We calculate elliptic and triangular flow parameters of thermal photons using an event-by-event hydrodynamic model with fluctuating initial conditions at 200A GeV Au+Au collisions at RHIC and at 2.76A TeV Pb+Pb collisions at the LHC for three different centrality bins. The photon elliptic flow shows strong centrality dependence where v2(pT ) increases towards peripheral collisions both at RHIC and at the LHC energies. However, the triangular flow parameter does not show significant dependence to the collision centrality. The elliptic as well as the triangular flow parameters found to underestimate the PHENIX data at RHIC by a large margin for all three centrality bins. We calculate pT spectrum and anisotropic flow of thermal photons from 200A GeV Cu+Cu collisions at RHIC for 0-20% centrality bin and compare with the results with those from Au+Au collisions. The production of thermal photon is found to decrease significantly for Cu+Cu collisions compared to Au+Au collisions. However, the effect of initial state fluctuation is found to be more pronounced for anisotropic flow resulting in larger v2 and v3 for Cu+Cu collisions. We study the correlation between the anisotropic flow parameters and the corresponding initial spatial anisotropies from their event by event distributions at RHIC and at the LHC energies. The linear correlation between v2 and ǫ2 is found be stronger compared to the correlation between v3 and ǫ3. In addition, the correlation coefficient is found to be larger at LHC than at RHIC.

We calculate pT spectra and elliptic flow for tip-tip and body-body configurations of full overlap uranium-uranium (U+U) collisions using a hydrodynamic model with smooth initial density distribution and compare the results with those obtained from Au+Au collisions at RHIC. Production of thermal photons is seen to be significantly larger for tip-tip collisions compared to body-body collisions of uranium nuclei in the region pT > 1 GeV. The difference in the results for the two configurations of U+U collisions depends on the initial energy deposition which is yet to be constrained precisely from hadronic measurements. The thermal photon spectrum from body-body collisions is found to be close to the spectrum from most central Au+Au collisions at RHIC. The elliptic flow parameter calculated for body-body collisions is found to be large and comparable to the v2(pT ) for mid-central collisions of Au nuclei. On the other hand, as expected, the v2(pT ) is close to zero for tip-tip collisions. The qualitative nature of the photon spectra and elliptic flow for the two different orientations of uranium nuclei is found to be independent of the initial parameters of the model calculation. We show that the photon results from fully overlapping U+U collisions are complementary to the results from Au+Au collisions at RHIC.

Recently the effect of nucleon shadowing on the Monte-Carlo Glauber initial condition was studied and its role on the centrality dependence of elliptic flow (v2) and fluctuations in initial eccentricity for different colliding nuclei were explored. It was found that the results with shadowing effects are closer to the QCD based dynamical model as well as to the experimental data. Inspired by this outcome, in this work we study the transverse momentum (pT ) spectra and elliptic flow of thermal photons for Au+Au collisions at RHIC and Pb+Pb collisions at LHC by incorporating the shadowing effects in deducing the initial energy density profile required to solve the relativistic hydrodynamical equations. We find that the thermal photon spectra remain almost unaltered, however, the elliptic flow of photon is found to be enhanced significantly due to shadowing effects.

The Cu + Au collisions are expected to provide interesting new aspects in the understanding of anisotropic flow in heavy ion collisions due to asymmetric initial geometry on the transverse plane as well as due to the significant difference in number of participants in the two colliding nuclei. We calculate directed flow coefficient v 1 of thermal photons from 200 A GeV Cu + Au collisions at RHIC using a hydrodynamic model with fluctuating initial conditions and compare the results with the elliptic and triangular flow parameters. The photon v 1(p T) is found to be non-zero and shows a different nature compared to the elliptic and triangular flow parameters as a function of p T. We see a negative v 1 for p T < 2 GeV/c, which becomes positive and rises slowly with larger p T (>2 GeV/c) values. The QGP radiation completely dominates the v 1 in the region 1 < p T < 6 GeV/c whereas, the contribution from hadronic phase is only marginal. In addition, the v 1 is found to be more sensitive to the initial formation time of the plasma compared to the elliptic and triangular flow parameters. Photon v 1 from Au + Au collisions is also calculated and compared with the v 1 from Cu + Au collisions. These results indicate that fluctuations play the most significant role in the determination of photon v 1 as the results from symmetric and symmetric collisions are found to be close to each other. We suggest that a simultaneous measurements of photon v n coefficients (n = 1, 2, 3) can provide valuable information about the initial state produced in heavy ion collisions.