Energy loss of leading hadrons and direct photon production in evolving quark-gluon plasma

Abstract

We calculate the nuclear modification factor of neutral pions and the photon yield at high ${p}_{T}$ in central Au-Au collisions at the Relativistic Heavy-Ion Collider (RHIC) ($\sqrt{s}=200\phantom{\rule{0.3em}{0ex}}\text{GeV}$) and Pb-Pb collisions at the Large Hadron Collider (LHC) ($\sqrt{s}=5500\phantom{\rule{0.3em}{0ex}}\text{GeV}$). A leading-order accurate treatment of jet energy loss in the medium has been convolved with a physical description of the initial spatial distribution of jets and a one dimensional hydrodynamic expansion. We reproduce the nuclear modification factor for pions ${R}_{\mathit{AA}}$ at RHIC, assuming an initial temperature ${T}_{i}=370\phantom{\rule{0.3em}{0ex}}\text{MeV}$ and a formation time ${\ensuremath{\tau}}_{i}=0.26$ fm/c, corresponding to $\mathit{dN}/\mathit{dy}=1260$. The resulting suppression depends on the particle rapidity density $\mathit{dN}/\mathit{dy}$ but weakly on the initial temperature. The jet energy loss treatment is also included in the calculation of high ${p}_{T}$ photons. Photons coming from primordial hard N-N scattering are the dominant contribution at RHIC for ${p}_{T}>5\phantom{\rule{0.3em}{0ex}}\text{GeV}$, whereas at the LHC, the range $8<{p}_{T}<14\phantom{\rule{0.3em}{0ex}}\text{GeV}$ is dominated by jet-photon conversion in the plasma.