Measurements of heavy flavor quark (charm and bottom) correlations in heavy ion collisions are instrumental to understand the flavor dependence of energy loss mechanisms in hot and dense QCD media. Experimental measurements of these correlations in b
aseline $p$+$p$ collisions are crucial to understand the contributions of perturbative and non-perturbative QCD processes to the correlation functions and further help in interpreting correlation measurements in heavy ion collisions. In this paper, we investigate $D$-$bar{D}$ meson correlations and $D$ with one particle from $D$ meson decay daughter correlations using PYTHIA Event Generator in $p$ + $p$ collisions at $sqrt{s}$ = 200, 500 and 5500 GeV. Charm/bottom events are found to contribute mainly to the away side/near side pattern of $D$-electron correlations, respectively. In the energy region of RHIC, $D$-$bar{D}$ correlations inherit initial $c$-$bar{c}$ correlations and $Brightarrow DX$ decay contribution is insignificant. Furthermore, Bottom quark correlations are suggested to be applicable at LHC energy, as the bottom contributions on $D$ related correlations are relatively large.
RHIC-STAR is a mid-rapidity collider experiment for studying high energy nuclear collisions. The main physics goals of STAR experiment are 1) studying the properties of the strongly coupled Quark Gluon Plasma, 2) explore the QCD phase diagram structu
re. In these proceedings, we will review the recent results of heavy ion physics at STAR.
We present the $v_2$ measurement at midrapidity from Au+Au collisions at $sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, 39 and 62.4 GeV for inclusive charged hadrons and identified hadrons ($pi^{pm}$, $K^{pm}$, $K_{S}^{0}$, $p$, $bar{p}$, $phi$, $Lambda$, $bar
{Lambda}$, $Xi^{-}$, $bar{Xi}^{+}$, $Omega^{-}$, $bar{Omega}^{+}$) up to 4 GeV/$c$ in $p_{T}$. The beam energy and centrality dependence of charged hadron $v_2$ are presented with comparison to higher energies at RHIC and LHC. The identified hadron $v_{2}$ are used to discuss the NCQ scaling for different beam energies. Significant difference in $v_{2}(p_{T})$ is observed between particles and corresponding anti-particles for $sqrt{s_{NN}} <$ 39 GeV. These differences are more pronounced for baryons compared to mesons and they increase with decreasing energy.
A systematic study is presented for centrality, transverse momentum ($p_T$) and pseudorapidity ($eta$) dependence of the inclusive charged hadron elliptic flow ($v_2$) at midrapidity($|eta| < 1.0$) in Au+Au collisions at $sqrt{s_{NN}}$ = 7.7, 11.5, 1
9.6, 27 and 39 GeV. The results obtained with different methods, including correlations with the event plane reconstructed in a region separated by a large pseudorapidity gap and 4-particle cumulants ($v_2{4}$), are presented in order to investigate non-flow correlations and $v_2$ fluctuations. We observe that the difference between $v_2{2}$ and $v_2{4}$ is smaller at the lower collision energies. Values of $v_2$, scaled by the initial coordinate space eccentricity, $v_{2}/varepsilon$, as a function of $p_T$ are larger in more central collisions, suggesting stronger collective flow develops in more central collisions, similar to the results at higher collision energies. These results are compared to measurements at higher energies at the Relativistic Heavy Ion Collider ($sqrt{s_{NN}}$ = 62.4 and 200 GeV) and at the Large Hadron Collider (Pb + Pb collisions at $sqrt{s_{NN}}$ = 2.76 TeV). The $v_2(p_T)$ values for fixed $p_T$ rise with increasing collision energy within the $p_T$ range studied ($< 2 {rm GeV}/c$). A comparison to viscous hydrodynamic simulations is made to potentially help understand the energy dependence of $v_{2}(p_{T})$. We also compare the $v_2$ results to UrQMD and AMPT transport model calculations, and physics implications on the dominance of partonic versus hadronic phases in the system created at Beam Energy Scan (BES) energies are discussed.
We present measurement of elliptic flow, $v_2$, for charged and identified particles at midrapidity in Au+Au collisions at $sqrt{s_{NN}}$ = 7.7 - 39 GeV. We compare the inclusive charged hadron $v_2$ to those from transport model calculations, such a
s UrQMD model, AMPT default model and AMPT string-melting model. We discuss the energy dependence of the difference in $v_2$ between particles and anti-particles. The $v_2$ of $phi$ meson is observed to be systematically lower than other particles in Au+Au collisions at $sqrt{s_{NN}}$ = 11.5 GeV.
