Collision centrality and $\tau_0$ dependence of the emission of thermal   photons from fluctuating initial state in ideal hydrodynamic calculation
  Fluctuations in the initial QCD matter density distribution are found to enhance the production of thermal photons significantly in the range 2 \leq pT \leq 4 GeV/c compared to a smooth initial state averaged profile in ideal hydrodynamic calculation for 200 AGeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and 2.76 ATeV Pb+Pb collisions at the Large Hadron Collider (LHC). The thermal emission of photons is strongly dependent on the initial temperature of the system where the presence of 'hotspots' in the initial state translates into enhanced production of photons compared to a smooth profile. The effect of fluctuations in the initial state is found to be stronger for peripheral collisions and for lower beam energies. The pT spectra are found to be quite sensitive to the value of the initial formation time of the plasma which is not known unambiguously and which may vary with collision centralities at a particular beam energy. Increase in the value of the formation time lowers the production of thermal photons compared to the results from a shorter formation time. However, the relative enhancement from fluctuating initial tates (compared to a smooth initial state) is found to be stronger for the larger values of formation time. The pT spectra alone are found to be insufficient to quantify the fluctuations in the initial density distribution due to the uncertainties in the initial conditions. A suitably normalized ratio of central-to-peripheral yield as a function of collision centrality and pT can be a useful measure of the fluctuation size scale. 
Rupa Chatterjee, Hannu Holopainen, Thorsten Renk, and Kari J. Eskola
nucl-th hep-ph