Hybrid model calculations of direct photons in high-energy nuclear collisions

Abstract

Direct photon emission in heavy-ion collisions is calculated within a relativistic micro $+$ macro hybrid model and compared to the microscopic transport model, the Ultra-relativistic Quantum Molecular Dynamics model. In the hybrid approach, the high-density part of the evolution is replaced by an ideal three-dimensional hydrodynamic calculation. This allows the effects of viscosity and full local thermalization, in comparison with the transport model of the ideal fluid dynamics, to be examined. The origin of high-${p}_{\ensuremath{\perp}}$ photons as well as the impact of elementary high-$\sqrt{s}$ collisions is studied. The contribution of different production channels and nonthermal radiation to the spectrum of direct photons is further explored. Detailed comparisons to the measurements by the WA98 Collaboration are also undertaken.