Heavy ﬂavor measurements at the STAR experiment

. In ultra-relativistic heavy-ion collisions at RHIC, a new state of nuclear matter with extreme properties is produced — the strongly interacting quark-gluon plasma. Heavy quarks, predominantly produced at early stages of collisions owing to their large masses, provide an exceptional probe for exploring the hot and dense medium created in such collisions. The Heavy Flavor Tracker and Muon Telescope Detector have been fully operational since 2014 and have signiﬁcantly improved STAR’s capabilities in measuring both open and hidden heavy ﬂavor hadrons in heavy-ion collisions. We present an overview of recent heavy ﬂavor results obtained at the STAR experiment. of charm quark di ﬀ usion coe ﬃ cients between 2 and ∼ 12 for the QGP temperature between T c and 2T c . The measurements of J / ψ with the MTD via the di-muon channel are consistent with the di-electron channel results and show a distinct rising trend in the R AA with transverse momentum in 20-60% centrality. The suppression observed at high p T in 0-20% central collisions is consistent with dissociation due to color screening. The measurements of high- p T J / ψ R AA and ratio of yields of di ﬀ erent Υ states indicate a lower medium temperature reached at RHIC than at LHC.


Introduction
Heavy flavor quarks are predominantly produced early in heavy-ion collisions in hard scattering processes. They experience the full evolution of the system. These features make heavy quarks ideal probes for studying the properties of the strongly coupled Quark-Gluon Plasma (sQGP) at RHIC. Two new detectors dedicated to heavy flavor measurements, the Heavy Flavor Tracker (HFT) [1] and the Muon Telescope Detector (MTD) [2], were fully installed at the STAR experiment in the beginning of 2014. In this proceedings we will report recent results from STAR using the HFT and MTD.
The MTD is made of multi-gap resistive plate chambers placed at the outermost layer of STAR behind the solenoidal magnet, which is used as a hadron absorber. It is designed for muon triggering and identification with timing resolution ∼100 ps for p T > 1.2 GeV/c. It has a geometrical acceptance of 45% in azimuth within |η| < 0.5.

Open heavy flavor measurements
The total charm production cross sections at mid-rapidity (|η| < 1) have been measured by STAR in p+p and Au+Au collisions at √ s NN = 200 GeV, and are found to scale with the number of binary nucleon+nucleon collisions [4]. This observation supports the expectation that heavy flavor quarks are mainly produced in initial hard parton scatterings at RHIC. Relatively long lifetimes of heavy flavor quarks enable them to experience the whole evolution of the system. Therefore they can serve as calibrated probes to study QGP. Shown in figure 1 is the D 0 nuclear modification factor (R AA ) in 0-10% Au+Au collisions at √ s NN  [4]. The vertical bars (brackets) represent statistical (systematic) uncertainties, while the grey shaded areas indicate the systematic uncertainties from p+p reference for the R AA obtained from the HFT data. The latter were not present for the published results due to cancellation of efficiency corrections between the p+p and Au+Au data. The boxes around unity denote the overall normalization uncertainties in the Au+Au and p+p data, respectively. The new R AA measurement confirms the strong suppression of D 0 yields at high transverse momenta, which is comparable to the suppression observed for light hadrons and indicates strong charm interactions with the medium. of ( m 2 0 + p 2 T −m 0 )/n q , where m 0 is the hadron mass and n q the number of constituent quarks. The D 0 v 2 /n q is below those of light hadrons [5], but still consistent within uncertainties, suggesting non-zero charm quark flow. The measurements of D 0 R AA and v 2 are compared to theoretical calculations. As seen in figure 3, the TAMU [6] and SUBATECH [7] models describe both the D 0 R AA and v 2 . The DUKE [8] model describes the R AA well but underestimates the v 2 . These models use different approaches to describe interactions of charm quarks in the medium and allow to extract transport coefficients. The TAMU model uses a non-perturbative T-matrix approach with potentials obtained from Lattice QCD calculations to describe two-body interactions as a function of the transferred 4-momentum. The SUBATECH group employs a pQCD approach with the Hard Thermal Loop approximation for soft collisions. The charm quark diffusion coefficient 2πT D s extracted from the TAMU (SUBATECH) model ranges between 2-∼10 (2-4) for T/T c = 1-2. The DUKE group uses a generalized Langevin approach with transport properties tuned to LHC data, i.e. 2πT D s = 7. Figure 4 shows the diffusion coefficient extracted from different model calculations [7,[9][10][11][12][13][14] compared to the yellow band inferred from the STAR data, which are consistent with the lattice QCD calculations [13,14] within large uncertainties.

Quarkonium measurements with the MTD
While the heavy quark production can be calculated by pQCD, the evolution into quarkonia from quark-antiquark pair is a non-perturbative process. Precise measurements of quarkonium production in p+p collisions can help to understand quarkonium production mechanisms. The situation in heavyion collisions is more complicated. Quarkonia may dissociate owing to the color screening of the heavy quark-antiquark potential by the QGP. Such a dissociation depends on the temperature of the QGP and varies for different quarkonium states of different binding energies. On the other hand, cold nuclear matter effects (CNM), such as shadowing/anti-shadowing of parton distribution functions, initial-state parton energy loss and final-state nuclear absorption, along with regeneration via coalescence of deconfined charm quarks may also play a role. Therefore quarkonium production can be used to constrain the QGP temperature once other effects are understood.    [15,16] within uncertainties. The dependence of J/ψ R AA on the number of participating nucleons (N part ) has also been measured. The left (right) panel of figure 6 shows the abovementioned di-muon results (red stars) for p T > 0 (5) GeV/c compared to the results from PHENIX [17] in Au+Au collisions at the same centre-of-mass energy (black open circles) as well as the measurements from ALICE [18] and CMS [19]   contributions to low p T J/ψ at the LHC with much higher cc yields. At high p T , the R AA at both RHIC and LHC have a decreasing trend towards more central collisions, and the LHC results are lower than those at RHIC. This is likely due to higher temperature reached at the LHC which leads to stronger J/ψ dissociation. Two different transport models [20,21], both including dissociation and regeneration, can qualitatively describe the measured R AA . However, there seems to be a tension between the model calculations and experimental data at high p T . Shown in the left panel of figure 7 is the reconstructed Υ signal from the di-muon decay channel using the same MTD data described above. Compared to the di-electron channel, the di-muon channel has the advantage of better invariant mass resolution, thanks to the significantly suppressed bremsstrahlung radiation. The unlike-sign (red points) spectrum is fitted to the sum of Υ(1S), Υ(2S) and Υ(3S) signal distributions, BB, Drell-Yan and combinatorial background distributions. The shapes of the signal distributions are Gaussian functions with means fixed to the PDG values, and widths determined from detector simulations, while the shape of BB and Drell-Yan background is determined from PYTHIA. Both the shape and normalization of the combinatorial background are obtained by fitting the like-sign (black points) spectrum. Shown in the right panel are the ratio of the excited Υ state yield with respect to the ground state yield, obtained from the abovementioned fit, the worldwide p+p data and CMS measurements [22,23] in Pb+Pb collisions at √ s NN = 2.76 TeV. There is an indication of larger Υ(2S + 3S)/Υ(1S) ratio at RHIC than that at the LHC.

Summary
STAR has carried out the first measurements with the recently installed HFT and MTD in Au+Au collisions. The measurements of D 0 R AA and v 2 indicate strong interactions between charm quarks and the medium created in heavy-ion collisions. Model calculations which can simultaneously describe the R AA and v 2 give a range of charm quark diffusion coefficients between 2 and ∼12 for the QGP temperature between T c and 2T c . The measurements of J/ψ with the MTD via the di-muon channel are consistent with the di-electron channel results and show a distinct rising trend in the R AA with transverse momentum in 20-60% centrality. The suppression observed at high p T in 0-20% central collisions is consistent with dissociation due to color screening. The measurements of high-p T J/ψ R AA and ratio of yields of different Υ states indicate a lower medium temperature reached at RHIC than at LHC.