Overview of heavy-flavour measurements in ALICE∗

L.V.R. van Doremalen on behalf of the ALICE Collaboration

Utrecht University, The Netherlands

ALICE is devoted to the study of the properties of the Quark-GluonPlasma (QGP). This state of matter is created in ultra-relativistic heavy-ioncollisions at the LHC. Heavy quarks are considered effective probes of theQGP since, due to their large masses, they are produced in hard scatteringprocesses and experience the full evolution of the hot and dense mediumwhile interacting with its constituents. The heavy-quark measurementsprovide insights on processes like in-medium energy loss and hadroniza-tion. Measurements in proton-proton collisions provide the baseline forinterpreting heavy-ion collision results and constitute an excellent test ofpQCD calculations. In addition, proton-nucleus collisions allow separatingcold nuclear matter effects from those due to the deconfined strongly in-teracting matter created in heavy-ion collisions. In this contribution, anoverview of recent ALICE results for open heavy flavours, quarkonia, andheavy-flavour jets is presented.

PACS numbers: 12.38.Mh, 25.75.-q, 25.75.Nq

1. Introduction

At the extremely high temperatures reached in heavy-ion collisions, aphase-transition occurs from ordinary nuclear matter to a QGP state inwhich quarks and gluons are not confined into hadrons. The quark forma-tion time during the collision is proportional to the inverse of the quarkmass [1]. Therefore, heavy quarks are generated early during the collisionand can experience the full evolution of the medium [2]. The quarks loseenergy while moving through the medium by collisional and radiative pro-cesses. This energy loss is expected to depend on the path length, theQGP density,the parton colour charge (Casimir factor), and the quark mass(dead-cone effect) [3, 4]. Because of this, the following energy loss hierarchyis expected: ∆Eloss(g) > ∆Eloss(u,d) > ∆Eloss(c) > ∆Eloss(b).

∗ Presented at Excited QCD 2020

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Fig. 1. Left: RAA of non-strange D mesons in central Pb–Pb collisions compared

with theoretical calculations. Right: Ratio of RAA of non-prompt D0 mesons over

the RAA of prompt D0 mesons. The data is compared with models with different

energy loss for charm and beauty. Copyright CERN, reused with permission.

The nuclear modification factor (RAA) quantifies the medium effectsthat affect the heavy quarks when they traverse the medium. This factor,defined as

RAA =1

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coll 〉 representsthe average number of binary nucleon-nucleon collisions in Pb–Pb collisionsfor a given centrality interval. If heavy quarks do not lose energy in themedium RAA = 1, while it drops below unity if they do. Heavy quarks arealso expected to be affected by the collective motion of the medium. Thisgives rise to an anisotropic flow usually described by the components of aFourier expansion of the azimuthal distribution of the outgoing particles.The second coefficient of this expansion is called elliptic flow (v2).

2. Open heavy flavour

The left panel in Fig. 1 shows a comparison of the RAA of non-strangeD-mesons in central Pb–Pb collisions with theoretical calculations. The lowmomentum reach in central collisions allows setting stringent constraints onenergy-loss models for central Pb–Pb collisions. Models without shadowing,like the BAMPS model [5], overestimate the RAA spectrum at low pT.

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Fig. 2. Left: RAA in central Pb–Pb collisions for multiple types of particle species.

Right: Λ+c / D0 ratio as a function of multiplicity for several pT intervals. Copyright

CERN, reused with permission.

The models can be tested more rigorously by requiring a descriptionof multiple observables, like RAA and v2, at the same time, over a widemomentum range, and in different centrality intervals [6, 7]. This shows thataccurate modeling of data requires a combination of collisional and radiativeenergy loss, hadronization via coalescence, cold-nuclear-matter effects, anda realistic description of the medium evolution.

The right panel shows the ratio of the RAA of non-prompt D0-mesonsover the RAA for prompt D0-mesons. Prompt D0-mesons, which come di-rectly from the charm quarks produced in the initial collision, and non-prompt D0-mesons, which are produced later by the decay of beauty hadrons,show a different RAA at intermediate pT. Models with different energy lossfor charm and beauty can describe within uncertainties the ratio of non-prompt over prompt D0-meson RAA. This is an indication that energy lossdepends on the quark mass.

The left panel in Fig. 2 shows the RAA for different particle species witha hierarchy that is consistent with the expected difference in energy loss forcharm versus light-flavour and gluons. Strange D-mesons and Λc baryonsshow a hint of lower suppression, compared to non-strange D-mesons, thatmay point at recombination effects. ”Models that include hadronization viacoalescence reproduce DS data within uncertainties.

The right panel in Fig. 2 shows the Λ+c / D0 ratio as a function of multi-

plicity in pp, p–Pb, and Pb–Pb collisions for several pT intervals. This ratioshows an enhancement at low pT compared to e+e− collider measurementsin which Λ+

c / D0 ≈ 0.1 [8]. The multiplicity dependence of the Λ+c / D0

ratio shows that the enhancement remains higher than electron-positron col-

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Fig. 3. Left: RAA as a function of multiplicity for inclusive J/ψ in two rapidity

intervals. Right: RAA as a function of 〈Npart〉 for two Υ states along with model

predictions. Copyright CERN, reused with permission.

lider measurements even for low-multiplicity pp collisions, suggesting thatcharm-quark recombination with quarks from the surrounding hadronic en-vironment may already occur in small systems.

3. Quarkonium

At high temperatures colour screening in the QGP results in the sup-pression of quarkonium production [9]. Different quarkonium states havedifferent binding energies, which results in the expectation of a sequentialmelting of states when colliding nuclei at higher energies [10]. On the otherhand, the cc̄ multiplicity increases at higher collision energies. This leadsto the expectation of an enhancement of quarkonia production via recom-bination at hadronization.

The left panel of Fig. 3 shows the RAA as a function of multiplicityfor inclusive J/ψ-mesons in two rapidity intervals. This RAA measurementhas a significantly improved precision and pT reach compared to previousmeasurements [11]. At higher multiplicities the RAA at midrapidity is higherthan at forward rapidity. This observation may suggest that recombinationeffects are stronger at midrapidity, where the charm-quark density is higher.

The centrality dependence of the RAA is shown in the right panel of Fig.3. The data show a slight bottomonium centrality dependence and matchwell with the model predictions [12]. A stronger suppression of Υ(2S) thanΥ(1S) is observed.

For J/ψ-mesons, measurements show a positive v2 in a large pT rangeat forward rapidity. This is illustrated in the left panel of Fig. 4. Thebottomonium v2 is consistent with zero, however more data are needed fora conclusive interpretation on the difference between J/ψ and bottomoniumv2

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Fig. 4. Left: v2 as a function of pT for inclusive J/ψ. Right: Υ(1S) v2 as a function

of pT compared with inclusive J/ψ v2 and different models [13]. Copyright CERN,

reused with permission.

4. Heavy-flavour jets

Jets originate from hard parton-parton interactions. In ALICE heavy-flavour tagged jets are measured down to low jet pT (5 GeV/c). The study ofjets provides experimental data for gluon-to-hadron fragmentation functionsand gluon PDF at low x. The study of jet quenching provides additionalinformation to further characterise parton energy loss in the QGP.

Fig. 5 shows the first measurement of the Λ+c probability density distri-

bution of the parallel jet momentum fraction (zch|| ) compared to data. The

Pythia 8 SoftQCD model has the best agreement with data.

Jets with beauty hadrons were reconstructed exploiting the displacedimpact parameter of b-hadron decay tracks to the primary vertex. Theobserved yields are consistent with POWHEG. The nuclear modificationfactor in p–Pb (RpPb) for B-tagged jets is shown in the right panel of Fig.5. No cold-nuclear-matter effects are observed within uncertainties usingB-tagged jets.

REFERENCES

[1] Open Heavy-Flavor Production in Heavy-Ion Collisions, X. Dong, Y. Lee,and R. Rapp, arXiv:1903.07709 (2019).

[2] Quark-gluon plasma formation time and direct photons from heavy ion col-lisions, F.M. Liu, S.X. Liu, Phys. Rev. C 89, 034906 (2014).

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Fig. 5. Left: probability density distribution of the parallel jet momentum fraction

(zch|| ) for Λ+c -tagged jets compared to expectations from Monte Carlo generators.

Right: RpPb for B-tagged jets with a comparison of measurements by ALICE and

CMS [14]. Copyright CERN, reused with permission.

[3] Heavy-quark colorimetry of QCD matter, Y.L. Dokshitzer, D.E. Kharzeev,Phys. Lett. B519, 199 (2001).

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sNN = 5.02 TeV, ALICE Collaboration, arXiv:1804.09083 (2019).

[7] D-meson azimuthal anisotropy in mid-central Pb–Pb collisions at√sNN =

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[8] Combined analysis of charm-quark fragmentation-fraction measurements, M.Lisovyi, A. Verbytskyi, O. Zenaiev, Eur. Phys. J. C (2016) 76:397.

[9] Color Deconfinement and Charmonium Production in Nuclear Collisions, L.Kluberg, H. Satz, arXiv:0901.3831, (2009).

[10] Sequential charmonium dissociation, F. Karsch, D. Kharzeev, H. Satz, Phys.Lett. B637, 75 (2006).

[11] Centrality and transverse momentum dependence of inclusive J/ψ productionat midrapidity in Pb-Pb collisions at

√sNN = 5.02 TeV, ALICE Collabora-

tion, arXiv:1910.14404, (2019).

[12] Color screening and regeneration of bottomonia in high-energy heavy-ioncollisions, X. Du, M. He, R. Rapp, Phys. Rev. C 96, 054901 (2017).

[13] Measurement of Υ(1S) elliptic flow at forward rapidity in Pb–Pb collisionsat√sNN = 5.02 TeV, ALICE Collaboration, arXiv:1907.03169, (2019).

[14] Transverse momentum spectra of inclusive b jets in pPb collisions at√sNN =

5.02 TeV, CMS Collaboration, Phys. Lett.B754(2016) 59, arXiv:1510.03373.