Ultra-peripheral Collisions in Star

We refer to ultra-peripheral heavy ion collisions, as those nuclear interactions with a impact parameter, b, larger than twice the nuclear radius RA, where no hadronic nucleon-nucleon collisions occur . The large charge of a relativistic heavy nucleus is a strong source of quasi-real photons. Exclusive ρ meson production AuAu→AuAuρ0 can then be described by the vector meson dominance model : a photon emitted by one nucleus fluctuates to a virtual quark-anti quark pair; this intermediate state scatters diffractively from the other nucleus, emerging as a vector meson. The diagram for this process is shown in Fig. 1a. Here, the gold nuclei remain in their ground state. Additional photon exchange can yield nuclear excitation and the subsequent emission of single or multiple neutrons, as shown in Fig. 1b for the process AuAu → AuAuρ. Photon and Pomeron can couple coherently to the spatially extended