Photon Production in Thermally-Anisotropic Nuclear Matter Produced in Fermi-Energy Heavy-Ion Collisions Using Coarse Graining*

Understanding the phase diagram of nuclear matter under extreme conditions is a key objective in contemporary science and can be pursued through heavy-ion collisions (HICs) over a large range of collision energies. Penetrating probes like photons, which can escape the medium unscathed, are particularly valuable, including the investigation of mechanism toward local thermalization in the collision process.

While evidence for thermalization has been observed at ultra-/relativistic collision energies, this process is less understood at lower beam energies. We have calculated photon emission spectra from HICs at Fermi energy to investigate this question, by utilizing a coarse graining procedure of transport simulations from constrained- molecular dynamics transport, in connection with thermal emission rates computed from thermal field theory. An important ingredient in this study are off-equilibrium effects due to the initial longitudinal motion which we have encoded in parameterized fits to the time-evolving nucleon distribution functions. We have found that indeed most high- energy photons are produced during the initial stages of maximal compression facilitated by the incoming collective motion in primordial collisions of protons and neutrons. This furthermore helps to quantify the scale of thermalization which we found to be significantly longer than in previous works using other methods. We also discuss the comparison of our calculated spectra of thermal and prompt photons to experimental data.