with charged-pion and charged-particle RAA is reported in the same section, along with detailed comparisons with model calculations, including a simultaneous comparison of the RAA and elliptic flow v2. Conclusions are drawn in section 7. 2 Experimental apparatus and data sample A description of the ALICE experimental apparatus and its performance in pp, p–Pb and Pb–Pb collisions can be found in [28,29]. The main detectors used in the present analysis – 2 – JHEP10(2018)174 Centrality class hTAAi(mb−1)Nevents 0–10% 23.07 ±0.44 10.4×106 30–50% 3.90 ±0.11 20.8×106 60–80% 0.417 ±0.014 20.8×106 Table 1. Average nuclear overlap function and number of events for the three centrality classes used in the analysis. are the V0 detector, the Inner Tracking System (ITS) [30], the Time Projection Chamber (TPC) [31] and the Time Of Flight (TOF) detector [32], located inside a large solenoidal magnet providing a uniform magnetic field of 0.5 T parallel to the LHC beam direction (zaxis in the ALICE reference system), and the Zero Degree Calorimeters (ZDC) [33], located at z=±112.5 m from the nominal interaction point. The analysed sample consists of Pb–Pb collision data recorded with a minimum-bias interaction trigger that required coincident signals in both scintillator arrays of the V0 detector [34]. The V0 detector consists of two scintillator arrays, which cover the full azimuth in the pseudorapidity intervals −3.7< η < −1.7 and 2.8< η < 5.1. Events produced by the interaction of the beams with residual gas in the vacuum pipe were rejected offline using the V0 and the ZDC timing information. Only events with a reconstructed interaction point (primary vertex) within ±10 cm from the centre of the ITS detector along the beam line were used in the analysis. For the data sample considered in this paper, the probability of in-bunch collision pileup (i.e. collisions with two or more simultaneous interactions per bunch crossing) was negligible, while the request of at least a hit in one of the two innermost layers of the ITS rejected tracks produced in out-of-bunch pileup collisions. Collisions were divided into centrality classes, determined from the sum of the V0 signal amplitudes and defined in terms of percentiles of the hadronic Pb–Pb cross section. In order to relate the centrality classes to the collision geometry, the distribution of the V0 summed amplitudes was fitted with a function based on the Glauber model [18– 21] combined with a two-component model for particle production [35], which decomposes particle production in nucleus-nucleus collisions into the contributions due to soft and hard interactions. The centrality classes used in the present analysis, together with the corresponding average nuclear overlap function hTAAi[36] and the number of events (Nevents) in each class, are summarised in table 1. The corresponding integrated luminosity is about Lint ≈13 µb−1[37]. 3 Data analysis The D mesons and their charge conjugates were reconstructed in the decay channels D0→K−π+(with branching ratio, BR, of (3.93 ±0.04)%), D+→K−π+π+(BR of (9.46 ±0.24)%), D∗+→D0π+(BR of (67.7±0.5)%) and D+ s→φπ+→K+K−π+(BR of (2.27 ±0.08)%) [38]. D0, D+and D+ scandidates were defined using pairs and triplets of tracks with proper charge-sign combination having |η|<0.8, pT>0.4 GeV/c, a minimum number of 70 (out of 159) associated space points in the TPC and at least two hits – 3 – JHEP10(2018)174 (out of six) in the ITS, with at least one in the two innermost layers. D∗+candidates were formed by combining D0candidates with tracks having |η|<0.8, pT>0.1 GeV/c and at least three associated hits in the ITS. For D+ scandidate selection, one of the two pairs of opposite-sign tracks was required to have an invariant mass compatible with the φmass (mφ= 1019.461 ±0.019 MeV/c2[38]). In particular, the difference between the reconstructed K+K−invariant mass and φmass was required to be less than 5–10 MeV/c2 depending on the D+ spTinterval. This selection preserves 70–85% of the D+ ssignal. The selection of tracks with |η|<0.8 limits the D-meson acceptance in rapidity, which, depending on pT, varies from |y|<0.6 for pT= 1 GeV/c to |y|<0.8 for pT>5 GeV/c. A pT-dependent fiducial acceptance cut, |yD|< yfid(pT), was therefore applied to the D-meson rapidity. The value of yfid(pT) increases from 0.6 to 0.8 in the range 1 < pT<5 GeV/c, and the variation can be described according to a second-order polynomial function. For pT>5 GeV/cone has yfid = 0.8. The selection strategy is similar to the one used in previous analyses [25,39] and is mainly based on the separation between primary and secondary vertex, the displacement of the tracks from the primary vertex and the pointing of the reconstructed D-meson momentum to the primary vertex. In comparison to previous analyses, additional selection criteria were exploited. In particular, the normalised difference between the measured and expected transverse-plane impact parameters of each of the decay particles (already introduced in [40]) and the transverse-plane impact parameter to the primary vertex (dxy 0) of the D-meson candidates were used. Besides the rejection of the combinatorial background, a selection based on the latter two variables has the advantage to suppress significantly the fraction of D mesons coming from beauty-hadron decays (feed-down) and hence reduce the associated systematic uncertainty. The cut values on the selection variables were optimised in each centrality class independently, in order to obtain a large statistical significance of the D-meson signals, while keeping the selection efficiency of promptly produced D mesons as large as possible. Further background reduction was obtained by applying particle identification for charged pions and kaons with the TPC and TOF detectors. A ±3σwindow around the expected mean values of specific ionisation energy loss dE/dxin the TPC gas and time-of-flight from the interaction point to the TOF detector was used for the identification, where σis the resolution on these two quantities. In central collisions, a 2 σ selection was used for D∗+and D+(for pT<3 GeV/c) candidates. For D+ scandidates, tracks without a TOF signal (mostly at low momentum) were identified using only the TPC information and requiring a 2 σcompatibility with the expected dE/dx. The stricter PID selection strategy was needed due to the large background of track triplets and, in case of D+ s, because of its short lifetime, which limits the effectiveness of the geometrical selections on the displaced decay-vertex topology. The D0, D+and D+ sraw yields were obtained from binned maximum-likelihood fits to the candidate invariant-mass (M) distributions, while for the D∗+the mass difference ∆M=M(Kππ)−M(Kπ) distributions were used. Examples for these distributions are shown in figure 1for the centrality class 0–10%. The D0, D+and D+ scandidate invariantmass distributions were fitted with a function composed of a Gaussian term for the signal and an exponential function to describe the background shape, with the exception of the – 4 – JHEP10(2018)174 1.75 1.8 1.85 1.9 1.95 2 ) 2 c) (GeV/π(KM 0.3 0.4 0.5 0.6 0.7 0.8 6 10× ) 2 cCounts / (8 MeV/ ALICE + π - K→ 0 D and charge conj. c < 2 GeV/ T p1 < = 5.02 TeV NN s Pb, −0-10% Pb 2 c 2 MeV/± = 1868 µ2 c = 12 MeV/σ 1939±S = 14044 1.75 1.8 1.85 1.9 1.95 2 ) 2 c) (GeV/π(KM 2− 1− 0 1 2 3 4 5 6 3 10× ) 2 cCounts - bkg fct. / (8 MeV/ + π - K→ 0 D and charge conj. c < 2 GeV/ T p1 < 2 c 2 MeV/± = 1868 µ2 c = 12 MeV/σ 1939±S = 14044 1.75 1.8 1.85 1.9 1.95 2 ) 2 c) (GeV/ππ(KM 0.4 0.6 0.8 1 1.2 1.4 3 10× ) 2 cCounts / (14 MeV/ + π + π - K→ + D and charge conj. c < 10 GeV/ T p8 < 2 c 1 MeV/± = 1871 µ2 c 1 MeV/± = 17 σ 89±S = 1158 0.14 0.142 0.144 0.146 0.148 0.15 0.152 0.154 ) 2 c) (GeV/π(KM) - ππ(KM 0 10 20 30 40 50 60 70 ) 2 cCounts / (0.8 MeV/ + π + π - K→ + π 0 D→ *+ D and charge conj. c < 36 GeV/ T p24 < 2 c 0.2 MeV/± = 145.6 µ2 c 0.2 MeV/± = 0.9 σ 17±S = 73 1.8 1.85 1.9 1.95 2 2.05 2.1 2.15 ) 2 c) (GeV/π(KKM 100 150 200 250 300 350 ) 2 cCounts / (10 MeV/ + π + K - K→ + πφ → s + D and charge conj. c < 6 GeV/ T p4 < 2 c 3 MeV/± = 1968 µ2 c 2 MeV/± = 11 σ 34±S = 161 + D+ s D 1.8 1.85 1.9 1.95 2 2.05 2.1 2.15 ) 2 c) (GeV/π(KKM 0 5 10 15 20 25 30 ) 2 cCounts / (14 MeV/ + π + K - K→ + πφ → s + D and charge conj. c < 16 GeV/ T p12 < 2 c 3 MeV/± = 1969 µ2 c 3 MeV/± = 17 σ 9±S = 44 + D + s D Figure 1. Invariant-mass distributions for the four D-meson species in selected pTintervals for the centrality class 0–10%. Fitted values for the meson mass µ, width σand raw yield Sare also given. Top row: D0mesons with 1 < pT<2 GeV/c, before (left) and after (right) subtraction of the background fit function. For this pTinterval, the width of the Gaussian used to describe the signal is fixed to the value obtained in the simulations. Middle row: D+mesons with 8 < pT<10 GeV/c and D∗+mesons (difference of M(Kππ) and M(Kπ)) with 24 < pT<36 GeV/c. Bottom row: D+ s mesons with 4 < pT<6 GeV/c and 12 < pT<16 GeV/c; the D+→K+K−π+signal is visible on the left of the D+ ssignal. D0pTintervals 1–2 GeV/c and 2–3 GeV/c, where the background was found to be better described by a second-order polynomial function (a fourth-order polynomial was used in 1–2 GeV/c for the 0–10% centrality class). The ∆Mdistribution of D∗+candidates was fitted with a Gaussian function for the signal and a threshold function multiplied by an exponential for the background (a√∆M−mπ·eb(∆M−mπ), where mπis the pion mass and aand bare free parameters). The contribution of signal candidates that are present in the invariant-mass distribution of the D0meson with the wrong decay-particle mass assignment (reflection), was parametrised by fitting the simulated reflection invariant-mass distributions with a double Gaussian function, and it was included in the total D0fit function. The ratio between the reflected signal and the yields of the D0was taken from simulations – 5 – JHEP10(2018)174 (typically 2–5% of the raw yield, depending on pT) [39]. The Monte Carlo simulation used for this study is the same one used to determine the reconstruction efficiency, as described in the following dedicated paragraph. In addition, given the critical signal extraction induced by the small signal-to-background ratio of the D0meson in 1 < pT<2 GeV/c, the width of the Gaussian used to describe the signal was fixed to the value obtained in the simulations. The Gaussian widths obtained from the simulations were found to be consistent with those extracted from the data in the full pTrange, for all measured centrality classes, with deviations of at most 10–15%. In the fit to the D+ s-candidate invariant-mass distribution, an additional Gaussian was used to describe the D+→K+K−π+signal on the left of the D+ ssignal. The statistical significance S/√S + B of the observed signals, estimated within 3 standard deviations, varies from 5 to 33 depending on the D-meson species, the pTinterval, and the centrality class. The D-meson raw yields were corrected in order to obtain the pT-differential yields of prompt D mesons dND dpT|y|<0.5 = fprompt(pT)·1 2ND+D raw (pT)|y|<yfid(pT) ∆pT·αy(pT)·(Acc ×)prompt(pT)·BR ·Nevents .(3.1) The raw yields ND+D raw were divided by a factor of two to obtain the charge-averaged (particle and antiparticle) yields. To correct for the contribution of feed-down from beautyhadron decays, the raw yields were multiplied by the fraction of promptly produced D mesons, fprompt (see eq. (3.2)). Furthermore, they were divided by the product of prompt D-meson acceptance and efficiency (Acc ×)prompt, by the branching ratio BR of the decay channel, by the transverse momentum interval width ∆pTand by the number of events Nevents. The (Acc ×)prompt correction includes the tracking efficiency, the acceptance of pions and kaons, and the kinematical and topological selection efficiency of D mesons. The factor αy(pT) = yfid(pT)/0.5 normalises the corrected yields measured in |y|< yfid(pT) to one unit of rapidity |y|<0.5, assuming a flat rapidity distribution for D mesons in |y|< yfid(pT). This assumption was validated to the 1% level with simulations for pp collisions [41,42] and it is justified also for Pb–Pb collisions. For example, measurements of the prompt and non-prompt J/ψ RAA in Pb–Pb collisions at √sNN = 2.76 TeV do not exhibit a significant rapidity dependence [43]. The correction for acceptance and efficiency (Acc ×)prompt was determined using Monte Carlo simulations with a detailed description of the detector and its response, based on the GEANT3 transport package [44]. The underlying Pb–Pb events at √sNN = 5.02 TeV were simulated using the HIJING v1.383 generator [45] and D-meson signals were added using the PYTHIA v6.421 generator [46] with Perugia-2011 tune. Each simulated PYTHIA pp event contained a cc or bb pair, and D mesons were forced to decay into the hadronic channels of interest for the analysis. In the most central event class, the pTdistribution of D mesons in the MC simulation for pT>2 GeV/c was weighted in order to match the shape measured in data for D0mesons in finer pTintervals with respect to those used in the analysis. In the centrality classes and pTranges where an analysis in finer pTintervals was not possible, the simulated D-meson pTdistribution was weighted to match the shape – 6 – JHEP10(2018)174 given by model calculations. In particular, fixed-order plus next-to-leading-log perturbative QCD calculations (FONLL) [47,48] multiplied by the RAA(pT) of D mesons computed using the BAMPS model (which implements both elastic and radiative processes) for the 30–50% centrality class [49–51] were used for the corresponding centrality class. For the pTintervals 1–2 GeV/c and 16–50 GeV/c in the 0–10% centrality class and for the 60–80% centrality class, where the RAA is nearly flat in the measured pTinterval, only the FONLL calculations were used. Figure 2shows the acceptance-times-efficiency (Acc ×ε) for prompt and feed-down D mesons with rapidity |y|< yfid(pT) in the centrality class 0–10%, after the aforementioned pT-distribution weighting procedure. The difference between the (Acc×ε) factor for prompt and feed-down D mesons arises from the geometrical selections applied, given the different decay topology of D mesons coming from B decays. In particular, the feed-down D mesons are on average more displaced from the primary vertex due to the large B-meson lifetime (cτ ≈500 µm [38]) and therefore are more efficiently selected by the majority of the analysis cuts (e.g. for D0and D∗+in most of the pTintervals). On the contrary, the selections on the difference between measured and expected decay-track impact parameters and on the D-meson impact parameter reject more feed-down D mesons, thus reducing the feed-down efficiencies as compared to the previous analyses (e.g. for D+and D+ s). The (Acc ×ε) is higher for more peripheral collisions, by up to a factor larger than two at low pT, since less stringent selections can be applied because of the lower combinatorial background. The fprompt factor was obtained, following the procedure introduced in [22], by subtracting the contribution of D mesons from beauty-hadron decays from the measured raw yield in each pTinterval. It was estimated using perturbative QCD calculations, efficiencies from MC simulations, and an hypothesis on the RAA of feed-down D mesons. The expression for fprompt reads: fprompt = 1−ND+D feed-down raw ND+D raw = 1−Rfeed-down AA ·hTAAi·dσ dpTFONLL, EvtGen feed-down,|y|<0.5·∆pT·αy·(Acc×)feed-down ·BR·Nevents 1 2ND+D raw . (3.2) In this expression, ND+D raw is the measured raw yield and ND+D feed-down raw is the estimated raw yield of D mesons from beauty-hadron decays. In detail, the beauty-hadron production cross section in pp collisions at √s= 5.02 TeV, estimated with FONLL calculations [52], was folded with the beauty-hadron→D + Xdecay kinematics using the EvtGen package [53] and multiplied by hTAAiof the corresponding centrality class, by the (Acc ×ε) for feed-down D mesons, and by the other factors introduced in eq. (3.1). In addition, the nuclear modification factor of D mesons from beauty-hadron decays was accounted for. The comparison of the RAA of prompt D mesons (Rprompt AA ) at √sNN = 2.76 TeV [24] with that of J/ψ from B-meson decays [43] at the same energy measured by the CMS collaboration indicates that prompt charmed hadrons are more suppressed than non-prompt charmed hadrons. The RAA values differ by a factor of about two in central collisions at a transverse momentum of about 10 GeV/c [24] and this difference is described by model – 7 – JHEP10(2018)174 1 10 )c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 /D + D ALICE |<0.5y| = 5.02 TeV NN s 0-10% Pb-Pb, = 5.02 TeV NN s30-50% Pb-Pb, = 5.02 TeV NN s60-80% Pb-Pb, = 7 TeVspp, Eur. Phys. J. C77 (2017) no.8, 550 Eur. Phys. J. C77 (2017) no.8, 550 2.7% BR uncertainty not shown± 1 10 )c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 /D *+ D ALICE |<0.5y| = 5.02 TeV NN s 0-10% Pb-Pb, = 5.02 TeV NN s30-50% Pb-Pb, = 5.02 TeV NN s60-80% Pb-Pb, = 7 TeVspp, Eur. Phys. J. C77 (2017) no.8, 550 Eur. Phys. J. C77 (2017) no.8, 550 0.7% BR uncertainty not shown± 0 2 4 6 8 10 12 14 16 18 20 )c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 /D + s D ALICE |<0.5y| = 5.02 TeV NN s 0-10% Pb-Pb, = 5.02 TeV NN s30-50% Pb-Pb, = 5.02 TeV NN s60-80% Pb-Pb, = 7 TeVspp, Eur. Phys. J. C77 (2017) no.8, 550 Eur. Phys. J. C77 (2017) no.8, 550 3.7% BR uncertainty not shown± 0 2 4 6 8 10 12 14 16 18 20 )c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 + /D + s D ALICE |<0.5y| = 5.02 TeV NN s 0-10% Pb-Pb, = 5.02 TeV NN s30-50% Pb-Pb, = 5.02 TeV NN s60-80% Pb-Pb, = 7 TeVspp, Eur. Phys. J. C77 (2017) no.8, 550 Eur. Phys. J. C77 (2017) no.8, 550 4.3% BR uncertainty not shown± Figure 4. Ratio of prompt D-meson yields as a function of pT. Statistical (bars) and systematic (boxes) uncertainties are shown. are larger in Pb–Pb than in pp collisions, in all three centrality classes, however the measurements in the two systems are compatible within about one standard deviation of the combined uncertainties. The RAA of prompt D0, D+and D∗+mesons is shown in the left-hand panels of figure 5, from central (top) to peripheral (bottom) collisions. The nuclear modification factors of the three D-meson species are compatible within statistical uncertainties, which are obtained by propagating those on the Pb–Pb yields and those of the pp reference. Their average was computed using the inverse of the quadratic sum of the relative statistical and uncorrelated systematic uncertainties as weights, in the pTintervals where more than one D-meson species is available, (figure 5, right-hand panels). The systematic uncertainties were propagated through the averaging procedure, considering the contributions from the – 14 – JHEP10(2018)174 5 10 15 20 25 30 35 40 45 50 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R 0 D + D + D* ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp 5 10 15 20 25 30 35 40 45 50 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp + , D* + , D 0 Average D + s D 5 10 15 20 25 30 35 40 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R 0 D + D + D* ALICE = 5.02 TeV NN s 30-50% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp 5 10 15 20 25 30 35 40 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 30-50% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp + , D* + , D 0 Average D + s D 5 10 15 20 25 30 35 40 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R 0 D + D + D* ALICE = 5.02 TeV NN s 60-80% Pb-Pb, |<0.5y| 5 10 15 20 25 30 35 40 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 60-80% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp + , D* + , D 0 Average D + s D Figure 5.RAA of prompt D0, D+and D∗+mesons (left-hand panels) and of prompt D+ smesons compared with the average RAA of the non-strange D-meson states available in each pTinterval (right-hand panels) for the 0–10%, 30–50% and 60–80% centrality classes. Statistical (bars), systematic (empty boxes), and normalisation (shaded box around unity) uncertainties are shown. Filled markers are obtained with the pp rescaled reference, empty markers with the pT-rescaled reference. – 15 – JHEP10(2018)174 tracking efficiency, the beauty-hadron feed-down subtraction and the FONLL-based √sscaling of the pp cross section from √s= 7 TeV to √s= 5.02 TeV as fully correlated uncertainties among the three D-meson species. The average nuclear modification factors in the 0–10% and 30–50% centrality classes (top and middle right-hand panels of figure 5) show a suppression that is maximal at pT= 6–10 GeV/c, where a reduction of the yields by a factor of about 5 and 2.5 with respect to the binary-scaled pp reference is observed in the two centrality classes, respectively. The suppression gets smaller with decreasing pT for pT<6 GeV/c, and RAA is compatible with unity in the interval 1 < pT<3 GeV/c. The average RAA in the 60–80% centrality class shows a suppression by about 20–30%, without a pronounced dependence on pT. The RAA of prompt D+ smesons is shown in the right-hand panels of figure 5, where it is compared with the average RAA of non-strange D mesons: the values are larger for D+ smesons, but the two measurements are compatible within one standard deviation of the combined uncertainties, as is the case for the ratios shown in figure 4. The average RAA of prompt D0, D+and D∗+in the 10% most central collisions is compared with a measurement of prompt D0mesons by the CMS collaboration [26] in the rapidity interval |y|<1 in figure 6(left panel): the measurements are compatible in the common pTinterval 2–50 GeV/c. In the right panel of figure 6, the nuclear modification factor of D mesons at √sNN = 5.02 TeV in the 0–10% centrality class is compared with the same measurement at √sNN = 2.76 TeV [23].1The measurement at √sNN = 5.02 TeV have total uncertainties reduced by a factor of about two and extended pTcoverage from 36 to 50 GeV/c. The suppression is compatible within uncertainties at the two energies, as also observed for charged particles [59]. The close similarity of the RAA measurements at the two energies was predicted by the Djordjevic model [54] (figure 6, right panel), and it results from the combination of a higher medium temperature at 5.02 TeV (estimated to be about 7% higher than at 2.76 TeV), which would decrease the RAA by about 10%, with a harder pTdistribution of charm quarks at 5.02 TeV, which would increase the RAA by about 5% if the medium temperature were the same as at 2.76 TeV. As explained in section 1, the measurement of the RAA of open-charm mesons is essential to understand in-medium parton energy loss, in particular its colour-charge and quark-mass dependence. In figure 7, the RAA of prompt D mesons is compared with that of charged particles in the same pTintervals, at the same energy and in the same centrality classes [59]. The ratio of their nuclear modification factors is displayed in the bottom panels, for the three centrality classes. The RAA of D mesons and charged particles differ by more than 2 σof the combined statistical and systematic uncertainties in all the pTintervals within 3 < pT<8 GeV/c in central collisions. The difference is less than 2 σin this range for semi-central collisions, while the two RAA are the same within 1 σfor pT>10 GeV/c in both central and semi-central collisions. In the 60–80% class the measurements are compatible in the common pTinterval. The interpretation of the difference observed for 1The TAA used to compute the D-meson RAA at √sNN = 2.76 TeV in the 0–10% centrality class and its uncertainty were updated with respect to [23] according to the values reported in ref. [36]. – 16 – JHEP10(2018)174 1 10 2 10 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y, | + , D* + , D 0 Average D |<1, PLB 782 (2018) 474-496y CMS, | 0 D 5 10 15 20 25 30 35 40 45 50 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R = 5.02 TeV NN s = 2.76 TeV, JHEP 03 (2016) 081 NN s Djordjevic =5.02 TeV NN s =2.76 TeV NN s ALICE Pb-Pb 0-10% Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp Figure 6. Left panel: average RAA of prompt D0, D+and D∗+mesons by ALICE compared to RAA of prompt D0mesons by CMS [26] in the 0–10% centrality class and at √sNN = 5.02 TeV. Statistical (bars), systematic (empty boxes), and normalisation (shaded box around unity) uncertainties are shown. Right panel: average RAA of D0, D+and D∗+mesons compared with the Djordjevic model [54] in the 0–10% centrality class at two collision energies. Statistical (bars), systematic (empty boxes), and normalisation (shaded box) uncertainties are shown. 5 10 15 20 25 30 35 )c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 AA R 80%−60 5 10 15 20 25 30 35 )c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 AA R 50%−30 | < 0.5y, | *+ , D + , D 0 Average D | < 0.8 η Charged particles, | arXiv:1802.09145 | < 0.5y, | *+ , D + , D 0 Average D | < 0.8 η Charged particles, | arXiv:1802.09145 10 20 30 40 50 )c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 AA R ALICE = 5.02 TeV NN s Pb-Pb, 10%−0 5 10 15 20 25 30 35 )c (GeV/ T p 0 0.5 1 1.5 2 2.5 3 ch AA R/ D AA R 5 10 15 20 25 30 35 )c (GeV/ T p 0 0.5 1 1.5 2 2.5 3 ch AA R/ D AA R 10 20 30 40 50 )c (GeV/ T p 0 0.5 1 1.5 2 2.5 3 ch AA R/ D AA R Figure 7. Average RAA of prompt D0, D+and D∗+mesons in the 0–10% (left), 30–50% (middle) and 60–80% (right) centrality classes at √sNN = 5.02 TeV compared to the RAA of charged particles in the same centrality classes [59]. The ratios of the RAA are shown in the bottom panels. Statistical (bars), systematic (empty boxes), and normalisation (shaded box around unity) uncertainties are shown. – 17 – JHEP10(2018)174 pT<8 GeV/c in central and semi-central collisions is not straightforward, because several factors can play a role in defining the shape of the RAA. In presence of a colour-charge and quark-mass dependent energy loss, the harder pT distribution and the harder fragmentation function of charm quarks compared to those of light quarks and gluons should lead to similar values of D-meson and pion RAA, as discussed in [60]. Since the pions are the dominant contribution in the inclusive chargedparticle yields, this statement is expected to be still valid for the comparison of the D- meson and the charged particle RAA. In addition, it should be considered that the yield of light-flavour hadrons could have a substantial contribution up to transverse momenta of about 2–3 GeV/c from soft production processes, such as the break-down of participant nucleons into quarks and gluons that subsequently hadronise. This component scales with the number of participants rather than the number of binary collisions. Finally, the effects of radial flow and hadronisation via recombination, as well as initial-state effects, could affect D-meson and light-hadron yields differently at a given pT. The average RAA of the three non-strange D-meson species in the three centrality classes are compared with theoretical models in figure 8. Models based on heavy-quark transport and models based on perturbative QCD calculations of high-pTparton energy loss are shown in the left and in the right panels, respectively. Transport models in the left panels include: BAMPS el. [57], POWLANG [61] and TAMU [55], in which the interactions are only described by collisional (i.e. elastic) processes; BAMPS el.+rad. [57], LBT [58], MC@sHQ+EPOS2 [62] and PHSD [63], in which also energy loss from medium-induced gluon radiation is considered, in addition to collisional process. In the right panels, the CUJET3.0 [64] and Djordjevic [54] models include both radiative and collisional energy loss processes, while the SCET [65] model implements medium-induced gluon radiation via modified splitting functions with finite quark masses.2All models, with the exception of BAMPS and CUJET3.0, include a nuclear modification of the parton distribution functions. The LBT, MC@sHQ, PHSD, POWLANG and TAMU models include a contribution of hadronisation via quark recombination, in addition to independent fragmentation. Most of the models provide a fair description of the data in the region pT<10 GeV/c in central collisions (except for BAMPS el., where the radiative term is missing), but many of them (LBT, PHSD, POWLANG and SCET) provide a worse description of non-central collisions. In the high-pTregion above 10 GeV/c only the BAMPS el.+rad., CUJET3.0, Djordjevic, MC@sHQ+EPOS2 and SCET models can describe the data in central collisions. The CUJET3.0 and Djordjevic models provide a fair description of the RAA in all three centrality classes for pT>10 GeV/c, where radiative energy loss is expected to be the dominant interaction mechanism, suggesting that the dependence of radiative energy loss on the path length in the hot and dense medium is well understood. In figure 9, the non-strange and strange D-meson RAA are compared with the models that provide both observables. An increase of the D+ sRAA is expected in the two models, PHSD and TAMU, in particular for pT<5 GeV/c, with respect to non-strange D mesons. 2The SCET curves reported here differ from those of ref. [65] because the latter used an extrapolation of the charged-particle multiplicity at √sNN = 5.02 TeV, while now the measured values are used. – 18 – JHEP10(2018)174 1 10 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp + , D* + , D 0 Average D BAMPS el.+rad. BAMPS el. POWLANG HTL PHSD LBT TAMU MC@sHQ+EPOS2 ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y| 5 10 15 20 25 30 35 40 45 50 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp + , D* + , D 0 Average D Djordjevic CUJET3.0 g=1.9-2.0 M,G SCET 1 10 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 30-50% Pb-Pb, |<0.5y| + , D* + , D 0 Average D BAMPS el.+rad. BAMPS el. POWLANG HTL PHSD LBT TAMU MC@sHQ+EPOS2 ALICE = 5.02 TeV NN s 30-50% Pb-Pb, |<0.5y| 5 10 15 20 25 30 35 40 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 30-50% Pb-Pb, |<0.5y| + , D* + , D 0 Average D Djordjevic CUJET3.0 g=1.9-2.0 M,G SCET 1 10 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 60-80% Pb-Pb, |<0.5y| + , D* + , D 0 Average D POWLANG HTL PHSD LBT TAMU ALICE = 5.02 TeV NN s 60-80% Pb-Pb, |<0.5y| 5 10 15 20 25 30 35 40 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 60-80% Pb-Pb, |<0.5y| + , D* + , D 0 Average D Djordjevic CUJET3.0 g=1.9-2.0 M,G SCET Figure 8. Average RAA of D0, D+and D∗+mesons compared with model calculations. The three rows refer to the 0–10%, 30–50% and 60–80% centrality classes. The left panels show models based on heavy-quark transport, while the right panels show models based on pQCD energy loss. Model nomenclature and references: BAMPS [57], CUJET3.0 [64], Djordjevic [54], LBT [58], MC@sHQ+EPOS2 [62], PHSD [63] POWLANG [61], SCET [65], TAMU [55]. Some of the models are presented with two lines with the same style and colour, which encompass the model uncertainty band. – 19 – JHEP10(2018)174 5 10 15 20 25 30 35 40 45 50 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp + , D* + , D 0 Average D + s D + , D* + , D 0 PHSD, Average D + s PHSD, D + , D* + , D 0 TAMU, Average D + s TAMU, D Figure 9. Average RAA of D0, D+and D∗+mesons and RAA of D+ smesons in the 0–10% centrality class compared with the PHSD [63] and TAMU [55] model calculations. This increase is induced by hadronisation via quark recombination in the QGP, as well as by different interaction cross sections for non-strange D and for D+ sin the hadronic phase of the system evolution. In the transverse momentum interval covered by the D+ s measurement (pT>4 GeV/c), the PHSD model predicts the effect to be very small, while the TAMU model predicts a sizeable difference of about 30% up to about 8 GeV/c, similar to the trend shown by the data. The simultaneous comparison of RAA and elliptic flow v2measurements at √sNN = 5.02 TeV [66] with models can provide more stringent constraints to the implementation of the interaction and hadronisation processes for heavy quarks in the QGP. The comparison with models that compute both observables is shown in figure 10 for the RAA and v2, in the 0–10% and 30–50% centrality classes, respectively. The TAMU model overestimates RAA and underestimates v2at high pT, probably because it does not include radiative energy loss. The BAMPS el. model overestimates the maximum flow while underestimating the RAA value at high pT. The radiative energy loss contribution in BAMPS el.+rad. improves the description of RAA but gives v2values lower than the data. The LBT, PHSD, POWLANG and MC@sHQ models provide instead a fair description of v2. Nevertheless, energy loss is overestimated at high pTin the 0–10% centrality classes (but also in semicentral events) by PHSD, POWLANG and LBT, while at low pTthe measured RAA is slightly higher than what predicted within LBT, PHSD and MC@sHQ. 7 Summary We have presented measurements of the pT-differential production yields of prompt D0, D+, D∗+and D+ smesons at central rapidity in Pb–Pb collisions in the three centrality classes 0–10%, 30–50% and 60–80% at a centre-of-mass energy per nucleon pair √sNN = 5.02 TeV. – 20 – JHEP10(2018)174 5 10 15 20 25 30 35 40 45 50 ) c (GeV/ T p 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 AA R ALICE = 5.02 TeV NN s 0-10% Pb-Pb, |<0.5y| Filled markers: pp rescaled reference -extrapolated reference T pOpen markers: pp + , D* + , D 0 Average D TAMU PHSD POWLANG HTL MC@sHQ+EPOS2 LBT BAMPS el.+rad. BAMPS el. )c (GeV/ T p 0 2 4 6 8 10 12 14 16 18 20 22 24 |>0.9}η∆{EP, | 2 v 0 0.1 0.2 0.3 average + , D* + , D 0 D Syst. from data Syst. from B feed-down LBT BAMPS el.+rad. BAMPS el. TAMU PHSD POWLANG HTL MC@sHQ+EPOS2 = 5.02 TeV NN sPb, −50% Pb−30 |<0.8y|ALICE Figure 10. Average RAA of D0, D+and D∗+mesons in the 0–10% centrality class (left) and their average elliptic flow v2in the 30–50% centrality class (right) [66], compared with models that have predictions for both observables at low pT. The average RAA of the three non-strange D-meson species shows minimum values of 0.2 and 0.4 in the centrality classes 0–10% and 30–50%, respectively, at pTof 6–10 GeV/c. RAA increases for pT<6 GeV/c, and it is compatible with unity at 1 < pT<3 GeV/c. The average RAA values are compatible with those measured at √sNN = 2.76 TeV and they have smaller uncertainties by a factor of about two, as well as extended pTcoverage up to 50 GeV/c in central collisions. The similarity of the RAA values at the two energies was predicted by the Djordjevic model, and it results from the combination of a higher medium temperature at 5.02 TeV (estimated to be about 7% higher than at 2.76 TeV) with a harder pTdistribution of charm quarks at 5.02 TeV. In central and semi-central collisions the average RAA of non-strange D mesons is compatible with that of charged particles for pT>6 GeV/c, while it is larger at lower pT. The RAA of D+ smesons have generally larger central values than those of the average of non-strange D mesons, but the two measurements are compatible within about one standard deviation of the combined uncertainties. The RAA of non-strange D mesons at high pT(above 10 GeV/c) is fairly described in the three centrality classes by model calculations that include both radiative and collisional energy loss. This indicates that the centrality dependence of radiative energy loss, which is the dominant contribution at high pT, is under good theoretical control. The RAA in the transverse momentum region below 10 GeV/c is described by several transport model calculations in central collisions, but most models fail in describing the centrality dependence of RAA and in describing simultaneously RAA and the elliptic flow coefficient v2. Therefore, the measurements provide significant constraints for the understanding of the interaction of charm quarks with the high-density QCD medium, especially at low and intermediate pT, where the RAA is the result of a more complex interplay among several effects. 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 – 21 – JHEP10(2018)174 teams for the outstanding performance of the LHC complex. The ALICE collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: A.I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences and Nationalstiftung f¨ur Forschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq), Universidade Federal do Rio Grande do Sul (UFRGS), Financiadora de Estudos e Projetos (Finep) and Funda¸c˜ao de Amparo `a Pesquisa do Estado de S˜ao Paulo (FAPESP), Brazil; Ministry of Science & Technology of China (MSTC), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC), China; Ministry of Science and Education, Croatia; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research — Natural Sciences, the Carlsberg Foundation and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat `a l’Energie Atomique (CEA) and Institut National de Physique Nucl´eaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium f¨ur Bildung, Wissenschaft, Forschung und Technologie (BMBF) and GSI Helmholtzzentrum f¨ur Schwerionenforschung GmbH, Germany; General Secretariat for Research and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy Government of India (DAE), Department of Science and Technology, Government of India (DST), University Grants Commission, Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; Indonesian Institute of Science, Indonesia; Centro Fermi — Museo Storico della Fisica e Centro Studi e Ricerche Enrico Fermi and Istituto Nazionale di Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology, Nagasaki Institute of Applied Science (IIST), Japan Society for the Promotion of Science (JSPS) KAKENHI and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnolog´ıa, through Fondo de Cooperaci´on Internacional en Ciencia y Tecnolog´ıa (FONCICYT) and Direcci´on General de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Cat´olica del Per´u, Peru; Ministry of Science and Higher Education and National Science Centre, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics and Romanian National Agency for Science, Technology and Innovation, Romania; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the Russian Federation and National Research Centre Kurchatov Institute, Russia; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National – 22 – JHEP10(2018)174 Research Foundation of South Africa, South Africa; Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Cubaenerg´ıa, Cuba and Centro de Investigaciones Energ´eticas, Medioambientales y Tecnol´ogicas (CIEMAT), Spain; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; National Science and Technology Development Agency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Education Commission under NRU project of Thailand, Thailand; Turkish Atomic Energy Agency (TAEK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America. 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Vrl´akov´a39, B. Wagner24, H. Wang63, M. Wang7, Y. Watanabe130,129, M. Weber110, S.G. Weber103, A. Wegrzynek36, D.F. Weiser101, S.C. Wenzel36, J.P. Wessels141, U. Westerhoff141, A.M. Whitehead122, J. Wiechula69, J. Wikne23, G. Wilk84, J. Wilkinson53, G.A. Willems141,36, M.C.S. Williams53, E. Willsher107, B. Windelband101, W.E. Witt127, R. Xu7, S. Yalcin77, K. Yamakawa45, S. Yano45, Z. Yin7, H. Yokoyama130,78, I.-K. Yoo20, J.H. Yoon60, V. Yurchenko3, V. Zaccolo58, A. Zaman16, C. Zampolli36, H.J.C. Zanoli118, N. Zardoshti107, A. Zarochentsev137, P. Z´avada67, N. Zaviyalov105, H. Zbroszczyk139, M. Zhalov95, X. Zhang7, Y. Zhang7, Z. Zhang131,7, C. Zhao23, V. Zherebchevskii137, N. Zhigareva64, D. Zhou7, Y. Zhou88, Z. Zhou24, H. Zhu7, J. Zhu7, Y. Zhu7, A. Zichichi29,11, M.B. Zimmermann36, G. Zinovjev3, J. Zmeskal110, S. Zou7 iDipartimento DET del Politecnico di Torino, Turin, Italy ii M.V. Lomonosov Moscow State University, D.V. Skobeltsyn Institute of Nuclear, Physics, Moscow, Russia iii Department of Applied Physics, Aligarh Muslim University, Aligarh, India iv Institute of Theoretical Physics, University of Wroclaw, Poland 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, National Academy of Sciences of Ukraine, 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, Lyon, France 9Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Havana, Cuba 10 Centro de Investigaci´on y de Estudios Avanzados (CINVESTAV), Mexico City and M´erida, Mexico 11 Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche ‘Enrico Fermi’, Rome, Italy 12 Chicago State University, Chicago, Illinois, United States 13 China Institute of Atomic Energy, Beijing, China 14 Chonbuk National University, Jeonju, Republic of Korea – 31 – JHEP10(2018)174 15 Comenius University Bratislava, Faculty of Mathematics, Physics and Informatics, Bratislava, Slovakia 16 COMSATS Institute of Information Technology (CIIT), Islamabad, Pakistan 17 Creighton University, Omaha, Nebraska, United States 18 Department of Physics, Aligarh Muslim University, Aligarh, India 19 Department of Physics, Ohio State University, Columbus, Ohio, United States 20 Department of Physics, Pusan National University, Pusan, Republic of Korea 21 Department of Physics, Sejong University, Seoul, Republic of Korea 22 Department of Physics, University of California, Berkeley, California, United States 23 Department of Physics, University of Oslo, Oslo, Norway 24 Department of Physics and Technology, University of Bergen, Bergen, Norway 25 Dipartimento di Fisica dell’Universit`a ‘La Sapienza’ and Sezione INFN, Rome, Italy 26 Dipartimento di Fisica dell’Universit`a and Sezione INFN, Cagliari, Italy 27 Dipartimento di Fisica dell’Universit`a and Sezione INFN, Trieste, Italy 28 Dipartimento di Fisica dell’Universit`a and Sezione INFN, Turin, Italy 29 Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Bologna, Italy 30 Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Catania, Italy 31 Dipartimento di Fisica e Astronomia dell’Universit`a and Sezione INFN, Padova, Italy 32 Dipartimento di Fisica ‘E.R. Caianiello’ dell’Universit`a and Gruppo Collegato INFN, Salerno, Italy 33 Dipartimento DISAT del Politecnico and Sezione INFN, Turin, Italy 34 Dipartimento di Scienze e Innovazione Tecnologica dell’Universit`a del Piemonte Orientale and INFN Sezione di Torino, Alessandria, Italy 35 Dipartimento Interateneo di Fisica ‘M. Merlin’ and Sezione INFN, Bari, Italy 36 European Organization for Nuclear Research (CERN), Geneva, Switzerland 37 Faculty of Engineering and Science, Western Norway University of Applied Sciences, Bergen, Norway 38 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Prague, Czech Republic 39 Faculty of Science, P.J. ˇ Saf´arik University, Koˇsice, Slovakia 40 Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany 41 Gangneung-Wonju National University, Gangneung, Republic of Korea 42 Gauhati University, Department of Physics, Guwahati, India 43 Helmholtz-Institut f¨ur Strahlen- und Kernphysik, Rheinische Friedrich-Wilhelms-Universit¨at Bonn, Bonn, Germany 44 Helsinki Institute of Physics (HIP), Helsinki, Finland 45 Hiroshima University, Hiroshima, Japan 46 Hochschule Worms, Zentrum f¨ur Technologietransfer und Telekommunikation (ZTT), Worms, Germany 47 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 48 Indian Institute of Technology Bombay (IIT), Mumbai, India 49 Indian Institute of Technology Indore, Indore, India 50 Indonesian Institute of Sciences, Jakarta, Indonesia 51 INFN, Laboratori Nazionali di Frascati, Frascati, Italy 52 INFN, Sezione di Bari, Bari, Italy 53 INFN, Sezione di Bologna, Bologna, Italy 54 INFN, Sezione di Cagliari, Cagliari, Italy 55 INFN, Sezione di Catania, Catania, Italy 56 INFN, Sezione di Padova, Padova, Italy 57 INFN, Sezione di Roma, Rome, Italy 58 INFN, Sezione di Torino, Turin, Italy 59 INFN, Sezione di Trieste, Trieste, Italy – 32 – JHEP10(2018)174 60 Inha University, Incheon, Republic of Korea 61 Institut de Physique Nucl´eaire d’Orsay (IPNO), Institut National de Physique Nucl´eaire et de Physique des Particules (IN2P3/CNRS), Universit´e de Paris-Sud, Universit´e Paris-Saclay, Orsay, France 62 Institute for Nuclear Research, Academy of Sciences, Moscow, Russia 63 Institute for Subatomic Physics, Utrecht University/Nikhef, Utrecht, Netherlands 64 Institute for Theoretical and Experimental Physics, Moscow, Russia 65 Institute of Experimental Physics, Slovak Academy of Sciences, Koˇsice, Slovakia 66 Institute of Physics, Bhubaneswar, India 67 Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 68 Institute of Space Science (ISS), Bucharest, Romania 69 Institut f¨ur Kernphysik, Johann Wolfgang Goethe-Universit¨at Frankfurt, Frankfurt, Germany 70 Instituto de Ciencias Nucleares, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico 71 Instituto de F´ısica, Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, Brazil 72 Instituto de F´ısica, Universidad Nacional Aut´onoma de M´exico, Mexico City, Mexico 73 iThemba LABS, National Research Foundation, Somerset West, South Africa 74 Johann-Wolfgang-Goethe Universit¨at Frankfurt Institut f¨ur Informatik, Fachbereich Informatik und Mathematik, Frankfurt, Germany 75 Joint Institute for Nuclear Research (JINR), Dubna, Russia 76 Korea Institute of Science and Technology Information, Daejeon, Republic of Korea 77 KTO Karatay University, Konya, Turkey 78 Laboratoire de Physique Subatomique et de Cosmologie, Universit´e Grenoble-Alpes, CNRS-IN2P3, Grenoble, France 79 Lawrence Berkeley National Laboratory, Berkeley, California, United States 80 Lund University Department of Physics, Division of Particle Physics, Lund, Sweden 81 Nagasaki Institute of Applied Science, Nagasaki, Japan 82 Nara Women’s University (NWU), Nara, Japan 83 National and Kapodistrian University of Athens, School of Science, Department of Physics, Athens, Greece 84 National Centre for Nuclear Research, Warsaw, Poland 85 National Institute of Science Education and Research, HBNI, Jatni, India 86 National Nuclear Research Center, Baku, Azerbaijan 87 National Research Centre Kurchatov Institute, Moscow, Russia 88 Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 89 Nikhef, National institute for subatomic physics, Amsterdam, Netherlands 90 NRC Kurchatov Institute, IHEP, Protvino, Russia 91 NRNU Moscow Engineering Physics Institute, Moscow, Russia 92 Nuclear Physics Group, STFC Daresbury Laboratory, Daresbury, United Kingdom 93 Nuclear Physics Institute of the Czech Academy of Sciences, ˇ Reˇz u Prahy, Czech Republic 94 Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States 95 Petersburg Nuclear Physics Institute, Gatchina, Russia 96 Physics department, Faculty of science, University of Zagreb, Zagreb, Croatia 97 Physics Department, Panjab University, Chandigarh, India 98 Physics Department, University of Jammu, Jammu, India 99 Physics Department, University of Rajasthan, Jaipur, India 100 Physikalisches Institut, Eberhard-Karls-Universit¨at T¨ubingen, T¨ubingen, Germany 101 Physikalisches Institut, Ruprecht-Karls-Universit¨at Heidelberg, Heidelberg, Germany 102 Physik Department, Technische Universit¨at M¨unchen, Munich, Germany 103 Research Division and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum f¨ur Schwerionenforschung GmbH, Darmstadt, Germany 104 Rudjer Boˇskovi´c Institute, Zagreb, Croatia 105 Russian Federal Nuclear Center (VNIIEF), Sarov, Russia – 33 – JHEP10(2018)174 106 Saha Institute of Nuclear Physics, Kolkata, India 107 School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 108 Secci´on F´ısica, Departamento de Ciencias, Pontificia Universidad Cat´olica del Per´u, Lima, Peru 109 Shanghai Institute of Applied Physics, Shanghai, China 110 Stefan Meyer Institut f¨ur Subatomare Physik (SMI), Vienna, Austria 111 SUBATECH, IMT Atlantique, Universit´e de Nantes, CNRS-IN2P3, Nantes, France 112 Suranaree University of Technology, Nakhon Ratchasima, Thailand 113 Technical University of Koˇsice, Koˇsice, Slovakia 114 Technische Universit¨at M¨unchen, Excellence Cluster ‘Universe’, Munich, Germany 115 The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland 116 The University of Texas at Austin, Austin, Texas, United States 117 Universidad Aut´onoma de Sinaloa, Culiac´an, Mexico 118 Universidade de S˜ao Paulo (USP), S˜ao Paulo, Brazil 119 Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil 120 Universidade Federal do ABC, Santo Andre, Brazil 121 University College of Southeast Norway, Tonsberg, Norway 122 University of Cape Town, Cape Town, South Africa 123 University of Houston, Houston, Texas, United States 124 University of Jyv¨askyl¨a, Jyv¨askyl¨a, Finland 125 University of Liverpool, Department of Physics Oliver Lodge Laboratory, Liverpool, United Kingdom 126 University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, Split, Croatia 127 University of Tennessee, Knoxville, Tennessee, United States 128 University of the Witwatersrand, Johannesburg, South Africa 129 University of Tokyo, Tokyo, Japan 130 University of Tsukuba, Tsukuba, Japan 131 Universit´e Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France 132 Universit´e de Lyon, Universit´e Lyon 1, CNRS/IN2P3, IPN-Lyon, Villeurbanne, Lyon, France 133 Universit´e de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France, Strasbourg, France 134 Universit´e Paris-Saclay Centre d ´ Etudes de Saclay (CEA), IRFU, Department de Physique Nucl´eaire (DPhN), Saclay, France 135 Universit`a degli Studi di Pavia, Pavia, Italy 136 Universit`a di Brescia, Brescia, Italy 137 V. Fock Institute for Physics, St. Petersburg State University, St. Petersburg, Russia 138 Variable Energy Cyclotron Centre, Kolkata, India 139 Warsaw University of Technology, Warsaw, Poland 140 Wayne State University, Detroit, Michigan, United States 141 Westf¨alische Wilhelms-Universit¨at M¨unster, Institut f¨ur Kernphysik, M¨unster, Germany 142 Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary 143 Yale University, New Haven, Connecticut, United States 144 Yonsei University, Seoul, Republic of Korea – 34 –