Photons from Ultra-Relativistic Heavy Ion Collisions

Abstract

It is believed that a novel state of matter - Quark Gluon Plasma (QGP) will be transiently produced if normal hadronic matter is subjected to sufficiently high temperature and/or density. We have investigated the possibility of QGP formation in the ultra-relativistic collisions of heavy ions through the electromagnetic probes - photons and dileptons. The formulation of the real and virtual photon production rate from strongly interacting matter is studied in the framework of Thermal Field Theory. Since signals from the QGP will pick up large backgrounds from hadronic matter we have performed a detailed study of the changes in the hadronic properties induced by temperature within the ambit of the Quantum Hadrodynamic model, gauged linear and non-linear sigma models, hidden local symmetry approach and QCD sum rule approach. The possibility of observing the direct thermal photons and lepton pairs from quark gluon plasma has been contrasted with that from hot hadronic matter with and without medium effects for various mass variation scenarios. The effects of medium induced modifications have also been incorporated in the evolution dynamics through the equation of state. We find that the in-medium effects on the hadronic properties in the framework of the Quantum Hadrodynamic model, Brown-Rho scaling and Nambu scaling scenarios are conspicuously visible through the low invariant mass distribution of dileptons and transverse momentum spectra of photons. We have compared our evaluation of the photon and dilepton spectra with experimental data obtained by the WA80, WA98 and CERES Collaborations in the heavy ion experiments performed at the CERN SPS. Predictions of electromagnetic spectra for RHIC energies have also been made.