Direct photon emission from hadronic sources: Hydrodynamics vs. Transport theory

TL;DR

This paper compares direct photon emission in heavy-ion collisions using a microscopic transport model and a hybrid hydrodynamic approach, analyzing the effects of viscosity and different production channels against experimental data.

Contribution

It introduces a hybrid model combining transport and hydrodynamics to study photon emission, highlighting viscosity effects and channel contributions.

Findings

Transport and hydrodynamic models produce comparable photon spectra.

Viscosity influences photon yield and spectrum shape.

Comparison with WA 98 data validates the hybrid approach.

Abstract

Direct photon emission in heavy-ion collisions is calculated within the relativistic microscopic transport model UrQMD. We compare the results from the pure transport calculation to a hybrid-model calculation, where the high-density part of the evolution is replaced by an ideal 3-dimensional fluiddynamic calculation. The effects of viscosity, present in the transport model but neglected in ideal fluid-dynamics, are examined. We study the contribution of different production channels and non-thermal collisions to the spectrum of direct photons. Detailed comparison to the measurements by the WA~98-collaboration are undertaken.