Soft electromagnetic radiations from relativistic heavy ion collisions

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 (pT < 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 MT , 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. PACS number(s): 12.38. Mh, 13.85.Qk, 25.75.+r

Soft electromagnetic radiations from an equilibrating quark-gluon plasma

“…where M 0 is the matrix element for ab → cd and J µ is the real(virtual) photon current [13,14]. Using kinetic theory one can write down the rate of production of a photon per unit time per unit volume at a temperature T as…”

“…To compensate this one needs to insert a phase space correction factor Φ, defined as the ratio of three-body phase space to two-body phase space. This can be obtained [12,14,21] as…”

“…It has been discussed in Ref. [12,14] that the result grossly over-estimates without the phase space correction; (2) the approximate electromagnetic current has been used; (3) also simply a factor unity in the exit channel has been assigned instead of considering the appropriate statistical factors. With proper inclusion, like in the present calculation, of these factors should lead to an agreement with the thermal field theory result.…”

“…[12] it has been shown that the description of the CERES data will improve to some extent, when one adds hadronic bremsstrahlung contribution. For a fully equilibrated plasma, the production of low mass dileptons and soft photons due to bremsstrahlung process has been studied in great detail in the literature [13][14][15]. However, in the recently formulated selfscreened parton cascade (SSPC) model [3] early hard scattering produce a medium which screens the long range color fields associated with softer interactions.…”

Open charm, photon, and dilepton production in an increasingly strongly interacting parton plasma

“…Likewise, enhancement in soft dilepton production could be due to quark-gluon plasma formation [3,7] which would provide very good signals. However, background from bremsstrahlung off partons and pions [13] and also pion decays [5] and emission from hadronic scattering [14] must also be considered. In the end, one has little choice but to rely on excess in a certain restricted momentum range of the produced electromagnetic radiation.…”