Direct photon-charged hadron coincidence measurements in STAR

Abstract

The multiplicities of charged particle with transverse momentum (3 $<$ $p_T^{h^{\pm}}$ $<$ 16 $\mathrm GeV/c$) and pseudorapidity ($\mid\eta\mid$ $\leq$ 1.0) associated with direct photons and $\pi^{0}$ of high transverse momentum (8 $<$ $p_T^{\gamma, \pi^{0}}$ $<$ 16 $\mathrm GeV$) at mid-rapidity ($\mid\eta\mid$ $\leq$ 0.9) have been measured over a broad range of centralities for $Au+Au$ collisions and $p+p$ collisions at $\sqrt{s_{NN}}$ = 200 $\mathrm GeV$ in the STAR experiment. A transverse shower-shape analysis in the STAR Barrel Electromagnetic Calorimeter Shower Maximum Detector is used to discriminate between the shower of single photon and that of photons from the decays of high $p_T$ $\pi^{0}$s. The relative azimuthal positions of the associated particles with respect to the trigger particle are constructed, and the associated charged hadron yields per direct $\gamma$ are extracted. An agreement is observed between the measured suppression for direct $\gamma$-trigger associated-particle yields in $Au+Au$ compared to that in $p+p$ and theoretical calculations, although the uncertainties are large. Within the current uncertainties, the suppression is similar to the previously observed suppression in single-particle yields as well as in hadron-triggered associated-particle yields.