Thermal photon radiation in high multiplicity p+Pb collisions at the Large Hadron Collider

TL;DR

This paper presents the first hydrodynamic calculation of thermal photon radiation in high multiplicity p+Pb collisions at the LHC, showing that thermal photons can serve as a signature of quark-gluon plasma formation in small systems.

Contribution

It introduces a novel hydrodynamic model calculation of thermal photon emission in small collision systems, highlighting their potential as signatures of quark-gluon plasma.

Findings

Thermal photons can increase low p_T photon spectrum by 2-3 times.

Thermal photon emission indicates the presence of a hot quark-gluon plasma.

Small systems can reach temperatures comparable to larger nucleus-nucleus collisions.

Abstract

The collective behaviour of hadronic particles has been observed in high multiplicity proton-lead collisions at the Large Hadron Collider (LHC), as well as in deuteron-gold collisions at the Relativistic Heavy-Ion Collider (RHIC). In this work we present the first calculation, in the hydrodynamic framework, of thermal photon radiation from such small collision systems. Owing to their compact size, these systems can reach temperatures comparable to those in central nucleus-nucleus collisions. The thermal photons can thus shine over the prompt background, and increase the low $p_T$ direct photon spectrum by a factor of 2-3 in 0-1% p+Pb collisions at 5.02 TeV. This thermal photon enhancement can therefore serve as a clean signature of the existence of a hot quark-gluon plasma during the evolution of these small collision systems, as well as validate hydrodynamic behavior in small systems.