No abstract

The production of dielectron pairs due to bremsstrahlung of quarks is calculated in a hot quark-gluon plasma. It is shown that multiple scattering of quarks leads to an infrared stable result for dielectron emission rates. The influence of the Landau-Pomeranchuk effect, due to destructive interference of virtual photons, is calculated and analyzed. For the soft electromagnetic signal from a QCD plasma, this effect is more important than the one due to the thermal quark mass; it furthermore reduces significantly the dielectron production rates. It is also shown that in the low invariant-mass region the contributions from quark and pion virtual bremsstrahlung are of the same order of magnitude.PACS number(s): 12.38.Mh, 12.38.Cy, 25.75.f r

Intensive radiation of magnetic bremsstrahlung type (synchrotron radiation) resulting from the interaction of escaping quarks with the collective confining colour field is discussed as a new possible mechanism of observed direct photon anisotropyThe mighty wealth of experimental data on relativistic heavy ion collisions collected in the different experiments in recent years (even before putting LHC in operation) is reasonably well described (but less well understood) in the framework of approach based on the relativistic hydrodynamic equations [1,2]. In particular, a (nearly) perfect hydrodynamics has successfully predicted an existence of radial and elliptic flows, their dependence on centrality, mass, beam energy and transverse momentum. Crucial moment of this approach is that the respective liquid possesses rather special transport properties. Indeed, the ratio of its shear viscosity coefficient η to the entropy density s, i.e. η/s, develops very small magnitude. Obviously, any microscopic interpretation of new experimental data at this energy scale should take into account this novel theoretical background but also to answer the most exciting question what is that fluid entity.Measuring the photon radiation in ultrarelativistic collisions of heavy nuclei has been suggested as one of the most indicative signals of producing new state of matter many years ago [3,4]. In this context the recent measurements by the PHENIX Collaboration which show the azimuthal anisotropy of produced direct photons very close to the hadron one [5] are rather exciting. This result appears to be in a serious contradiction with expected dominance of photon production from quark gluon plasma at an early stage of ion collision at the top RHIC (Brookhaven) and now available LHC (CERN) energies. The observed temperature of "anomalous" photon radiation (about T ave ≃ 220 Mev) is in accordance with the PHENIX Collaboration measurements [6] at the energy √ s = 200 GeV of heavy ion collisions. This temperature magnitude being considered as a result of averaging over the entire evolution of the matter created in nuclear collisions is noticeably higher than the phase transition temperature (this statement is wandering over the all phenomenological papers albeit we understand the lattice QCD declares 1) e-mail: snigirev@lav01.sinp.msu.ru the presence of a cross-over only [7]) and obviously supports the scenario of photon radiation from quark gluon plasma. Forming a gluon condensate which radiates the photons at the early stage of collisions is considered [8] as another alternative explanation of high photon source temperature measured.However, in both these scenarios the photon azimuthal anisotropy is declared to be small [9] and insufficient to explain the experimental data mentioned. For the time being this new result of the PHENIX Collaboration promoted great interest in both experimental and theoretical studies and several phenomenological suggestions [10,11,12,13,14] are under discussion to understand an origin of this exciting observation...