Open heavy-flavour production from small to large collision systems with ALICE at the LHC
OOpen heavy-flavour production from small to largecollision systems with ALICE at the LHC
Fabio Catalano † , ∗ Politecnico di Torino,Corso Duca degli Abruzzi 24, Torino, 10129, ItalyINFN sez. Torino,Via Pietro Giuria 1, Torino, 10125, Italy
E-mail: [email protected]
Heavy quarks are effective probes of the hot and dense nuclear matter, the quark–gluon plasma,produced in ultra-relativistic heavy-ion collisions. Due to the short time scale characterisingtheir production, heavy quarks experience the whole evolution of the system. In particular,measurements of heavy-flavour hadron production in Pb–Pb collisions at LHC energies give insightinto the mechanisms of heavy-quark transport in the deconfined matter. In small hadronic systems,pp and p–Pb collisions, heavy-flavour measurements provide the baseline for observations of hot-medium effects in heavy-ion collisions, as well as tests of perturbative quantum chromodynamiccalculations and measurements of cold-nuclear-matter effects. In this contribution, recent ALICEresults on open heavy-flavour hadron production in pp, p–Pb and Pb–Pb collisions at variousenergies are discussed. New measurements are presented both for fully-reconstructed charmedhadrons and for single electrons from heavy-flavour hadron decays, also investigating the beauty-quark production via the measurement of D mesons and electrons from beauty-hadron decays.
HardProbes20201-6 June 2020Austin, Texas ∗ Speaker † on behalf of the ALICE Collaboration © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] J u l pen heavy-flavour production with ALICE at the LHC Fabio Catalano
1. Introduction
Due to their large masses, charm and beauty quarks (heavy quarks) are mainly produced inhard-scattering processes between partons of the colliding nucleons. Therefore, their productioncan be described by perturbative quantum chromodynamic (pQCD) calculations down to zerotransverse momentum ( p T ). In ultra-relativistic heavy-ion collisions extreme temperatures arereached and lattice QCD calculations predict a phase transition of nuclear matter to a colour-deconfined medium, called quark–gluon plasma (QGP). Heavy quarks are produced in the initialstages of the nucleus–nucleus (Pb–Pb) collision, before the QGP formation, and experience thefull evolution of the system while propagating through the medium and strongly interacting withthe QGP constituents. In Pb–Pb collisions the measurement of hadrons containing heavy quarksprovides crucial information on the in-medium parton energy loss. The comparison of heavy-flavour and light-flavour hadrons gives insight into the colour-charge and quark-mass dependenceof energy loss. Moreover, the hadronisation mechanism of heavy quarks in the medium can beinvestigated comparing the production of different heavy-flavour hadron species, e.g., charmedbaryons and mesons, or hadrons with and without strange-quark content [1]. Measurements ofopen heavy-flavour hadron production in proton-proton collisions are relevant tests of pQCD modelcalculations.In ALICE, charmed hadrons are reconstructed at midrapidity ( | y | < .
8) via the hadronic decaychannels: D → K − π + , D + → K − π + π + , D ∗ + → D π + → K − π + π + , D + s → φπ + → K − K + π + ,Λ + c → pK and their charge conjugates. Particle candidates are built from pairs or triplets oftracks with the proper charge combination. Kinematic and geometrical selections on the displaceddecay-vertex topology, together with particle identification, are applied to reduce the combinatorialbackground. The charmed-hadron raw yields are obtained from an invariant-mass analysis andthe reconstruction efficiencies are estimated using Monte Carlo simulations [2]. In addition,heavy-flavour hadrons are studied through the measurement of electrons produced in their semi-leptonic decays. Electrons are identified at midrapidity using the information provided by ALICEcentral-barrel detectors [3]. The hadron contamination and electrons from non-heavy-flavoursources, mainly photon conversions and Dalitz decays of light neutral mesons, are subtracted fromthe measured inclusive yield, which is then corrected for the acceptance and selection efficiency[4]. Exclusive measurements of prompt charmed hadrons, originated from the hadronisation ofcharm quarks produced in the initial collision, and non-prompt ones, which are produced frombeauty-hadron decays, are possible thanks to the longer proper mean-life of hadrons containingbeauty quarks. This allows the ALICE experiment to assess beauty-quark production through themeasurement of non-prompt D mesons and electrons from beauty-hadron decays.
2. Open heavy-flavour production in pp collisions
The production cross section of prompt [2] and non-prompt D mesons and of electrons fromsemi-leptonic heavy-flavour hadron decays [4] is measured at midrapidity in pp collisions at √ s = .
02 TeV. In the left panel of Fig. 1, prompt and new measurements of non-prompt D + mesonsare compared, respectively, to FONLL [5] predictions and to FONLL with the B → D + Xdecay kinematics described by the PYTHIA8 package [6]. In the right panel, the cross section of2 pen heavy-flavour production with ALICE at the LHC
Fabio Catalanoheavy-flavour decay electrons is compared to theoretical calculations. The measurements are well − − −
10 110 ) c - b G e V µ ) ( y d T p / ( d σ d | < 0.5 y , | + Prompt DDataEPJC (2019) 79:388FONLL | < 0.5 y , | + Non-prompt DDataFONLL + PYTHIA8 Decayer
ALICE Preliminary = 5.02 TeV s pp, ± ± ) c (GeV/ T p m ode l da t a ) c (GeV/ T p m ode l da t a ALI−PREL−344575
ALI-PUB-327771
Figure 1:
Left: prompt and non-prompt D + -meson cross sections in pp collisions at √ s = .
02 TeV comparedto FONLL predictions. Right: e ± from heavy-flavour hadron decays compared to FONLL predictions. described by pQCD calculations. The prompt D + -meson measurement lies on the upper part of theFONLL prediction uncertainty band, while non-prompt D + mesons are in good agreement with thecentral predictions. A similar behaviour is observed for heavy-flavour decay electrons, where thecross section is on the upper edge of FONLL predictions for p T < / c and moves towards thecentral values at high p T , where beauty-hadron decays are the dominant contribution.
3. D + s -meson abundance as a function of particle multiplicity Figure 2 shows the yield ratio between D + s and D mesons measured by the ALICE experimentin p–Pb [7] and Pb–Pb [8] collisions at √ s NN = .
02 TeV, as a function of the charged-particlemultiplicity for different p T intervals, together with new measurements in pp at √ s =
13 TeVand in Pb–Pb collisions. The D + s / D ratio in pp collisions does not show a dependence on theevent multiplicity and it is in agreement with the expected value considering the charm-quarkfragmentation fractions measured in e + e − collisions at LEP [9]. The measurement in minimumbias p–Pb collisions is compatible to what is observed in pp collisions at similar multiplicity values.An increase of the D + s / D ratio with respect to pp and p–Pb is observed in Pb–Pb collisions at p T < / c . This higher production of D + s mesons in Pb–Pb is expected if charm quarkshadronise via coalescence with a quark of the QGP medium, where the production of s¯s pairs isenhanced [1].
4. Open heavy-flavour nuclear modification factor
The production of prompt charmed hadrons and electrons from heavy-flavour hadron decays[4] is measured in Pb–Pb collisions at √ s NN = .
02 TeV and compared to pp collisions through thenuclear modification factor R AA ( p T ) = ( d N AA / d p T )/((cid:104) N AA coll (cid:105) · d N pp / d p T ) ; where d N AA / d p T andd N pp / d p T are the p T -differential yields measured in nucleus–nucleus and pp collisions, respectively,and (cid:104) N AA coll (cid:105) is the average number of binary interactions in a nucleus–nucleus collision. In the left3 pen heavy-flavour production with ALICE at the LHC Fabio Catalano |<0.5 η | 〉η /d ch N d 〈 / D + s D c < 8 GeV/ T p |<0.5 η | 〉η /d ch N d 〈 / D + s D c < 6 GeV/ T p |<0.5 η | 〉η /d ch N d 〈 / D + s D c < 4 GeV/ T p ALICE Preliminary ± = 13 TeV s pp, SPD multiplicity classes = 5.02 TeV NN s Pb Minimum Bias, − parXiv:1906.03425 = 5.02 TeV NN s Pb (2015), − PbJHEP 10 (2018) 174V0 multiplicity classes = 5.02 TeV NN s Pb (2018), − PbV0 multiplicity classes |<0.5 η | 〉η /d ch N d 〈 / D + s D c < 24 GeV/ T p
12 < 10 |<0.5 η | 〉η /d ch N d 〈 / D + s D c < 12 GeV/ T p ALI−PREL−342747
Figure 2: D + s / D ratio measured in pp, p–Pb and Pb–Pb collisions as a function of the charged-particlemultiplicity and for different p T intervals. panel of Fig. 3, the measured R AA of strange and non-strange D mesons, Λ + c and charged particles[10] in central Pb–Pb collisions are reported. A strong suppression of the charmed and light-flavour
10 20 ) c (GeV/ T p AA R ALICE Preliminary | < 0.5 y = 5.02 TeV, | NN s Pb, −
10% Pb − Filled markers: pp measured reference-extrapolated reference T p Open markers: pp +c Λ *+ , D + , D Average D s+ D | < 0.8, JHEP 1811 (2018) 13 y charged particles, | ALI−PREL−330734 ) c (GeV/ T p AA R e → c, b arXiv:1910.09110 [nucl-ex] e → c) → b ( | < 0.8 η | | < 0.6 η | ALICE Preliminary = 5.02 TeV NN s Pb, −
10% Pb − ALI−PREL−308477
Figure 3:
Left: R AA comparison between Λ + c , non-strange D mesons, D + s and charged particles in the 0–10%centrality class. Right: R AA comparison of e ± from heavy-flavour hadron and beauty-hadron decays. hadron R AA is observed, as expected in the presence of the QGP due to in-medium energy loss.The R AA of non-strange D mesons is higher than that of charged particles below 4 GeV / c , whilethey are compatible at higher p T . This behaviour can be explained by the mass and colour-chargedependence of energy loss. However, also other factors play a role, such as the different initial p T distributions and fragmentation functions of charm and light quarks and the different effects ofhadronisation via recombination and radial flow [8]. Finally, there is an indication of a smallersuppression of D + s mesons and Λ + c baryons than non-strange D mesons at p T < / c . In theright panel of Fig. 3, the R AA of electrons from semi-leptonic beauty-hadron decays in central Pb–Pbcollisions is compared to that of inclusive heavy-flavour decay electrons. The R AA of electrons4 pen heavy-flavour production with ALICE at the LHC Fabio Catalanocoming from beauty hadrons is above the inclusive one in all the p T intervals below 10 GeV / c , evenif compatible within uncertainties, pointing to a possible mass dependence of heavy-quark energyloss, where beauty quarks lose less energy in the medium than charm quarks.
5. Conclusions
The ALICE collaboration has measured the production of charmed hadrons and electronsfrom heavy-flavour hadron decays in different collision systems and at different centre-of-massenergies. In addition, the production of beauty quarks has been investigated with the measurementof non-prompt D mesons and electrons from beauty-hadron decays.In pp collisions, the measured D-meson and heavy-flavour electron cross sections are com-patible with pQCD calculations. In Pb–Pb collisions, the indication of a higher D + s / D ratio thanin pp, at low p T , is in agreement with the charm-quark hadronisation via quark recombination inthe QGP. The D-meson R AA is higher than that of charged particles at p T < / c , suggestinga colour-charge and quark-mass dependence of the energy loss. Furthermore, an indication of asmaller energy loss of beauty quarks than charm quarks is observed through the measurement ofelectrons from beauty-hadron decays. References [1] I. Kuznetsova and J. Rafelski, “Heavy flavor hadrons in statistical hadronization of strangeness-richQGP,”
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