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Charged-particle spectra associated with direct photon ($gamma_{dir} $) and $pi^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $sqrt{s_{_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between $gamma_{dir}$ and $pi^0$. Assuming no associated charged particles in the $gamma_{dir}$ direction (near side) and small contribution from fragmentation photons ($gamma_{frag}$), the associated charged-particle yields opposite to $gamma_{dir}$ (away side) are extracted. At mid-rapidity ($|eta|<0.9$) in central Au+Au collisions, charged-particle yields associated with $gamma_{dir}$ and $pi^0$ at high transverse momentum ($8< p_{T}^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $|eta|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $gamma_{dir}$ and $pi^0$, and independent of the $gamma_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.
We review the charged particle and photon multiplicity, and transverse energy production in heavy-ion collisions starting from few GeV to TeV energies. The experimental results of pseudorapidity distribution of charged particles and photons at differ
Hard photon emitted from energetic heavy ion collisions is of very interesting since it does not experience the late-stage nuclear interaction, therefore it is useful to explore the early-stage information of matter phase. In this work, we have prese
Fast parton probes produced by hard scattering and embedded within collisions of large nuclei have shown that partons suffer large energy loss and that the produced medium may respond collectively to the lost energy. We present measurements of neutra
Parity-odd domains, corresponding to non-trivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the