Dilepton production and elliptic flow from an anisotropic quark-gluon plasma

Abstract

The emission of real photons from a momentum-anisotropic quark-gluon plasma (QGP) is affected by both the collective flow of the radiating medium and the modification of local rest frame emission rate due to the anisotropic momentum distribution of partonic degrees of freedom. In this paper, we first calculate the photon production rate from an ellipsoidally momentum-anisotropic QGP including hard contributions from Compton scattering and quark pair annihilation and soft contribution calculated using the hard thermal loop (HTL) approximation. We introduce a parametrization of the nonequilibrium rate in order to facilitate its further application in yield and flow calculations. We convolve the anisotropic photon rate with the space-time evolution of QGP provided by 3+1d anisotropic hydrodynamics (aHydro) to obtain the yield and the elliptic flow coefficient v 2 of photons from QGP generated at Pb-Pb collisions at LHC at 2.76 TeV and Au-Au collisions at RHIC at 200 GeV. We investigate the effects of various parameters on the results. In particular we analyze the sensitivity of results to initial momentum anisotropy.

Photon production and elliptic flow from a momentum-anisotropic quark-gluon plasma

“…The QGP photon yield in central collisions is several times higher than in peripheral ones, but their v 2 shows a change in sign and is negative for p T > 2 GeV. The same behavior at central collisions was found for dilepton v 2 in our previous paper [87]. The origin of negative photon v 2 from anisotropic QGP can be understood as the result of the interplay between two sources of momentum anisotropy of the generated particles: (1) the collective flow of the strongly interacting medium and (2) the anisotropic rate of photon production in LRF of each fluid element.…”

“…For the first time, we included transverse momentum anisotropy using an ellipsoidal parametrization of the QGP parton distribution functions. This is an extension of previous results of photon production including only longitudinal momentum anisotropy [51] and in line with our previous study of dilepton emission [87]. To calculate soft contributions to the photon rate, we utilized our previous results for quark self-energy in anisotropic QGP [81] advancing in the direction of including more information about collective excitations of the strongly interacting matter in phenomenological studies.…”

“…In this paper, the goal is not to provide phenomenological parameter tuning, and we used the same fT 0 ; η=sg for cases with different initial momentum anisotropy. In a previous paper [87], we showed that retuning T 0 for an initially anisotropic QGP does not change the interpretation of the results on electromagnetic emission.…”

“…In order to calculate the yield and elliptic flow coefficient of real photons emitted from the QGP, we convolve the LRF anisotropic photon production rate with the space-time evolution of the strongly interacting fluid provided by 3 þ 1d aHydro with a quasiparticle equation of state and smooth Glauber initial conditions [70,71]. The method is basically the same as the convolution of dilepton production with aHydro evolution described in [87]. We use the aHydro parameter values tuned to reproduce soft hadrons spectra for the LHC [70] and RHIC [73].…”

“…The bands in the plots show the spatial standard deviation of the ξ 1 values. We compare the evolution for cases with and without initial momentum anisotropy where both cases are tuned to give the best and similarly accurate fits to the LHC soft hadronic spectra [87]. The curves of hξ 1 i and its deviation in both cases converge at late times, but in the case with a relatively small initial anisotropy ξ 1 ðτ 0 Þ ¼ 3, the values of ξ 1 grow to much larger values at early times.…”