Elliptic flow of thermal photons inAu+Aucollisions atsNN=200

Abstract

The transverse momentum (p t ) dependence, the centrality dependence, and the rapidity dependence of the elliptic flow of thermal photons in Au + Au collisions at √ s NN = 200 GeV are predicted on the basis of a three-dimensional ideal hydrodynamic description of the hot and dense matter. The elliptic flow parameter v 2 , i.e., the second Fourier coefficient of azimuthal distribution, of thermal photons first increases with p t and then decreases for p t > 2 GeV/c, because of the weak transverse flow at the early stage. The p t -integrated v 2 first increases with centrality, reaches a maximum at about 50% centrality, and then decreases. The rapidity dependence of the elliptic flow v 2 (y) of direct photons (mainly thermal photons) is very sensitive to the initial energy density distribution along the longitudinal direction, which provides a useful tool to extract the realistic initial condition from measurements.

Elliptic flow of thermal photons in heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider

“…The fragmentation photon component, however, is expected to be suppressed due to the quenching of partonic jets in QCD matter in A + A collisions. In addition to this, in A + A collisions the jet (parton)-matter interactions, i.e., the jet-photon conversion [5,6] and collision-induced photon emission from high-energy partons, can produce photons which are important in the mid-p T region [7][8][9]. Finally, the hot medium itself emits thermal photons, which are expected to be important in the few-GeV region and below, as discussed in the hydrodynamical studies of Refs.…”

“…Finally, the hot medium itself emits thermal photons, which are expected to be important in the few-GeV region and below, as discussed in the hydrodynamical studies of Refs. [7,8,[10][11][12][13].…”

“…As shown earlier in Refs. [14][15][16][17], this results in a positive elliptic flow for the thermal photons. Because the net contribution from other sources to photon v 2 is expected to be very small or even negative [9], a large (hadron-like) photon v 2 measured in the few-p T region and below should thus serve as a signature of thermal photon dominance.…”

Study the early stage of heavy ion collisions with direct photons

“…In our previous work, 10 we successfully explained the p t spectra of direct photons from AuAu collisions measured by PHENIX with a 3+1D idea hydrodynamical evolution constrained with hadronic data, and made a detailed investigation on the competition among the four main direct photon sources. The jet-photon conversion(JPC) contribution increases with system size.…”

“…16 Now we should include nuclear shadowing effect and EMC effect as in Ref. 10. Conventionally, thermal photon production is an integration of photon emission rate over the whole space and time of the system evolution, c.f., Ref.…”