U. W. Heinz, J. Liu and C. Shen, arXiv:1403.8101 [nucl-th]

Abstract: Measurements of thermal photons emitted from the rapidly expanding hot and dense medium (“Little Bang”) formed in ultra relativistic heavy-ion collisions, and their current theoretical interpretation, are reviewed.

C. Shen, U. Heinz, J. F. Paquet and C. Gale, arXiv:1403.7558 [nucl-th]

Abstract: Photons are a penetrating probe of the hot and dense medium created in heavy-ion collisions. We present state-of-the-art calculations of viscous photon emission from nuclear collisions at RHIC and LHC. Thermal photons anisotropic flow coefficients v_n are computed, both with and without accounting for viscous corrections to the standard thermal emission rates. These corrections are found to have a larger effect on the v_n coefficients than the viscous suppression of hydrodynamic flow anisotropies. For thermal photons, the ratio v_2{SP}/v_3{SP} shows stronger sensitivity to the quark-gluon plasma (QGP) shear viscosity than for hadrons, and it can thus serve as a sensitive QGP viscometer.

C. Shen, U. W. Heinz, J. F. Paquet, I. Kozlov and C. Gale, arXiv:1308.2111 [nucl-th].

Abstract: We present state-of-the-art calculations of viscous photon emission from nuclear collisions at RHIC and LHC. Fluctuating initial density profiles are evolved with event-by-event viscous hydrodynamics. Momentum spectra of thermal photons radiated by these explosively expanding fireballs and their p_T-differential anisotropic flow coefficients v_n(p_T) are computed, both with and without accounting for viscous corrections to the standard thermal emission rates. Viscous corrections to the rates are found to have a larger effect on the v_n coefficients than the viscous suppression of hydrodynamic flow anisotropies. Since photons are found to be more sensitive to the quark-gluon plasma (QGP) shear viscosity than hadrons, their anisotropic flow coefficients v_n serve as a sensitive QGP viscometer.