Scholarly article on topic 'D mesons suppression in Pb–Pb collisions at measured by ALICE'

D mesons suppression in Pb–Pb collisions at measured by ALICE Academic research paper on "Physical sciences"

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Nuclear Physics A
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{"Heavy flavor" / Charm / Heavy-Ion / QGP / LHC}

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

Abstract The measurement of the prompt charm mesons D 0 , D + , D ⁎ + , and their antiparticles, in Pb–Pb collisions at the LHC, at a centre-of-mass energy s NN = 2.76 TeV , with the ALICE detector, using 2010 data, is presented. The R AA of the three meson species show a suppression of a factor 3–4, for transverse momenta larger than 5 GeV/c, in the 20% most central collisions. The suppression is reduced for peripheral collisions. The results are compared with those of charged particles, charged pions, and non-prompt J / ψ , and also with theoretical calculations.

Academic research paper on topic "D mesons suppression in Pb–Pb collisions at measured by ALICE"

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Nuclear Physics A 910-911 (2013) 285-288

NUCLEAR PHYSICS

www.elsevier.com/locate/nuclphysa

D mesons suppression in Pb-Pb collisions at VsNN = 2.76 TeV measured by

Zaida Conesa del Valle for the ALICE Collaboration

European Organization for Nuclear Research (CERN), Geneva, Switzerland

Abstract

The measurement of the prompt charm mesons D0, D+, D*+, and their antiparticles, in Pb-Pb collisions at the LHC, at a centre-of-mass energy sJsNN = 2.76 TeV, with the ALICE detector, using 2010 data, is presented. The raa of the three meson species show a suppression of a factor 3-4, for transverse momenta larger than 5 GeV/c, in the 20% most central collisions. The suppression is reduced for peripheral collisions. The results are compared with those of charged particles, charged pions, and non-prompt J/ty, and also with theoretical calculations.

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

1. Introduction

Charm and beauty production in proton-proton collisions at the LHC is an important tool to test pQCD calculations in a new energy domain. Their spectra in heavy-ion interactions are influenced by the formation of hot and dense QCD matter. Due to their relative large mass and the difference between quark and gluon color charge, the medium effects for heavy flavor hadrons differ from those of light hadrons. RHIC results show that they effectively lose energy in the medium through (elastic and/or inelastic) interactions, but a possible quark-mass hierarchy has not yet been elucidated. LHC measurements at higher energies, with larger cross sections and better capabilities to separate charm and beauty production, can help to answer these questions.

Open heavy flavor production can be measured with the ALICE experiment at the LHC in different colliding systems. The D meson reconstruction and analysis strategy using 2010 Pb-Pb data at ysNN = 2.76 TeV [1-3] is summarized here. The results are compared with those of charged particles [8], charged pions, and non-prompt J/ty [9], and also with theoretical calculations [11-19].

2. Measurements in Pb-Pb collisions at ysnn = 2.76 TeV with 2010 data

In heavy-ion collisions, the influence of the hot and dense QCD matter on the produced particles is commonly evaluated by comparing particle production in proton-proton (pp) and nucleus-nucleus (AA) collisions. It is usually presented in the form of the nuclear modification factor (RAA):

R 1 d^AA/d pt m

rAA(pT) = ^^ -Ya— ' (1)

(Taa) dŒpp/dpt

Email address: zaida.conesa.del.valle@cern.ch (Zaida Conesa del Valle for the ALICE Collaboration) © CERN for the benefit of the ALICE Collaboration.

0375-9474/ © 2013 CERN Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysa.2012.12.015

the ratio between the invariant yield in AA collisions (dN^/dpx), and the pp cross section (d<xpp/dpT) normalized by the average nuclear overlap function1 (<TAA)).

The data sample analyzed concerns minimum bias Pb-Pb collisions at ysNN = 2.76 TeV, collected in November and December 2010, with a £int = 5 nb-1. D0, D+ and D*+ mesons, and their antiparticles, were reconstructed in the central rapidity region in their hadronic decay channels : D0 ^ K-n+, D+ ^ K-n+n+, D*+ ^ D0n+. Only good quality tracks, with |nl < 0.8, at least 70 associated space points in the Time Projection Chamber (TPC), with ;X2/ndf< 2, and at least 2 (out of 6) associated hits in the Inner Tracking System (ITS), were selected to build the combinatorics. The selection of D0 (ct ~ 123 ^m) and D+ (ct ~ 312 ^m) candidates was based on the reconstruction of their secondary vertex topologies. Typical selection variables were: the candidate decay length, the distance and angles of the decay products, and the minimum pT of the decay products. In the D*+ meson case, the decay D0 secondary vertex topology was reconstructed. The selection cut values vary for each meson and pT bin, and were defined in order to have a large statistical significance of the signal and keep the selection efficiency as high as possible. The decay pions and kaons were selected using a ±3<x cut around the expected energy deposit in the TPC and the time of flight in the TOF detectors2. To build the D*+ candidates, no particle identification was required to the pions associated with D0 candidates. Acceptance and reconstruction efficiencies were evaluated with Monte Carlo simulations of minimum-bias Pb-Pb collisions produced with the HIJING v1.36 event generator. Prompt and feed-down (B decays) D meson signals were added using pp events from the PYTHIA v6.4.21 event generator with the Perugia-0 tuning [1]. A detailed description of the apparatus geometry and response, the experimental conditions, and their evolution with time was included. Prompt D meson yields were obtained by subtracting the contribution of D mesons from B decays. It was evaluated using the beauty cross sections from FONLL [4] calculations, the B—>D decay kinematics from the EvtGen package [5] and the Monte Carlo efficiencies for feed-down D mesons. This contribution was renormalized by the average nuclear overlap function in each centrality class, and the nuclear modification factor of feed-down D mesons (RAAd-down). To perform the correction, it was assumed that RAAd-down = ^AAmpt, while _RAAd-down was allowed to vary to evaluate the measurement systematics. More details on the analysis procedure and the systematic uncertainties determination can be found in ref. [1].

D0, D+ and D*+ mesons cross sections in pp collisions at = 7 TeV and 2.76 TeV were reported in ref. [2, 3]. Due to the limited statistics of the = 2.76 TeV data sample, the 7 TeV cross sections scaled down to 2.76 TeV were used as proton-proton reference. The scaling factor was evaluated as the ratio of the FONLL [4] cross sections at the two energies. The scaling factor uncertainties include the FONLL calculation uncertainties: 0.5mT < |R < 2mT,

0.5mT < |F < 2mT, 1.3 < mc < 1.7 GeV/c2, where |F = |R = mT and mT = p| + The scaling procedure was validated by comparing the scaled results to the measurements at 2.76 TeV [3], and by scaling the 7 TeV data to Tevatron energies and comparing to CDF data [6]. The scaling was also verified by using GM-VFNS calculations [7].

3. Results

The transverse momentum distributions dN/dpT and RAA(pT) of prompt D0, D+ and D*+ mesons in the 0-20% and 40-80% centrality classes were first presented in ref. [1]. Figure 1 shows RAA(pT) of prompt D mesons. The results of the three D meson species are in agreement within statistical uncertainties and show a suppression by a factor of 3-4 for pT > 5 GeV/c in the 0-20% centrality class. The centrality dependence was studied in more detail in wider pT bins and thinner centrality bins and reported in ref. [1]. A tendency to have a larger suppression in the most central collisions is observed, see also Fig. 2 (right).

The average D meson RAA(pT) was computed from the weighted average3 of D0, D+ and D*+ RAA(pT). The average D meson RAA is compared in Fig. 2 with charged particles [8] and non-prompt J/ifr from CMS [9]. RAA(pT) of the average D meson and charged particles [8] are similar in magnitude and pT trend, while non-prompt J/ifr shows a smaller suppression than that of charged particles. This observation can also be done in Fig. 2 (right), which displays

1 The nuclear overlap function, Taa, is defined as the convolution of the colliding ions nuclear density profiles, and it is evaluated in the Glauber model. The estimates of Taa in the Glauber model are in agreement with those computed in a Glauber Monte Carlo of the VZERO scintillators response, located at 2.8 < n < 5.1 and -3.7 < n < -1.7.

2 In the D*+ case, for the 0-20% centrality class, a ±2^ cut was applied to the D0 decay products.

3 Statistical uncertainties were used as weights to evaluate the average D meson RAA( pT).

Figure 1: Raa for prompt D0, D+ and D*+ in the 0-20% (left) and 40-80% (right) centrality classes [1]. Statistical (bars), systematic (empty boxes), and normalization (full box) uncertainties are shown. Horizontal error bars reflect bin widths, symbols were placed at the centre of the bin.

2r 1.8: 1.6: 1.4: 1.2-

1B ------

0.8: 0.6: 0.4: 0.2:

ALI-PUB-14258

ALICE 0-20% centrality Pb-Pb,V^nn = 2.76 TeV

Average D0, D+, D*+, |y|<0.5 Charged particles, |nl<0.8 CMS non-prompt J/y, |y|<2.4

•1.2r

12 14 16

p (GeV/c)

ALICE Pb-Pb, l[sNN = 2.76 TeV

^ Average D0, D*, D**, 6<p<12 GeV/c ^ Charged particles, 6<p<12 GeV/c CMS non-prompt J/y, p>6.5 GeV/c "

(CMS periph. point shown at (Npar^ weighted by N 3)

ALI-PUB-14707

common normalization uncertainty on ALICE data: 7% (peripheral) to 4% (central)

if. i i i i 11 i i i i 11 i i i i 11 i i i i 11 i i i i 11 i i i 11 i i r

0 50 100 150 200 250 300 350 400

< Nrt >

part '

Figure 2: Left: Average D meson (D0, D+ and D*+) Raa in the 0-20% centrality class vs. pt compared with charged particles and non-prompt J/ty [1]. Right: Average D meson Raa vs. centrality, represented by the average number of participant nucleons, (Npart), compared with charged particles and CMS non-prompt J/ty.

Raa vs. centrality, represented by the average number of participant nucleons (Npart), for pr > 6 GeV/c. However the comparison of non-prompt J/ifr and the average D meson raa is not yet conclusive and requires more precise and differential measurements.

The results in the 0-20% centrality class are compared in Fig. 3 (left) to the NLO (MNR) calculations [10] with EPS09 [11] shadowing. The magnitude of D meson suppression at pr > 5 GeV/c in the most central class can therefore not be explained by initial-state effects. Figure 3 (right) presents the ratio of the average D mesons and charged pions RAA(pr) in 0-20% centrality class. This ratio shows no clear pr dependence and is of about 1.5. Several theoretical models based on parton energy loss compute the charm nuclear modification factor [12-19]. Those having results for both D mesons and charged pions are shown in Fig. 3 (right) [12-15, 19]. All of them describe reasonably well this ratio and the D mesons and charged pions RAA(pT). However, it can be noted that a model based on the AdS/CFT correspondence seems to underestimate charm RAA and has limited predictive power for light hadron RAA.

Figure 3: Left: Average D meson (D0, D+ and D*+) Raa in the 0-20% centrality class vs. pt compared with NLO (MNR) calculations [10] with EPS09 [11] shadowing [1]. Right: Ratio of the average D meson and charged pion Raa vs pr compared with theoretical calculations [12-15, 19].

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