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Heavy flavor production in STAR

We report on the recent STAR results on heavy flavor production in Au+Au collisions at = 200, 62.4 and 39 GeV. We present the nuclear modification factor and elliptic flow of open charm mesons and electrons from semileptonic decays of heavy flavor hadrons. We also report on new measurements of energy dependence of J/ψ production. STAR data are compared to theoretical model calculations and physics implications are discussed.


Introduction
Heavy quarks, charm and bottom, are unique probes of the Quark Gluon Plasma (QGP) properties at RHIC because they are produced in the initial interactions with large momentum transfer, before the QGP phase, and they are expected to interact with the QGP differently from light quarks. Experimentally, we study heavy quark production via decay products of charmed and beauty hadrons. At the moment, there are two feasible techniques for such studies at RHIC: using electrons from semi-leptonic decay of open heavy flavor mesons (so called non-photonic electrons, NPE), or through hadronic decays of charmed mesons. In the first case, the yields are larger and a specialized trigger can be used, although the information about the parent meson kinematics is incomplete and such an electron sample is a mixture of electrons from charmed and beauty hadron decays. In the latter, we identify charmed mesons via hadronic decays (e.g. D 0 → K − π + ) which give access to the kinematic of parent meson, but it suffers from large combinatorial background if a vertex detector is not available.
Elliptic flow of heavy quarks is another important tool for studying the QGP properties. It can shed new light on possible collective behavior and degree of thermalization of nuclear matter created in heavy ion collisions. Simultaneous measurements of the c and b quark production and elliptic flow are crucial for understanding the nature of interactions of heavy quarks with the surrounding partonic medium, and the parton energy loss mechanism in general.
Measurements of the production of various quarkonia states can provide insight into thermodynamic properties of the QGP. Different states have different binding energy and therefore disassociate, due to the Debye screening of the quark-antiquark potential, at different temperatures. However, there are other effects that may affect the observed production, such as cold nuclear matter effects, final state nuclear absorption and statistical coalescence of quarkantiquark pairs. The study of quarkonia production for different colliding systems, collision energies and centralities can help to disentangle the interplay of these mechanisms and to understand the medium properties.
Besides studies of the QGP properties at top RHIC energy, we measure heavy quark production as a function of colliding energy in RHIC Beam Energy Scan program. Such measurements, complementary to the results for light quarks, provide us with additional means to map out the phase diagram of nuclear matter.
2. Data analysis STAR [1] is a large acceptance (|η| <1, full coverage in φ) multipurpose detector composed of several subsystems, which is well suited for the measurement of heavy quark production. In the analyses reported here, STAR Time Projection Chamber (TPC), Barrel Electromagnetic Calorimeter (BEMC) and Time-Of-Flight (TOF) detectors are used. We reconstruct quarkonia (J/ψ, Υ) via the di-electron decay channel. We identify charmed mesons via their hadronic decays (e.g. D 0 → K − π + ). The TPC is used for charged track reconstruction and particle identification via specific ionization energy loss (dE/dx). BEMC is used at high p T for electron selection and also for a high-p T (p T > 2 GeV) electron trigger, while TOF provides electron identification at low p T .  The left panel of Fig. 1 shows D 0 p T spectrum in p + p and Au+Au collisions at √ s NN = 200 GeV. The dashed lines represent a Levi fit to p + p data scaled by the number of binary collisions (N bin ) in a given centrality bin. The Levi fit describes the p + p data well; however, there are clear discrepancies when the fit is compared to Au+Au data. To quantify a modification of the D 0 production in Au+Au collisions, we calculate the nuclear modification factor R AA : yields in Au+Au and p+p collisions, respectively, and N bin is the number of binary collisions in a given Au+Au centrality class. The right panel of Fig. 1 shows the D 0 R AA for 0-10% most central and minimum bias Au+Au collusions. At high p T , D 0 production is significantly suppressed. Moreover, D 0 production is enhanced at intermediate p T (a "bump" at p T ∼ 1.5 GeV/c). The STAR data are compared to model predictions by He et al. [5] and Gossiaux et al. [6]. Both models assume that heavy quarks are strongly coupled with the surrounding partonic medium and predict a large suppression at high p T . The models qualitatively reproduce the shape of R AA as a function of p T . The enhancement at p T ∼ 1.5 GeV/c in those calculations is due to the charm quark and medium interaction as well as the radial flow of light quarks which coalesce with charm quarks to produce D mesons. However, a similar effect for other particles (protons, pions) is observed at intermediate p T in d+Au collisions [7] (the Cronin effect).  GeV are not corrected for J/ψ → e + e − contribution, which is as large as ∼ 20% at high p T at 200 GeV. Figure 3 shows the R AA and elliptic flow of non-photonic electrons. We observe a strong NPE suppression at high p T in central collisions, similar to D 0 and light hadrons (π ± ). Figure 3 (right panel) shows v 2 results obtained using two-and four-particle correlations (v 2 {2} and v 2 {4}, respectively). v 2 {2} and v 2 {4} have different sensitivity to the flow fluctuations and so called nonflow (correlations not related to the reaction plane) and thus provide an upper and lower limit on the average elliptic flow [9]. We compare our data to a few model calculations. The DGLV model with radiative energy loss only [10] underestimates the observed suppression; however, it gives a reasonable description of the data when collisional energy loss is added. The collisional dissociation model [11] and AdS/CFT correspondence calculations [12] can also describe the measured R AA reasonably well. In a partonic transport model, BAMPS [13,14], heavy quarks lose energy due to elastic collisions with the rest of the medium. The dasheddotted green line shows predictions for v 2 from the implementation of radiative and collisional energy loss from Gossiaux et al. [15,6]. He et al. employ the TMatrix interactions model [5] which is a non-perturbative approach to heavy quark energy loss. Each of these models predicts a non-zero v 2 of charm quarks, thus observed finite NPE v 2 suggest a finite elliptic flow of charm quarks which is acquired due to interactions with quarks and gluons in the QGP phase. At 200 GeV, v 2 increases with p T for p T > 3 GeV which is likely due to decay or jet-like correlations.  The left panel of Fig. 4 shows the nuclear modification factor as a function of centrality (represented by number of participants, N part ) for low−p T (p T < 5) GeV/c J/ψ in Au+Au  [17] which assumes two main components: suppression of directly produced J/ψ due to color screening and secondary production via recombination of c,c quarks. The model predicts very similar suppression for both 39 and 62 GeV, despite the significant difference in energy and thus different initial energy densities. In this model, larger direct suppression at higher energy is compensated by an enhanced regeneration rate due to the larger cross section for charm quarks. These calculations agree reasonably well with our data within errors. The right panel of Fig. 4 shows the nuclear modification factor as a function of centrality for Υ in Au+Au collisions at √ s NN = 200 GeV. The suppression increases with centrality and a significant suppression is observed in the most central collisions. Data are compared to a model by Strickland et al. [18], which assumes a thermal dissociation of Υ in a thermalized, expanding partonic medium, simulated with relativistic hydrodynamics. These calculations predict almost complete suppression of Υ(2S) and Υ(3S) states in the most central collisions. The model reproduces the STAR results well and the data favors the internal energy potential scenario.