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

¹ Institute for Theoretical Physics and Center for Extreme Matter and Emergent Phenomena, Utrecht University, Leuvenlaan 4, 3584 CE Utrecht, The Netherlands.
² Crete Center for Theoretical Physics, Institute of Theoretical and Computational Physics, Department of Physics University of Crete, 71003 Heraklion, Greece.
³ Crete Center for Quantum Complexity and Nanotechnology, Department of Physics, University of Crete, 71003 Heraklion, Greece.
⁴ APC, Univ Paris Diderot, Sorbonne Paris Cité, APC, UMR 7164 CNRS, F-75205 Paris, France.
⁵ Department of Physics, McGill University, 3600 University Street, Montreal, QC, H3A 2T8, Canada.
⁶ Theoretical Research Division, Nishina Center, RIKEN, Wako, Saitama 351-0198, Japan.

^a e-mail: dilunyang@gmail.com

Published online: 22 March 2017

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 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.