Direct photon production in Pb-Pb collisions at $\sqrt{s_\rm{NN}}$ = 2.76 TeV

TL;DR

This study measures direct photon production in Pb-Pb collisions at 2.76 TeV, revealing spectra consistent with perturbative QCD at high transverse momentum and indicating thermal photon emission at low transverse momentum.

Contribution

It provides the first combined measurement of direct photon spectra in Pb-Pb collisions at this energy, extending down to low transverse momentum with improved significance.

Findings

Agreement with pQCD for pT > 5 GeV/c

Exponential spectrum at low pT with inverse slope ~297 MeV

Significant thermal photon signal at low pT in central collisions

Abstract

Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76$ TeV was studied in the transverse momentum range $0.9 < p_\mathrm{T} < 14$ GeV$/c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^+e^-$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_\mathrm{T} \gtrsim 5$ GeV$/c$. Direct photon spectra down to $p_\mathrm{T} \approx 1$ GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_\mathrm{T} < 2.1$ GeV$/c$ is $2.6\sigma$ for the 0-20% class. The spectrum in this $p_\mathrm{T}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^\mathrm{stat}\pm 41^\mathrm{syst})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.