Direct photon production and flow at low transverse momenta in pp, p-Pb and Pb-Pb collisions

TL;DR

This paper reports measurements of low transverse momentum direct photons in various collision systems at the LHC, revealing no thermal photon signal in small systems but a significant excess in central Pb--Pb collisions, with flow measurements comparable to charged hadrons.

Contribution

First investigation of multiplicity dependence of direct photon production in p--Pb collisions and comprehensive analysis across different collision systems at the LHC.

Findings

No significant thermal photon signal in small systems.

Approximately 15% excess of direct photons in central Pb--Pb collisions at low pT.

Direct photon flow similar to charged hadron flow in Pb--Pb collisions.

Abstract

Low transverse momentum direct photon measurements have been carried out by the ALICE experiment at the CERN LHC in small collision systems (pp, $\sqrt{s}=2.76$ and 8 TeV and p--Pb, $\sqrt{s_\text{NN}}=5.02$ TeV) as well as in heavy-ion collisions (Pb--Pb, $\sqrt{s_\text{NN}}=2.76$ TeV). For the first time, also the multiplicity dependence of direct photon production was investigated in p--Pb collisions. Whereas in the small systems no significant thermal photon signal was observed, a $\sim15\%$ excess has been measured in central Pb--Pb collisions in the region of $p_{\rm T}<3$ GeV/$c$. A signal of prompt photon production at high transverse momentum consistent with binary scaling has been observed in all collision systems following NLO pQCD predictions. Direct photon flow has been measured in central and semi-central Pb--Pb collisions and found to be of similar size as the charged hadron and decay photon flow.