Universal Parametrization of Thermal Photon Rates in Hadronic Matter

TL;DR

This paper offers a universal, accurate parametrization of thermal photon emission rates from hadronic matter, simplifying complex microscopic calculations for use in high-energy heavy-ion collision models.

Contribution

It provides a comprehensive, fitted functional form for photon emission rates from hadronic matter, including in-medium rho mesons and Bremsstrahlung, applicable across a range of temperatures and chemical potentials.

Findings

Parametrization reproduces microscopic rates within 20% accuracy.

Includes contributions from in-medium rho mesons and pi-pi Bremsstrahlung.

Extends to chemical off-equilibrium conditions in heavy-ion collisions.

Abstract

Electromagnetic (EM) radiation off strongly interacting matter created in high-energy heavy-ion collisions (HICs) encodes information on the high-temperature phases of nuclear matter. Microscopic calculations of thermal EM emission rates are usually rather involved and not readily accessible to broad applications in models of the fireball evolution which are required to compare to experimental data. An accurate and universal parametrization of the microscopic calculations is thus key to honing the theory behind the EM spectra. Here we provide such a parametrization for photon emission rates from hadronic matter, including the contributions from in-medium rho mesons (which incorporate effects from anti-/baryons), as well as Bremsstrahlung from pi-pi scattering. Individual parametrizations for each contribution are numerically determined through nested fitting functions for photon energies from 0.2 to 5 GeV in chemically equilibrated matter of temperatures 100-180 MeV and baryon chemical potentials 0-400 MeV. Special care is taken to extent the parameterizations to chemical off-equilibrium as encountered in HICs after chemical freezeout. This provides a functional description of thermal photon rates within a 20% variation of the microscopically calculated values.