Charm hadron production at RHIC in combination model

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to account for the discrepancy of σNN cc ¯ measured by the two collaborations? A new method is proposed in Ref. [18] to determine σNN cc ¯ by measuring the spectrum of nonphotonic muon, so the ratio + − Rµ/cc ¯ , the inclusive branching ratio to muons ((µ + µ )/2) in AA reactions, is also required. In this paper, we quantitatively study the effects on the charm hadron ratios from the baryon enhancement and the strangeness enhancement in AA reactions within the quark combination model. We find the charm hadron ratios and the three key ratios Re/cc ¯ , RD0 /cc ¯ are substantially differ¯ , Rµ/cc ent from those in the pp( p ¯) reactions or e+ e− annihilations. Their variances with energy, centrality, and some parameters such as the yield ratio of primary charm vector meson to pseudoscalar meson Vc /Pc , the yield ratio of primary charm decuplet baryon to octet baryon Dc /Oc , the measure of net baryon number Nd¯/Nd and the strangeness suppression factor λs etc. are all investigated. Here λs denotes the number ratio of newly Nss Ns Ns ¯ ¯ ¯ produced strange to u(d) quarks, λs ≡ Nu2 . = N = N u ¯ +Nd d¯ u ¯ d¯ Predictions of charm hadron ratios for the upgrade of RHIC and for LHC are given. These ratios are important to measure σNN cc ¯ accurately in AA reactions at various energies in future, and may be helpful to decrease the discrepancy of σNN cc ¯ between STAR and PHENIX. Our predictions can be examined and the hadronization mechanism of open charm hadrons can be tested at RHIC and LHC.
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II.
QUARK COMBINATION MODEL
Our quark combination model (QCM) proposed some time ago [19, 20, 21, 22, 23] demands that quark(s) and/or antiquark(s) which are nearest in rapidity combine into a hadron. It has been shown that [20, 21] such a demand is in agreement with the fundamental requirement of QCD and uniquely determines the quark combination rule in the hadronization process. QCM has been successfully applied to e+ e− annihilations and pp( p ¯) collisions [20, 21, 22]. Recently, we have extended it to the RHIC reactions and have reproduced the global properties of hadrons such as hadronic multiplicities, pT spectra, elliptic flows and rapidity distributions [24]. QCM is particularly designed for describing the hadroniza-
Charm hadron production at RHIC in combination model
Tao Yao, Wei Zhou, and Qu-Bing Xie
School of Physics, Shandong University, Jinan, Shandong 250100, P. R. China (Dated: August 30, 2008) We investigate the charm hadron production in relativistic heavy-ion collisions with the quark combination ¯c ¯c ¯c Λc +Λ Λc +Λ Λc +Λ model. The pT dependencies of the charm baryon to meson ratios such as D − in 0 +D ¯ 0 , D+ +D− and D+ s +Ds Au+Au collisions at 200 GeV are obtained. The charm baryon enhancement in the intermediate pT range is very prominent, which further, together with the strangeness enhancement, affect the charm hadron ratios and lead to a ∼ 17% larger charm cross-section given by PHENIX. The dependencies of the charm hadron ratios on energy, centrality, and other parameters are also investigated. Predictions of the charm hadron ratios for the upgrade of RHIC and for LHC are presented.
arXiv:0809.0049v1 [nucl-th] 30 Aug 2008
PACS numbers: 25.75.-q, 25.75.Dw, 12.40.-y
I.
INTRODUCTION
Charm production in high energy collisions is one of the hot topics of both theory and experiment. Due to the large mass, charm quarks are believed to be produced mainly via initial gluon fusion in the early stage of relativistic heavy-ion collisions [1, 2]. They are also regarded as an unique tool to probe the hot dense matter or quark-gluon plasma(QGP) created in these collisions. For example, through the charm quark energy loss [3], the charm flow [4] and the J /ψ production (suppression or enhancement) [5, 6, 7] etc., one can learn much information of QGP. Since the running of RHIC, PHENIX and STAR collaborations have made many measurements on charm production [8, 9, 10, 11, 12, 13, 14, 15]. The binary scaling of the total charm cross-section σcc ¯ has been observed by the two collaborations. However, they give quite different value of the binary scaled charm cross-section σNN cc ¯ . There are two imin the experiments. One is portant ratios used to obtain σNN cc ¯ Re/cc ¯ , the key parameter to convert the mul¯ ≡ N(e+ +e− )/2 /Ncc tiplicity of nonphotonic electron into the charm cross-section. Here Ncc ¯ pairs created in the collisions. ¯ is the number of cc As pointed out in [16], Re/cc ¯ is sensitive to the charm hadron ratios (e.g. Λc /D0 ), so the different charm hadron ratios can lead to different Re/cc ¯ . Another ra¯ and hence different σcc tio RD0 /cc ¯ 0 )/2 /Ncc ¯ is used in deriving σcc ¯ by STAR, ¯ ≡ N(D0 +D also affected by the charm hadron ratios. Unfortunately, these ratios can not be obtained through theory calculations modelindependently and they are hardly measurable directly now in experiments due to the difficulty of charm hadron reconstruction in Au+Au collisions [52]. Then the two key ratios used in Au+Au collisions at RHIC are currently from the pp( p ¯) reactions or e+ e− annihilations. The enhancement of baryon production in the intermediate pT range observed in Au+Au reactions at RHIC suggests strongly coalescence/recombination (CO/RE) hadronization mechanism [17]. This mechanism can also result in an enhancement of charm baryons [53], so the charm hadron ratios in Au+Au collisions, and Re/cc ¯ , RD0 /cc ¯ , should be different + − from those in pp( p ¯) reactions or e e annihilations. Then one will ask: how large are the corrections to σcc ¯ from the two ratios in the different reactions? and are the effects able