The measurement of heavy-flavour production in ultra-relativistic heavy-ion collisions provides insights into the properties of the Quark-Gluon Plasma (QGP), the state of strongly-interacting matter characterised by high temperature

and energy density where quarks and gluons are deconfined. ALICE results on heavy-flavour production in p–Pb and Pb–Pb collisions are presented

Festanti, A.

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DOI 10.1393/ncc/i2019-19081-2Colloquia: EuNPC 2018IL NUOVO CIMENTO 42 C (2019) 81Heavy-flavour production measurements in heavy-ion collisionswith ALICE at the LHCA. Festanti on behalf of the ALICE CollaborationCERN - Geneva, Switzerlandreceived 5 February 2019Summary. — The measurement of heavy-flavour production in ultra-relativisticheavy-ion collisions provides insights into the properties of the Quark-Gluon Plasma(QGP), the state of strongly-interacting matter characterised by high temperatureand energy density where quarks and gluons are deconfined. ALICE results onheavy-flavour production in p–Pb and Pb–Pb collisions are presented.1. – IntroductionHeavy quarks are powerful probes to study the properties of the QGP formed inultra-relativistic heavy-ion collisions at the Large Hadron Collider (LHC). Charm andbeauty are mainly produced in hard scatterings before the QGP formation and theyexperience all stages of the medium evolution. Heavy quarks lose part of their energyinteracting with medium constituents via gluon radiation and collisional processes (see[1,2] for recent reviews). In the medium, heavy quarks of low momentum can hadronisevia recombination with other quarks, in addition to vacuum fragmentation [3, 4]. Re-combination, in the strange-quark rich environment, is expected to lead to an increaseof the production of D+s -meson in comparison to non-strange D mesons [5]. It may alsoenhance the charm baryon-to-meson production ratio relative to that in proton–proton(pp) collisions at low and intermediate momentum [6-8]. The energy loss and the dy-namics of heavy-quark hadronisation are studied using the nuclear modification factorRAA = (dNAA/dpT)/(〈TAA〉 × dσpp/dpT), which compares the transverse-momentum(pT) differential production yields in nucleus–nucleus collisions (dNAA/dpT) with thecross section in pp collisions (dσpp/dpT) scaled by the average nuclear overlap function(〈TAA〉). Heavy quarks can participate in the collective expansion of the system [9].Measurements of anisotropies in the azimuthal distribution of heavy-flavour hadrons aresensitive to these phenomena. The anisotropy is quantified through the second-orderFourier coefficient v2 = 〈cos[2(ϕ − Ψ2)]〉 of the azimuthal distribution of particle mo-menta relative to the symmetry-plane angles (Ψn), denoted as elliptic flow.The presence of a nucleus in the colliding system can modify the production of heavyquarks even if the QGP is not formed. Cold Nuclear Matter (CNM) effects are studiedvia the measurement of heavy-flavour production in p–Pb collisions.c© CERN on behalf of the ALICE CollaborationCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. FESTANTI on behalf of the ALICE COLLABORATION0 5 10 15 20 25 30 35)c (GeV/Tp0.20.40.60.811.21.41.61.8pPbR ALICE Preliminary  = 5.02 TeVNNsPb, −p<0.04cmsyPrompt D mesons, -0.96<*+, D+, D0Average D = 5.02 TeVsmeasured pp reference at CGC (Fujii-Watanabe)FONLL with EPPS16 nPDFKang et al.: incoherent multiple scattering broad + CNM ElossTVitev et al.: power corr. + kDukePOWLANG (HTL)POWLANG (lQCD))c (GeV/Tp0 5 10 15 20 25 30 35CPQ00.20.40.60.811.21.41.61.82 ALICE Preliminary = 5.02 TeVNNsPb, −p+, D*+, D0Average DCharged particles10% ZN energy−Central: 0100% ZN energy−Peripheral: 60Fig. 1. – Left: prompt D-meson RpPb in p–Pb collisions at√sNN = 5.02 TeV in −0.96 < ycms <0.04 compared to model calculations [11]. Right: prompt D-meson average central-to-peripheralratio compared to that of charged particles.Heavy-flavour production was studied with ALICE [10] in p–Pb collisions at√sNN =5.02 TeV and in Pb–Pb collisions at√sNN = 2.76, 5.02 TeV by measuring electronsand muons from semi-leptonic decays of heavy-flavour hadrons and fully reconstructedD-meson and Λ+c hadronic decays. Electron tracks were reconstructed and identifiedin the central rapidity region using the information provided by the Time ProjectionChamber (TPC), the Time Of Flight (TOF) detector and the electromagnetic calorimeter(EMCAL). Muon tracks were reconstructed in the forward Muon Spectrometer (−4 <η < −2.5). D mesons and Λ+c -baryons displaced decay vertices were reconstructed atmid-rapidity (|y| < 0.5) exploiting the high track spatial resolution of the Inner TrackingSystem (ITS). Charged pions, kaons and protons were identified using TPC and TOFsignals.2. – Results in p–Pb collisionsThe average of the nuclear modification factor RpPb of prompt D0, D+ and D∗+mesons is shown in the left panel of fig. 2 [11]. The RpPb was computed by dividing thepT-differential cross section in p–Pb collisions at√sNN = 5.02 TeV by the cross sectionmeasured in pp collisions at the same collision energy, multiplied by the Pb atomic massnumber. The D-meson RpPb is compatible with unity over the pT interval covered by themeasurement. The experimental results are described by models including CNM effectsand they disfavour a suppression larger than 10% in the interval 3 < pT < 12 GeV/c,in contrast to expectations of Duke and POWLANG HTL models, which assume theformation of a QGP also in p–Pb collisions and predict a moderate suppression at highpT. The right panel of fig. 2 shows the central-to-peripheral ratio (QCP), defined asthe ratio of the yields in the 0–10% class to that in the 60–80% centrality class, bothnormalised by the average nuclear overlap function in the corresponding centrality class,of prompt non-strange D mesons compared to that of charged particles. A similar trendis observed for the two measurements and the D-meson QCP is larger than unity by1.5 standard deviations of the statistical and systematic uncertainties in the interval3 < pT < 8 GeV/c.HEAVY-FLAVOUR PRODUCTION MEASUREMENTS ETC. 3)c (GeV/Tp10 20 30 40 50AAR00.511.52ALICE = 5.02 TeVNNsPb, PbPb reference from JHEP 04 (2018) 108p80%|<0.5, 0y, |+c10% (arXiv:1804.09083)|<0.5, 0y, |*+, D+, D0Average D10% (arXiv:1804.09083)|<0.5, 0y, |s+D10% (arXiv:1802.09145)|<0.8, 0charged particles, |)c (GeV/Tp1 10AAR00.20.40.60.811.21.41.61.82ALICE Preliminary = 5.02 TeVNNs0-10% Pb-Pb,  < 4y, 2.5 < μ →c,b  e→c,b | < 0.8yITS+TOF+TPC eID, || < 0.6yTPC+EMCal eID, |Fig. 2. – Left: RAA of prompt Λ+c in 0–80% Pb–Pb collisions at√sNN = 5.02 TeV [13] comparedto that of non-strange D mesons, D+s -meson, and charged particle in 0–10% centrality class.Right: RAA of electrons and muons from heavy-flavour hadron decays in 10% most centralPb–Pb collisions at√sNN = 5.02 TeV measured at mid and forward rapidity, respectively.3. – Results in Pb–Pb collisionsA strong suppression of the D-meson RAA for pT > 5 GeV/c was measured in the10% most central Pb–Pb collisions at√sNN = 5.02 TeV (fig. 3 left) [12]. A similarsuppression was observed for electrons and for muons coming from heavy-flavour hadrondecays measured at mid and forward rapidity, respectively (fig. 3 right). The left panel offig. 3 shows the comparison of prompt Λ+c -baryon RAA measured in the 0–80% centralityclass [13] to that of non-strange D mesons, D+s and charged particles [14] measured in the0-10% centrality class. The RAA of charged particles is smaller than that of D mesons bymore than 2σ (combined statistical and systematic uncertainties) up to pT = 8 GeV/c,while the results are compatible within 1σ for pT > 10 GeV/c. The observed difference isimportant to set constraints on the dependence of energy loss on the Casimir factor andon the quark mass [15]. Although compatible within 1σ, the D+s RAA values are largerthan those of non-strange D mesons. A hint of larger Λ+c RAA than that of non-strangeD mesons is observed.In the left panel of fig. 3 the Λ+c /D0 ratio measured in Pb–Pb collisions at√sNN =5.02 TeV [13] is compared to the results in pp and p–Pb collisions at√s = 7 TeV and√sNN = 5.02 TeV [16], respectively. The ratio measured in Pb–Pb collisions is higherthan that measured in pp and p–Pb collisions.The non-strange D-meson v2 measured in Pb–Pb collisions at√sNN = 5.02 TeV inthe 30–50% centrality class [17] is larger than zero in 2 < pT < 10 GeV/c (right panelof fig. 3). The result is described by models where the charm azimuthal anisotropy ismainly caused by processes like elastic collisions or hadronisation via coalescence thattransfer to charm particles the anisotropy of the hydrodinamically-expanding medium.The calculations that describe the data with χ2/ndf < 1 provide a charm quark thermal-isation time of τcharm ∼ 3−14 fm/c (mcharm = 1.5 GeV/c2), comparable to the estimateddecoupling time of the system [17].4 A. FESTANTI on behalf of the ALICE COLLABORATION)c (GeV/Tp0 2 4 6 8 10 120/D+ c0123|<0.5y = 5.02 TeV, |NNsPb, 80% Pb0(JHEP 04 (2018) 108)|<0.5y = 7 TeV, |spp, (JHEP 04 (2018) 108)<0.04y0.96< = 5.02 TeV, NNsPb, pALICE)c (GeV/Tp0 2 4 6 8 10 12 14 16 18 20 22 24|>0.9}ηΔ{EP, |2v00.10.20.3 average+, D*+, D0DSyst. from dataSyst. from B feed-downLBTBAMPS el.+rad.BAMPS el.TAMUPHSDPOWLANG HTLMC@sHQ+EPOS2 = 5.02 TeVNNsPb, −50% Pb−30|<0.8y| ALICEFig. 3. – Left: Λ+c /D0 ratio in Pb–Pb collisions at√sNN = 5.02 TeV [13] compared with themeasurements in pp and p–Pb collisions [16]. Right: average D0, D+ and D∗+ elliptic flow v2in Pb–Pb collisions at√sNN = 5.02 TeV in the 30–50% centrality class compared with modelcalculations [17].4. – ConclusionsThe suppression of heavy-flavour production measured in central Pb–Pb collisionsdemonstrates that charm quarks interact strongly with the high-density QCD medium.The results in p–Pb collisions confirm that the effects observed at intermediate/high pTin Pb–Pb collisions are induced by the presence of the QGP. The first measurement ofthe Λ+c -baryon production in Pb–Pb collisions shows a hint of a larger yield relativeto that of the D0 meson with respect to those measured in pp and p–Pb collisions, inline with what expected in a scenario where charm quarks hadronise, at least partly,via coalescence with light quarks from the medium. The measured non-zero D-mesonv2 indicates that low-momentum charm quarks take part in the collective motion of theQGP and that collisional interaction processes as well as recombination of charm andlight quarks contribute to the observed elliptic flow.REFERENCES[1] A. Andronic et al., Eur. Phys. J. C, 76 (2016) 107[2] F. Prino and R. Rapp, J. Phys. G, 43 (2016) 093002[3] V. Greco, C. M. Ko and R. Rapp, Phys. Lett. B, 595 (2004) 202[4] A. Andronic et al., Phys. Lett. B, 571 (2003) 36-44[5] I. Kuznetsova and J. Rafelski, J. Phys. G, 32 (2006) S499[6] S. Plumari et al., Eur. Phys. J. C, 78 (2018) 348[7] Y. Oh et al., Phys. Rev. C, 79 (2009) 044905[8] S. H. Lee et al., Phys. Rev. Lett. , 100 (2008) 222301[9] S. Batsouli, S. Kelly, M. Gyulassy and J. L. Nagle, Phys. Lett. B, 557 (2003) 26[10] [ALICE Collaboration], B. B. Abelev et al., Int. J. Mod. Phys. A, 29 (2014) 1430044[11] [ALICE Collaboration], preprint ALICE-PUBLIC-2017-008[12] [ALICE Collaboration], S. 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Heavy-flavour production measurements in heavy-ion collisions

with ALICE at the LHC

http://eprints.bice.rm.cnr.it/20481/1/ncc11821.pdf

Heavy-flavour production measurements in heavy-ion collisions with ALICE at the LHC

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