Photon Radiation in Fermi-Energy Heavy-Ion Collisions*

Electromagnetic radiation (photons and dileptons) has a long history as penetrating probes of the strongly interacting matter created in heavy-ion collisions, as it does not suffer significant final-state interactions.  At ultra-relativistic collision energies, the fireball is believed to reach local equilibrium facilitating the use of well-defined thermal emission rates which can illuminate properties of the produced QCD matter (such as its temperature, lifetime, and spectral properties). The situation is less clear at lower collisions energies. Here we utilize the concept of coarse-graining of the fireball formed in nuclear collisions at Fermi energies to investigate the radiation of photons in these reactions. Utilizing results of microscopic transport simulations for nucleon positions and momenta within the Constrained Molecular Dynamics (CoMD) model, we extract the time dependence of local temperatures and chemical potentials. For the longitudinal direction, off-equilibrium effects due to the primordial motion of the incoming nuclei are essential to be accounted for. The coarse-graining results are then used to compute the emission spectra of photons employing rates for nucleon-nucleon Bremsstrahlung. The results are compared to experimental data for photon energy spectra.