Towards azimuthal anisotropy of direct photons

Abstract

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...

Direct-photon spectrum and elliptic flow produced from Pb+Pb collisions at sNN=2.76 TeV at the CERN Large Hadron Collider within an integrated hydrokinetic model

“…It was suggested [18] that a considerable additional contribution to the photon production could come due to the boundary bremsstrahlung resulting from the interaction of escaping quarks with the collective confining color field at the surface of the QGP. Obviously, such a mechanism of "magnetic bremsstrahlung-like radiation" (synchrotron radiation in modern terminology) should manifest itself noticeably in the observed anisotropy of direct photons.…”

“…One of those mechanisms suggests, for example, that a very strong magnetic field created in noncentral heavy ion collisions can increase the photon radiation owing to the conformal anomaly [15] and synchrotron radiation [16,17]. Another mechanism contributing significantly to the observed anisotropy of direct photons refers to a "magnetic bremsstrahlung-like radiation" (synchrotron radiation in modern terminology) of quarks in the collective color field ensuring a confinement [18]. Additional productive sources of electromagnetic radiation when the QCD environment undergoes a confinement have been recently launched in Refs.…”

Elliptic flow due to radiation in heavy-ion collisions

“…It is pointed out in ref. [34] that the bremsstrahlung of quarks on the surface of QGP, pulled back by the confining force could produce photon radiation in comparable amount to those produced in the plasma phase.…”

“…There is an ongoing debate in the literature about the contributions of initial and final state asymmetries to the elliptic flow [34][35][36][37]. Emphasizing only a few examples, it is observed that in proton-nucleon collisions the CGC correlations could be directly visible in the measurable particle spectrum.…”

Effect of the magnetic field on the photon radiation from quark–gluon plasma in heavy ion collisions

“…In Ref. [24] it was argued that the observed photon asymmetry may be due to an intensive bremsstrahlung like synchrotron radiation resulting from the interaction of escaping quarks with the collective confining color field at the surface of the QGP. For this mechanism the asymmetry arises due to bigger surface emission from the almond-shaped QGP fireball along the direction of the impact parameter vector (as shown in Fig.…”