# Heavy-flavor measurements in heavy-ion collisions with the ALICE experimentAcademic research paper on "Physical sciences"

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## Abstract of research paper on Physical sciences, author of scientific article — Zaida Conesa del Valle

Abstract Open and hidden heavy-flavor measurements with the ALICE experiment at the LHC are reported. Emphasis goes to the recent results in p–Pb and Pb–Pb collisions at s N N = 5.02 and 2.76 TeV respectively. Heavy-flavor measurements are presented in the form of either the ratio of the production cross sections in heavy-ion and pp collisions normalized by the average number of nucleon-nucleon collisions, or the per-event yields as a function of charged-particle multiplicity. Possible interpretations of these results in pp, p–Pb or Pb–Pb collisions in terms of multi-parton interactions, gluon saturation, initial or final state energy loss, system collective motion, color-charge screening or recombination of uncorrelated quarks are discussed.

## Academic research paper on topic "Heavy-flavor measurements in heavy-ion collisions with the ALICE experiment"

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Nuclear and Particle Physics Proceedings 273-275 (2016) 1582-1587

www.elsevier.com/locate/nppp

Heavy-flavor measurements in heavy-ion collisions with the ALICE experiment

Zaida Conesa del Valle (for the ALICE Collaboration)

Institut de Physique Nucléaire d'Orsay (CNRS/IN2P3 - Université Paris-Sud, Orsay, France)

Abstract

Open and hidden heavy-flavor measurements with the ALICE experiment at the LHC are reported. Emphasis goes to the recent results in p-Pb and Pb-Pb collisions at t/sNN = 5.02 and 2.76 TeV respectively. Heavy-flavor measurements are presented in the form of either the ratio of the production cross sections in heavy-ion and pp collisions normalized by the average number of nucleon-nucleon collisions, or the per-event yields as a function of charged-particle multiplicity. Possible interpretations of these results in pp, p-Pb or Pb-Pb collisions in terms of multi-parton interactions, gluon saturation, initial or final state energy loss, system collective motion, color-charge screening or recombination of uncorrelated quarks are discussed.

Keywords: Heavy flavor, Charm, Quarkonia, Heavy Ion, QGP, LHC

1. Introduction

Heavy-flavor hadrons containing charm or beauty quarks provide information on the different mechanisms at play in hadronic collisions. The large mass of heavy quarks makes their production cross section calculable via perturbative Quantum ChromoDynamics (pQCD). The non-perturbative hadronization phase, i.e. the transition of heavy quarks to hadrons, is mimicked via the fragmentation functions. Heavy-flavor production measurements in pp collisions at the LHC are therefore a test of pQCD and collinear or kT factorization calculations in the high-energy regime [1-6]. Heavy-quark production in a nuclear environment, i.e. in p-Pb collisions, is influenced by the modification of the parton distributions in nuclei. The saturation of low fractional momentum (x) gluons becomes important at the LHC. These effects are modeled by either phenomenologi-cal modifications of the Parton Distribution Functions in nuclei (nPDFs) [7], or the Colour Glass Condensate (CGC) theory [8, 9]. Heavy quarks (Q) produced in nuclei might also undergo inelastic collisions causing a transverse momentum broadening [10, 11], or lose energy radiating gluons either before or after the QQ pair is formed [12, 13]. In addition, in Pb-Pb collisions

the formation of hot and dense QCD matter might also alter parton momentum distributions. Heavy quarks traversing extremely dense QCD matter lose energy via elastic or inelastic interactions with the medium constituents. Theoretical calculations of the radiative contribution predict it to be proportional to the Casimir coupling factor, implying a smaller energy loss for quarks than gluons [14-17]. Moreover, gluon bremsstrahlung off heavy quarks is expected to be suppressed at angles smaller than the ratio of the quark mass to its energy [18]. At a given parton energy, beauty quarks should then lose less energy than charm quarks. The formation of bound QQ (quarkonia) states is suppressed in this medium at extremely high temperature due to the Debye-like color-charge screening [19]. It has also been hypothesized that if heavy quarks are abundant in the medium, uncorrelated pairs could recombine into quarkonia bound states in the QGP or at the hadroniza-tion phase [20, 21]. In addition, if heavy quarks interact strongly with the medium or hadronize in it, they should inherit its azimuthal anisotropy [22]. This would result in an azimuthal anisotropy of both open and hidden heavy-flavor production.

In this report, open heavy-flavor measurements

(Sec. 2) concern heavy-flavor decay leptons (electrons and muons) and prompt D mesons reconstructed via their hadronic decays. Hidden heavy-flavor results (Sec. 3) refer to J/i^, f(2S) and Y mesons. J/i^'s are reconstructed both via their dielectron and dimuon decays, while f(2S) and Y are measured from their dimuon decay. These analyses exploit different detectors of the ALICE apparatus. The central barrel detector (|nl < 0.9) is equipped, among others, with an Inner Tracking System and a Time Projection Chamber that allow vertex finding, and particle tracking and identification. Their abilities are complemented by the Time-Of-Flight detector, the Transition Radiation Detector and the Electromagnetic Calorimeter that contribute to e/p/K/n particle identification. The forward muon spectrometer (-4.0 < n < -2.5) consists of a set of absorbers and a Muon Tracking and Trigger system that make possible muon reconstruction and identification. The VZERO scintillator arrays (-3.7 < n < -1.7 and 2.8 < n < 5.1) provide the information needed for trigger and centrality determination. The ALICE experimental setup is completed with a set of detectors that are not used in the analyses reported here.

Focus is given to the results obtained from the data samples of p-Pb and Pb-Pb collisions, at t/sNNN = 5.02 TeV and 2.76 TeV respectively, collected during the LHC Run-I. Most of the results are presented here in the form of the nuclear modification factor, RAB, i.e. the relative particle production rates in heavy-ion, or proton-ion, and pp data per nucleon-nucleon collision:

Rab(Pt) =

__d^AB/dpt

{tab > d^pp/dpt

where dNAB/dpT represents the particle yield in AB collisions, d^pp/dpT is the production cross section in pp collisions, and {TAB> is the nuclear overlap function of nucleus A, or proton, and nucleus B.

2. Open heavy-flavor production

The transverse momentum (pT) and rapidity (j) differential heavy-flavor decay lepton and prompt D-meson production was studied in p-Pb collisions [23]. In particular, Fig. 1 presents the heavy-flavor decay muon RpPb as a function of pT. Forward (backward) rapidity measurements refer to data collected in the p(Pb)-going direction and probe the Pb nuclei Bjorken-x (xBj) of O ~ 10-5(10-2), while mid-rapidity data probe xBj values of O ~ 10-4. RpPb is compatible with unity at high pT. In particular, prompt D mesons at mid-rapidity with pT > 2 - 3 GeV/c, and heavy-favor decay muons

1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I -

p-Pb \|sNN = 5.02 TeV, c,b decays 2.5<y <3.54 " : ALICE Preliminary cms ;

| systematic uncertainty on normalization

.....................

ALI-PREL-80422

PT (GeV/c)

p-Pb ]/s, : ALICE Preliminary

i 1 1 1 i 1 1 1 i 1 1 1 i 1 1 1 i 1 1 1 i. , = 5.02 TeV, c,b decays j

-4<y <-2.96

1........................

ALI-PREL-80434

NLO (MNR) with EPS09 shadowing systematic uncertainty on normalization

PT (GeV/c)

Figure 1: RpPb of heavy-flavor decay muons as a function of pT at forward and backward rapidity in p-Pb collisions at y= 5.02 TeV, as compared to pQCD calculations at NLO (MNR) [24] with the EPS09 nPDF parameterization [7].

at forward (backward) rapidity with pT > 2 - 3 GeV/c (pT > 6 - 8 GeV/c) show RpPb ~ 1. The results are in good agreement with models considering: (i) pQCD calculations with EPS09 shadowing [7, 24], (ii) CGC estimates [9], (iii) calculations including fcT-broadening, nuclear energy loss and nPDFs [12]. The calculations of (i) are available for mid, forward and backward rapidities, while (ii) and (iii) were only evaluated at mid rapidity. As a consequence, initial-state effects on open heavy-flavor production are expected to be small at high pT in heavy-ion collisions.

To complete this picture, open heavy-flavor production has also been studied in p-Pb collisions as a function of the multiplicity of particles produced in the collision. As an example, Fig. 2 shows the per-event yield of prompt D mesons (|j| < 0.5) as a function of the charged-particle multiplicity at mid-rapidity (|n| < 1) in pp collisions at yS = 7 TeV and p-Pb collisions at

0.4 0.2 0 -0.2 -0.4

ALICE Preliminary

+ pp, is = 7 TeV

p-Pb, = 5.02 TeV 2<pt <4 GeV/c

+7%/-3% (3.1%) in pp (p-Pb) normalization unc. not shown 6% (3%) unc. in pp (p-Pb) on dW/dn / < dW/dn > not shown

B fraction hypothesis in pp and p-Pb: x 1/2 (2) at low (high) multiplicity

ALI-PREL-76733

dWch/dn / <dW /dn>

Figure 2: D0 yields (|y| < 0.5) per event as a function of charged-particle multiplicity at mid rapidity (|nl < 1) in pp collisions at ^fs = 7 TeV and p-Pb collisions at ySNN = 5.02 TeV. Both the D-meson yields and the charged-particle multiplicity are shown normalized to their multiplicity integrated values.

<1.4 r

CC -1.2-

■ ALICE Preliminary D mesons 8<p <16 GeV/c, |y|<0.5 Correlated systematic uncertainties I I Uncorrelated systematic uncertainties

• CMS Preliminary Non-prompt J/y 6.5<p <30 GeV/c, |y|<1.2 I I Systematic uncertainties

CMS-PAS-HIN-12-014

0 50 100 150 200 250 300 350 400

< N . weighted with N „>

ALI-DER-52638 Part Oüll

Figure 3: RAA of prompt D mesons, measured by ALICE, and nonprompt J/ifr, measured by CMS [27], in Pb-Pb collisions at y^NN = 2.76 TeV as a function of the collision centrality, expressed in terms of the number of nucleons participating in the interaction.

D meson, |y |<0.5

Pb-Pb, ysNN = 2.76 TeV

ysNN = 5.02 TeV for 2 < pT < 4 GeV/c. In this representation, both the D-meson yields and the charged-particle multiplicity are normalized to their multiplicity integrated values. The relative D-meson yields increase with the relative charged-particle multiplicity. It has been conjectured that the origin of such behavior in pp data could be due to a larger contribution of Multi-Parton Interactions (MPIs) [4] in high-multiplicity events. Alternative scenarios consider percolation calculations or an increase of the QCD radiation associated to short distance processes. A similar trend is observed in p-Pb data, which remains to be understood. Two competing mechanisms could be at play, either the MPI's or the larger number of binary nucleon-nucleon collisions occurring in a high multiplicity p-Pb interaction.

Open heavy-flavor RAA measurements were also performed in Pb-Pb collisions as a function of pT and the collision centrality [25, 26]. Figure 3 displays the ALICE prompt D meson and the CMS non-prompt J/f [27] Raa at high pT as a function of centrality, represented by the average number of nucleons participating in the interaction. Open heavy-flavor production is suppressed at high pT in Pb-Pb reactions with respect to the binary scaled pp production cross section, and the magnitude of this suppression increases from peripheral to central events. This confirms that open heavy-flavor produc-

tion is affected by partonic energy loss, and the magnitude of the suppression depends on the medium density, increasing from peripheral to central collisions. In addition, in the semi-central and most central events high pT non-prompt J/f are less suppressed than prompt D mesons. Although in those measurements the pT and y intervals are different, the probed average pT of D and B hadrons is similar. This observation is consistent with various calculations including the quark-mass dependence of the energy loss [14-18].

The azimuthal anisotropy of particle production in Pb-Pb collisions has also been quantified evaluating the second coefficient of the Fourier decomposition of the azimuthal distribution, v2. Heavy-flavor decay lepton v2 results at mid and forward rapidity in semi-central Pb-Pb collisions are shown in Fig. 4 as a function of pT. The v2 values are similar at mid and forward rapidity. A positive v2 value is observed at intermediate pT, 1.5 < pT < 4 GeV/c, that is consistent with the positive prompt D-meson v2 in a comparable kinematic range [28, 29]. The magnitude of prompt D-meson v2 at intermediate pT is also comparable to the charged-particle v2, which suggests that low-pT charm quarks take part in the system collective motion. The high-pT v2 results are expected to convey information on the path-length dependence of the energy loss, but the current measurement precision is limited by the statistics.

cm 0.5 0.4 0.3 0.2 0.1 0 -0.1

ALI-PREL-77 62

1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1 I 1 1 1

ALICE Preliminary

-•- Heavy-flavour decay e±, ^{EP, |An| > 0.9}, |y| < 0.7 " -a- Heavy-flavour decay |±, v2{2}, 2.5 < y < 4

Pb-Pb, \fsNN = 2.76 TeV " 20-40% Centrality Class

2 4 6 8 10 12 14 PT (GeV/c)

Figure 4: Heavy-flavor decay electron (|j| < 0.7) and muon (2.5 < y < 4) azimuthal anisotropy, v2, for Pb-Pb collisions at ySNN = 2.76 TeV in the 20-40% centrality class as a function of the lepton pT.

3. Hidden heavy-flavor production

Charmonium (cc bound state) production has been measured in p-Pb collisions. The measurement of the J/l yield has been carried out in the dimuon and di-electron decay channels as a function of rapidity [30]. The pT dependence of the forward and backward rapidity yields has also been studied [31]. Figure 5 shows the J/l pT-integrated RpPb as a function of rapidity. Negative rapidity J/| originate from partons with large xBj values in the Pb nucleus and exhibit an RpPb close to unity. On the contrary, positive rapidity J/| correspond to partons with small xBj values in the Pb nucleus, which manifests on RpPb values smaller than unity. The results are compared to four different theoretical calculations including either one or a combination of: EPS09 nPDF parameterization, nuclear absorption, coherent parton energy loss in nuclei, or the CGC framework [7, 8, 13]. The measurements are well described by either of these calculations except that of the CGC, which can not give predictions at negative rapidities and seems to underestimate the positive rapidity RpPb value.

|(2S) measurements have also been examined as a function of y and pT in p-Pb collisions [31]. Figure 6 presents the double ratio of the |(2S) over J/| production cross sections in p-Pb collisions at t/sNNN = 5.02 TeV and pp collisions at yfs = 7 TeV as a function

Figure 5: J/| pT-integrated RpPb as a function of rapidity in p-Pb collisions at V^NN = 5.02 TeV [30].

J/ 1.4- ALICE, p-Pb ^iNN= 5.02 TeV, inclusive Jfy, -ST " • 2 03 < ycms < 3.53

02 1.2-

-4.46 < c < -2.s

^ 1 —

] 0.8 1

/ 0.6-

0.4 0.2

ALI-PUB-81973

PT (GeV/c)

Figure 6: Double ratio of the production cross section of |(2S ) over J/l in p-Pb collisions at ysNN = 5.02 TeV and pp collisions at V? = 7 TeV as a function of pT in two rapidity intervals [31].

of pT in two rapidity intervals. While initial-state models predict this double ratio to be equal to one within a few percent due to the slightly different xBj involved, the measurements evidence that the double ratio is below unity without a visible pT dependence. This observation presumably points to unexpected final-state effects that could differentiate between J/| and |(2S) in p-Pb collisions.

J/i production rates have also been measured in Pb-Pb collisions at tJsNNN = 2.76 TeV. J/| RAA was scrutinized as a function of y, pT, collision centrality and its azimuthal anisotropy [32-34]. Figure 7 (top) reports the pT-integrated J/| RAA as a function of cen-trality. J/i production is suppressed in the most cen-

tral Pb-Pb collisions at the LHC, but the suppression is smaller than the one measured at RHIC [35]. The pT dependence of J/f Raa in the most central collisions is shown in Fig. 7 (bottom). While at RHIC there is no visible pT dependence up to pT ~ 5 GeV/c, at the LHC there is a significant deviation, the low pT J/f's being less suppressed than the high pT ones, and the high pT ones showing a similar suppression than at RHIC. J/f azimuthal anisotropy was discussed in reference [34], suggesting positive v2 values in semi-central collisions at intermediate pT. The current partial understanding of these results assumes a large charmonium suppression in the hot and dense QCD matter, and considers that at high energies another mechanism counterbalances this suppression at low pT. A possible scenario for this offset is the recombination of uncorrelated c and c quarks either in the QGP or at hadronization.

The production of bottomonium (bib bound state) has also been measured at forward rapidity in Pb-Pb collisions. Y yields were measured in the dimuon decay channel as a function of rapidity and centrality. Y and J/f Raa are compared in Fig. 8 as a function of rapidity. Y production is more suppressed than J/f production. In addition, the comparison to CMS Y results [36] suggests a slightly larger suppression at forward than at mid-rapidity. J/f and Y pT-integrated RAA show a similar rapidity dependence. The state-of-the-art model calculations expect a smaller contribution of recombination processes for Y than for J/f production. These models are therefore not able to reproduce the Y RAA rapidity trend and underestimate its suppression at forward rapidity.

4. Summary

Heavy-flavor measurements in p-Pb and Pb-Pb collisions at the LHC with the ALICE detector at tJ&NNN = 5.02 and 2.76 TeV, respectively, have been summarized.

Open heavy-flavor production in p-Pb collisions is described reasonably well by pQCD calculations including initial-state effects. However, a complete modeling of all observables might require to consider, in addition to nPDF or CGC calculations, the possible influence of other effects such as: quark fragmentation functions, multi-parton interactions, feT-broadening or energy loss in cold nuclear matter.

Hidden heavy-flavor production in p-Pb collisions is also globally well reproduced by model calculations. Nevertheless, f(2S ) results suggest that some unexpected final-state effects that could distinguish among J/f and f (2S ) might be at play, besides the nPDF, CGC or coherent energy loss contributions.

1.4 1.2 1

0.8 0.6 0.4 0.2

ALI-DER-65274

<r 1.4 —

Inclusive J/y ^ ^V, Pb-Pb ^n = 2 76 TeV and Au-Au V®nn = 02 TeV

■ ALICE (PLB 734 (2014) 314), 2.5<y<4, 0<pT<8 GeV/c global syst.= ± 15%

□ PHENIX (PRC 84(2011) 054912), 1.2<y|<2.2, PT>0 GeV/c global syst = ± 9.2%

50 100 150 200 250 300 350 400

N part>

0.8 0.6 0.4 0.2

Pb-Pb SN = 2.76 TeV and Au-Au Sn = 02 Tev

■ ALICE J/y ^ |iV, 2.5<y<4, centrality 0%-20% global syst.= ± 8%

« PHENIX J/y ^ 1.2<y|<2.2, centrality 0%-20% global syst. = ± 10%

0 12 3

ALI-PUB-64810

PT (GeV/c)

Figure 7: J/| RAA at forward rapidity as measured by ALICE in Pb-Pb collisions at ySNN = 2.76 TeV and PHENIX in Au-Au collisions at ysNN = 0.2 TeV [32, 33, 35]. The pT-integrated RAA as a function of the collision centrality, expressed by the number of participating nucleons, is shown in the top figure. The variation as a function of pT in the 20% most central collisions is shown in the bottom plot.

< 1.4r--1-

^ I ALICE: Pb-Pb ^SNN = 2.76 TeV, Llnl = 69 ^b-1, 0%-90% 12- • Inclusive T(1S), pT > 0

Inclusive J/y, 0 < p < 8 GeV/c

1-.......................................................................

0.8 0.6 ; 0.41 0.21 0^

2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4

Figure 8: y and J/| pT-integrated RAA as a function of rapidity for Pb-Pb collisions at ysNN = 2.76 TeV in the 0-90% centrality range.

The results in Pb-Pb collisions evidence a suppression of heavy-flavor production at high pT with respect to that of pp collisions scaled by the average number of nucleon-nucleon interactions. Since initial-state effects are expected to be small at high pT for open heavy flavors, their suppression in the most central Pb-Pb interactions is assumed to come from final-state effects. This corroborates that heavy quarks experience partonic energy loss in the medium. The magnitude and central-ity dependence of high-pT prompt D-meson and nonprompt J/i RAA are consistent with model calculations considering quark mass dependent energy loss. The positive v2 values at intermediate pT in semi-central collisions prove open heavy-flavor production azimuthal anisotropy. The similarity with charged particle v2 in this kinematic range suggests that charm quarks participate in the system collective motion.

J/l production in Pb-Pb collisions is suppressed with respect to that in pp collisions scaled by the average number of nucleon-nucleon interactions. The pT-integrated RAA centrality dependence presents a flat distribution from semi-peripheral to the most central collisions, while the equivalent measurements at RHIC show an enhancement of the suppression with centrality. The production rate in Pb-Pb collisions decreases with increasing pT, and for pT > 5 GeV/c it is similar to the one measured at RHIC. These observations are interpreted considering a large suppression of charmonium in the medium, and another mechanism compensating this suppression at low pT at the LHC. The recombination of uncorrelated heavy quark pairs appears to be a plausible scenario. Recent Y pT-integrated results in Pb-Pb collisions exhibit RAA values smaller than the J/l ones, i.e. a larger suppression than J/|. State-of-the-art calculations are not able to reproduce these Y measurements.

In the near future, the LHC Run-II will bring an increase of the collision energy and luminosity that should allow more precise and differential measurements. In particular, open heavy-flavor studies down to pT ~ 0 and up to higher pT will be explored, as well as the properties of quarkonia.

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