Abstract
The thermal-photon emission from strongly coupled gauge theories at finite temperature is calculated by using holographic models for QCD in the Veneziano limit (V-QCD). These emission rates are then embedded in hydrodynamic simulations combined with prompt photons from hard scattering and the thermal photons from hadron gas to analyze the spectra and anisotropic flow of direct photons at RHIC and LHC. The results from different sources responsible for the thermal photons in the quark gluon plasma (QGP) including the weakly coupled QGP (wQGP) from perturbative calculations, strongly coupled $\mathcal{N}$=4 super Yang-Mills (SYM) plasma (as a benchmark for reference), and Gubser's phenomenological model mimicking the strongly coupled QGP (sQGP) are then compared. It is found that the direct-photon spectra are enhanced in the strongly coupled scenario compared with the ones in the wQGP, especially at intermediate and high momenta, which improve the agreements with data. Moreover, by using IP-glassma initial states, both the elliptic flow and triangular flow of direct photons are amplified at high momenta ($p_T$>2.5 GeV) for V-QCD, while they are suppressed at low momenta compared to wQGP. The distinct results in holography stem from the blue-shift of emission rates in strong coupling. In addition, the spectra and flow in small collision systems were evaluated for future comparisons. It is found that thermal photons from the deconfined phase are substantial to reconcile the spectra and flow at high momenta.
Highlights
The thermal-photon emission from strongly coupled gauge theories at finite temperature is calculated by using holographic models for QCD in the Veneziano limit (V-QCD)
In most of theoretical approaches, the emission rate from the perturbative calculation with hard thermal loop resummation initiated by Arnold, Moore, and Yaffe (AMY) [11, 12] is applied for thermal photons generated from the quark gluon plasma (QGP), where the next-to-leading order correction has been investigated in [13]
The prompt photons come from hard interactions in early times and the thermal photons incorporate the contributions from the QGP phase and hadronic phase
Summary
The local information in heavy ion collisions, the scenario could be modified for long-lived plasmas such as the cosmic plasma [3, 4]. The contributions from prompt photons and thermal photons from hadron gas are incorporated to compute both the spectra and flow of direct photons in both RHIC and LHC energies. In this proceeding, we review the major results found in [19]. In Sec., we present numerical results of direct-photon spectra and anisotropic flow in comparison with experimental measurements, in which we further highlight the results for small collision systems.
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