Anisotropic flow of thermal photons as a quark-gluon plasma viscometer

TL;DR

This paper presents advanced viscous hydrodynamic calculations of thermal photon emission in nuclear collisions, analyzing flow coefficients and viscous effects to better understand quark-gluon plasma properties.

Contribution

It introduces detailed viscous corrections to photon emission rates and explores their impact on flow harmonics, providing new insights into quark-gluon plasma diagnostics.

Findings

Viscous corrections significantly affect flow coefficients more than flow suppression.

The ratio v2/v3 offers a robust observable for plasma properties.

Predictions for photon directed flow v1 at RHIC and LHC energies.

Abstract

We present state-of-the-art calculations of viscous photon emission from nuclear collisions at RHIC and LHC. Fluctuating initial density profiles are evolved with event-by-event viscous hydrodynamics. Momentum spectra of thermal photons radiated by these explosively expanding fireballs and their $p_T$-differential anisotropic flow coefficients $v_n(p_T)$ are computed, both with and without accounting for viscous corrections to the standard thermal emission rates. Viscous corrections to the rates are found to have a larger effect on the $v_n$ coefficients than the viscous suppression of hydrodynamic flow anisotropies. The benefits of taking the ratio of elliptic to triangular flow, $v_2/v_3$, are discussed, and the spacetime regions which contribute dominantly to the photon flow harmonics are identified. The directed flow $v_1$ of thermal photons is predicted for RHIC and LHC energies.