Measurement of D-meson production in pp, p-Pb and Pb-Pb collisions with ALICE at the LHC
NNuclear Physics A 00 (2018) 1–5
NuclearPhysics A
Measurement of D-meson production in pp, p–Pb and Pb–Pbcollisions with ALICE at the LHC
A. Rossi a , for the ALICE Collaboration. a CERN
Abstract
Heavy quarks, i.e. charm and beauty, are considered calibrated probes for the strongly interacting deconfined medium (QuarkGluon Plasma, QGP) formed in heavy-ion collisions. Produced in hard scattering processes in the initial stages of the collision,they interact with the medium, lose energy and, depending on the coupling strength to the medium, take part in the collectivemotion of the QCD matter. ALICE measured the production of D , D ∗ + , D + and D + s mesons at central rapidity in pp, p–Pb andPb–Pb collisions at the LHC. The study of the modification of the transverse momentum di ff erential yields of charm particlesin Pb–Pb collisions with respect to pp collisions, quantified by the nuclear modification factor ( R AA ), can unravel details of theenergy loss mechanism, such as its dependence on the quark mass and on the path length the parton travels through the medium.A similar comparison between pp and p–Pb collision data ( R pPb ) is fundamental to disentangle e ff ects related to the presence ofthe hot medium from cold nuclear matter e ff ects. The degree of thermalization and coupling to the medium is investigated insemi-peripheral Pb-Pb collisions by measuring the elliptic flow coe ffi cient ( v ) at low p T . At high p T , v is sensitive to the path-length dependence of the energy loss. Results on the transverse momentum and centrality dependence of the D-meson elliptic flowand R AA will be presented. The comparison with the R AA of non-prompt J /ψ from B-meson decays measured with CMS will bediscussed. The preliminary results on D-meson R pPb and the dependence of D-meson yields on rapidity in p–Pb collisions will beshown. As an outlook, the analysis and the preliminary results on the azimuthal correlations of D-mesons and charged hadrons inpp collisions will be described. Keywords:
QGP, charm, heavy flavour, ALICE, energy loss, elliptic flow
1. Introduction
The comparison of open heavy-flavour hadron production in proton-proton, proton-Pb and Pb–Pb collisions atthe LHC o ff ers the opportunity to investigate the properties of the high-density colour-deconfined state of strongly-interacting matter (Quark Gluon Plasma, QGP) that is expected to be produced in high-energy collisions of heavynuclei [1]. Due to their large mass, charm and beauty quarks are created at the initial stage of the collision in hard-scattering processes with high virtuality ( Q (cid:38) c [ b ] ) involving partons of the incident nuclei. They interact withthe medium and lose energy via both inelastic (medium-induced gluon radiation, or radiative energy loss) [2, 3]and elastic (collisional energy loss) [4] processes. The loss of energy, sensitive to the medium energy density andsize, is expected to depend on the quark mass and be smaller for heavy quarks than for light quarks and gluonsfor most of the mechanisms considered in theoretical models. In particular, the parton and mass dependence ofradiative energy loss derives from the smaller colour coupling factor of quarks with respect to gluons, and fromthe ‘dead-cone e ff ect’, which reduces small-angle gluon radiation for heavy quarks with moderate energy-over-massvalues [5]. A sensitive observable is the nuclear modification factor, defined as R AA ( p T ) = d N AA / d p T (cid:104) T AA (cid:105) d σ pp / d p T , where N AA isthe yield measured in heavy-ion collisions, (cid:104) T AA (cid:105) is the average nuclear overlap function calculated with the Glaubermodel [7] in the considered centrality range, and σ pp is the production cross section in pp collisions. In-medium energy1 a r X i v : . [ h e p - e x ] S e p Nuclear Physics A 00 (2018) 1–5 (GeV/c) T p N u c l ea r m od i f i c a t i on f a c t o r *+ , D + , D Average of prompt D = 5.02 TeV NN sp Pb, <0.04 cms NN sPb Pb, |<0.5 cms Filled markers : pp rescaled reference extrapolated reference T Open markers: pp p
ALI−DER−54711 cms y c ) b G e V µ d y ( T / dp σ d <5 GeV/c T T T Systematic uncert.from Datafrom B feed down subtr. 3.4%) and BR syst. unc. not shown ± Normalization ( =5.02 TeV NN sp Pb, meson D Pbp (MNR+EPS09) pA R × (FONLL) dy σ d × A ALI−DER−61952
Figure 1. Left panel: comparison of average D , D ∗ + and D + nuclear modification factors measured in p–Pb collisions and in the 0-7.5% mostcentral Pb–Pb collisions. Right panel: p T , y -di ff erential cross-section for D production in p–Pb collisions as a function of the rapidity in the centreof mass system for three di ff erent p T ranges. The continuous and dashed lines represent expectations based on pQCD calculations including EPS09parametrization of nuclear PDF [15, 22, 24] (see text for more details). loss determines a suppression, R AA <
1, of hadrons at moderate-to-high transverse momentum ( p T (cid:38) / c ).The dependence of the energy loss on the parton nature (quark / gluon) and mass can be investigated by comparingthe nuclear modification factors of hadrons with charm ( R DAA ) and beauty ( R BAA ) with that of pions ( R π AA ), mostlyoriginating from gluon fragmentation at LHC energies. A mass ordering pattern R π AA ( p T ) < R DAA ( p T ) < R BAA ( p T )has been predicted [5, 6]. However, it is important to note that the comparison of heavy-flavour hadron and pion R AA cannot be interpreted directly as a comparison of charm, beauty, and gluon energy losses, due to the di ff erentparton fragmentation functions and slope of the p T -di ff erential cross sections (even in the absence of medium e ff ects).Moreover, at low p T , a significant fraction of pions does not come from hard-scattering processes.A R AA value di ff erent from unity can also originate from initial and final state “cold-nuclear matter” e ff ects, notrelated to the formation of a deconfined medium. At LHC energies, nuclear shadowing, which reduces the partondensity for gluons carrying a nucleon momentum fraction x below 10 − , is expected to be the most important forheavy-flavour production. A correct interpretation of heavy-ion results demands for the measurement of these e ff ectsvia the analysis of p–Pb data.In heavy-ion collisions with non-zero impact parameter the interaction region exhibits an azimuthal anisotropywith respect to the reaction plane ( Ψ RP ) defined by the impact parameter and the beam direction. Collective e ff ectsconvert this geometrical anisotropy into an anisotropy in momentum space that is reflected in the final state hadronazimuthal distribution [8]. The e ff ect, sensitive to the degree of thermalization of the system, can be evaluated bymeasuring the 2 nd coe ffi cient of the Fourier expansion of the particle azimuthal distribution, called elliptic flow ( v ).The measurement of D-meson v can provide, at low p T , fundamental information on the degree of thermalizationof charm quarks in the medium. At high p T , a non-zero v can originate from the path-length dependence of energyloss [9–14].The measurement of both ”hot” and ”cold” nuclear e ff ects requires the understanding of the production cross-sections of open heavy-flavour in pp collisions, used as a reference. The p T -di ff erential production cross sectionsof D mesons is well described by fix-order pQCD calculations relying on the collinear factorization approach, likeFONLL [15], GM-VFNS [16], or the k T -factorization approach [17]. The study of the azimuthal angular correlationsof D-meson and charged hadrons produced in the collisions o ff ers the opportunity of investigating charm productionin a more di ff erential way and can serve as a test for Monte Carlo generators simulating the full kinematic of charmproduction processes and parton shower.In these proceedings, the measurements of D-meson production in pp, p–Pb and Pb–Pb collisions performed bythe ALICE Collaboration are presented. 2 Nuclear Physics A 00 (2018) 1–5 〉 coll weighted with N part N 〈 AA R ALICE Preliminary D mesons<16 GeV/c, |y|<0.5 T = 2.76 TeV NN sPb Pb, CMS PAS HIN 12 014 ψ CMS Preliminary Non prompt J/<30 GeV/c, |y|<1.2 T ALI−DER−52638 〉 coll weighted with N part N 〈 AA R meson, |y|<0.5 DUncorrelated syst. uncertaintiesCorrelated syst. uncertainties, |y|<0.8 ± π = 2.76 TeV NN sPb Pb, meson, |y|<0.5 DUncorrelated syst. uncertaintiesCorrelated syst. uncertainties, |y|<0.8 ± π <3 GeV/c T ALI−DER−52746
Figure 2. Comparison of the centrality dependence of the nuclear modification factors of non-strange D mesons in 8 < p T <
16 GeV / c andJ /ψ from B-meson decays measured with CMS [26] in 6 < p T <
30 GeV / c (left panel) and of non-strange D mesons and charged pions in2 < p T < / c (right panel). The filled boxes represent the D-meson systematic uncertainties that are correlated in the di ff erent centralityintervals.
2. Analysis and results
Open charm production is measured in the ALICE experiment via an invariant mass analysis of the decay channelsD → K − π + (BR = c τ = µ m), D + → K − π + π + (BR = c τ = µ m), D ∗ + → D π + (BR = + s → φπ + → K + K − π + (BR = c τ = µ m) in pp collisions at √ s = .
76 and 7 TeV, p-Pbcollisions at √ s NN = .
02 TeV and Pb-Pb collisions at √ s NN = .
76 TeV. A similar analysis strategy was used inthe di ff erent collision systems [18–21], based on exploiting the ALICE high track spatial resolution in the vicinity ofthe primary vertex of the collision ( ∼ µ m at p T = / c ), granted by the Inner Track System silicon detector(ITS), to identify secondary decay vertices displaced by few hundred µ m from the interaction vertex. In order tofurther enhance the ratio between the D-meson signal and the large combinatorial background the measurements ofthe particle time-of-flight from the collision point to the Time Of Flight (TOF) detector and of the specific energy lossin the Time Projection Chamber (TPC) gas are used to identify kaons and pions. The pseudorapidity acceptance fortracks reconstructed in the ITS, TPC and TOF detectors is | η | < . R pPb . As for the measurements performed in pp and Pb–Pb [18, 21], the contribution of D mesons coming fromB-hadron decays is estimated and subtracted using the cross section of D mesons from B-meson decays calculatedwith FONLL [15], the reconstruction e ffi ciency for prompt and secondary D mesons and using a range of hypothesesfor the nuclear modification factor of feed-down D mesons ( R feed − downpPb ). In p–Pb collisions R feed − downpPb / R DpPb = . < R feed − downAA / R DAA < . ff ects based on EPS09 nuclear PDFparametrizations [22] and from the model based on the Color Glass Condensate (CGC) framework described in [23]. R pPb is compatible with unity in the range 1 < p T <
24 GeV / c . In the centre of mass system of the p–Pb collisions, themeasurement covers the rapidity range − . < y < .
04. Within the current statistical and systematic uncertainties,no evidence of a dependence of the production cross section on rapidity is observed within this window, as shown inthe right panel of the same figure for D in 2 < p T < / c , 5 < p T < / c and 8 < p T <
16 GeV / c . Themeasured p T , y -di ff erential cross-section is compatible with predictions obtained by scaling the y-di ff erential cross-section calculated with FONLL [15] by the Pb atomic mass number and by a R pPb estimated, as a function of p T ,on the basis of the MNR calculation [24] and EPS09 nuclear PDF parametrizations [22]. The latter R pPb estimatedescribes well the measured R pPb [25]. The comparison with the preliminary R AA measured in the 7.5% most centralPb–Pb collisions in 1 < p T <
36 GeV / c , shown in the left panel of Fig. 1, highlights that the suppression observed inPb–Pb collisions (about a factor 5 for p T ∼
10 GeV / c ) is predominantly induced by final state e ff ects due to charmquark energy loss in the medium.The preliminary comparison of the centrality dependence of the nuclear modification factors of non-strange D3 Nuclear Physics A 00 (2018) 1–5 (GeV/c) T p v |>2} η∆ {EP,| vCharged particles, {EP} v average, |y|<0.8, *+ , D + ,D Prompt DSyst. from dataSyst. from B feed down = 2.76 TeV NN sPb Pb, Centrality 30 50%
ALICE
ALI−PUB−48703 histo_2 (rad) φ∆ ) (r ad φ ∆ d a ss o c d N D N histo_2 average) *+ ,D D meson charged hadron azimuthal correlations (D = 5 nb int =7 TeV, Lspp, > 0.3 GeV/c assocT < 16 GeV/c, p DT η∆ | 14% scale uncertainty ± histo_2 DataPythia Perugia 0Pythia Perugia 2010Pythia Perugia 2011
ALI−DER−63803
Figure 3. Left panel (from [28]): comparison between D , D ∗ + and D + average v and charged hadron v measured in 30-50% central Pb–Pb collisions. Right panel: azimuthal correlations between D mesons (average of D and D ∗ + results) and charged hadrons in pp collisionsat √ s = ∆ φ . mesons and of J /ψ from B-meson decay measured with CMS [26], displayed in the left panel of Fig. 2, represents anindication for a stronger suppression of charm than beauty in central Pb–Pb collisions. The 8 < p T <
16 GeV / c rangewas chosen for D mesons in order to have a similar kinematic range than that of B mesons decaying in a J /ψ in themeasured 6 . < p T <
30 GeV / c range. For the subtraction of non-prompt D mesons from B-meson decays, on thebasis of CMS results, R feed − downAA / R DAA = < R feed − downAA / R DAA < R AA is compatible within uncertainties with the charged-hadron R AA for p T > / c in central Pb–Pb collisions.The same observation holds for more peripheral collisions, as also visible in the right panel of Fig. 2, where thecentrality dependence of D-meson and charged-pion nuclear modification factors is shown for 2 < p T < / c .In the left panel of Fig. 3 the first measurement of D-meson elliptic flow in heavy-ion collisions is shown. Themeasurement, performed with ALICE in the 30-50% centrality range [28], exploits the event plane method, in whichthe correlation of the particle azimuthal angle ( ϕ ) to the reaction plane Ψ RP is analyzed. The reaction plane is estimatedvia the event plane Ψ , which is obtained from the azimuthal distribution of a (sub-)sample of tracks in the event [29].The measurement represents a 5 σ observation of v > < p T < / c , with an average of themeasured values in this interval around 0.2. A positive v is also observed for p T > / c , which most likelyoriginates from the path-length dependence of the partonic energy loss, although the large uncertainties do not allowa firm conclusion. The measured D-meson v is comparable in magnitude to that of charged particles, which isdominated by light-flavour hadrons [30]. This suggests that low momentum charm quarks take part in the collectivemotion of the system.As an outlook, in the right panel of Fig. 3, the first measurement of the azimuthal correlations between D mesonsand charged particles in pp collisions at √ s = and D ∗ + correlations with charged particles, is compatiblewith the expectations obtained from the PYTHIA event generator [32] (tunes Perugia 0, Perugia 2010, and Peru-gia 2011 [33]), considering the large statistical and systematic uncertainties. The latter is dominated by the 14%uncertainty on the normalization. The larger statistics expected from upcoming runs at the LHC in 2014-2018, shouldallow for a precise measurement already in run 2 and the upgrade of the ALICE detector during the long shut downin 2018 [34] should give access to this observable also in Pb–Pb collisions with run 3 data. References [1] P. Braun-Munzinger and J. Stachel, Nature , 302-309 (2007).[2] M. Gyulassy and M. Plumer, Phys. Lett.
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