COHERENCE AND INCOHERENCE IN THE PHOTON AND DILEPTON PRODUCTION BY BREMSSTRAHLUNG IN RELATIVISTIC HEAVY-ION COLLISIONS

Abstract

Due to the strong collective deceleration during the initial stage of relativistic heavy-ion collisions, the nuclear matter irradiates real and virtual bremsstrahlung. We describe the process of bremsstrahlung emission in the framework of a semiclassical model in order to study coherence and incoherence effects in the production process. Guided by the intuitive notation of shock fronts being formed between the incident nuclei, we use a simple parametrization of the nuclear current density. The photon spectrum is studied up to photon energies of 300 MeV. In particular, a gradual transition from the coherent production process of low-energy photons to the incoherent one for hard photons is demonstrated. For heavy collision systems coherence effects in the photon spectra dominate, showing characteristic structures arising from shock fronts. The dilepton spectrum is described in first-order perturbation theory. Generally, dileptons are found to be produced incoherently. Only in the case of dielectron production with small invariant pair masses do moderate coherence effects survive.