Direct-Photon Spectra and Anisotropic Flow in Heavy Ion Collisions from Holography

TL;DR

This paper uses holographic models to calculate thermal-photon emission in strongly coupled gauge theories, embedding these in hydrodynamic simulations to analyze direct photon spectra and flow in heavy ion collisions at RHIC and LHC, showing enhanced spectra and flow features compared to weakly coupled models.

Contribution

It introduces a holographic approach to compute thermal-photon emission rates in strongly coupled QCD-like theories and integrates these into collision simulations, providing new insights into photon spectra and flow.

Findings

Enhanced photon spectra at intermediate and high momenta in strongly coupled scenarios.

Increased elliptic and triangular flow of direct photons at high momenta with holography.

Spectra and flow in small systems suggest substantial thermal photon contributions.

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.