Soft Electromagnetic Radiations from Heavy Ion Colisions

Abstract

The production of low mass dileptons and soft photons from thermalized Quark Gluon Plasma (QGP) and hadronic matter in relativistic heavy ion collisions is evaluated. A boost invariant longitudinal and cylindrically symmetric transverse expansion of the systems created in central collision of lead nuclei at CERN SPS, BNL RHIC, and CERN LHC, and undergoing a first order phase transition to hadronic matter is considered. A large production of low mass ($M<$ 0.3 GeV) dileptons, and soft photons ($p_T<$ 0.4 GeV) is seen to emanate from the bremsstrahlung of quarks and pions. We find an increase by a factor of 2--4 in the low mass dilepton and soft photon yield as we move from SPS to RHIC energies, and an increase by an order of magnitude as we move from SPS to LHC energies. Most of the soft radiations are found to originate from pion driven processes at SPS and RHIC energies, while at the LHC energies the quark and the pion driven processes contribute by a similar amount. The study of the transverse mass distribution is seen to provide interesting details of the evolution. We also find a unique universal behaviour for the ratio of $M^2$ weighted transverse mass distribution for $M=$ 0.1 GeV to that for $M=$ 0.2 and 0.3 GeV, as a function of $M_T$, for SPS, RHIC, and LHC energies, in the absence of transverse expansion of the system. A deviation from this universal behaviour is seen as a clear indication of the flow.