Enhancement of thermal photon production in event-by-event hydrodynamics

TL;DR

This paper shows that including event-by-event initial condition fluctuations in hydrodynamical models significantly increases thermal photon production, improving agreement with experimental data and highlighting photons as probes of early medium fluctuations.

Contribution

It demonstrates that initial condition fluctuations can enhance thermal photon yields by about a factor of 2, providing a new way to probe early-time dynamics in heavy-ion collisions.

Findings

Photon yield increases by a factor of 2 with fluctuations.

Enhanced agreement with PHENIX data.

Photon production is sensitive to hotspots in the medium.

Abstract

Thermal photon emission is widely believed to reflect properties of the earliest, hottest evolution stage of the medium created in ultra-relativistic heavy-ion collisions. Previous computations of photon emission have been carried out using a hydrodynamical medium description with smooth, averaged initial conditions. Recently, more sophisticated hydrodynamical models which calculate observables by averaging over many evolutions with event-by-event fluctuating initial conditions (IC) have been developed. Given their direct connection to the early time dynamics, thermal photon emission appears an ideal observable to probe fluctuations in the medium initial state. In this work, we demonstrate that including fluctuations in the IC may lead to an enhancement of the thermal photon yield of about a factor of 2 in the region $2 < p_T < 4$ GeV/$c$ (where thermal photon production dominates the direct photon yield) compared to a scenario using smooth, averaged IC. Consequently, a much better agreement with PHENIX data is found. This can be understood in terms of the strong temperature dependence of thermal photon production, translating into a sensitivity to the presence of 'hotspots' in an event and thus establishing thermal photons as a suitable probe to characterize IC fluctuations.