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

TL;DR

This paper develops a hybrid model combining relativistic hydrodynamics and microscopic transport to study direct photon emission in high-energy nuclear collisions, analyzing effects of viscosity, thermalization, and collision channels, and comparing results with experimental data.

Contribution

It introduces a hybrid model that integrates hydrodynamics with microscopic transport to better understand photon production mechanisms in nuclear collisions.

Findings

High-p_T photons originate from early collision stages.

Viscosity and thermalization significantly affect photon spectra.

The model's results align with WA98 experimental measurements.

Abstract

Direct photon emission in heavy-ion collisions is calculated within a relativistic micro+macro hybrid model and compared to the microscopic transport model UrQMD. In the hybrid approach, the high-density part of the evolution is replaced by an ideal 3-dimensional hydrodynamic calculation. This allows to examine the effects of viscosity and full local thermalization, in comparison of the transport model to the ideal fluid-dynamics. We study the origin of high-p_T photons as well as the impact of elementary high-sqrt(s) collisions. We further explore the contribution of different production channels and non-thermal radiation to the spectrum of direct photons. Detailed comparison to the measurements by the WA98-collaboration are also undertaken.