Empirical estimation the effects of flow on thermal photon angular distribution and spectra in nucleus-nucleus collisions at RHIC and LHC

TL;DR

This paper investigates how radial and longitudinal flow influence the shape of thermal photon spectra and elliptic flow in nucleus-nucleus collisions at RHIC and LHC, using experimental data and flow estimations.

Contribution

It provides an empirical analysis linking hadron flow data to thermal photon spectra and predicts significant elliptic flow for thermal photons at LHC energies.

Findings

Radial and longitudinal flow significantly affect photon spectra via Doppler effects.

Thermal photon elliptic flow closely matches that of hadrons.

Predicted large elliptic flow for thermal photons at LHC.

Abstract

Experimental data for hadron radial and elliptic flow are used to investigate their influence on the shape of the thermal photon spectrum at RHIC. Leaving alone the actual mechanism of photon production and its time evolution, we concentrate on the spectrum shape. Radial and longitudinal flow of the bulk can change significantly the observed photon energy spectrum via the Doppler Effect. Experimental thermal photon data are described by local frame temperature parameter which depends on the assumption of longitudinal and radial flow. From the observed hadron elliptic flow we estimate the modulation of radial flow parameter versus the angle relative to the reaction plane. Based on this we calculate elliptic flow parameter for thermal photons, which was found to be very close to that for hadrons. Considering very similar amplitude for hadron elliptic flow at LHC and RHIC, we demonstrate that thermal photons at LHC should show large elliptic flow as well. All of these considerations are also valid for the low invariant mass dilepton pairs.