Measurement of Direct Photons in pp and Pb-Pb Collisions with ALICE

TL;DR

This paper reports the measurement of direct photon spectra in Pb-Pb and pp collisions at LHC energies, revealing excess photons at low transverse momentum in central Pb-Pb collisions likely from thermal sources, contrasting with pQCD predictions.

Contribution

First measurement of direct photon spectra in Pb-Pb collisions at 2.76 TeV with ALICE, highlighting thermal photon contributions at low pT and comparing with pQCD predictions.

Findings

Clear direct-photon signal below 4 GeV/c in central Pb-Pb collisions

Thermal photon contribution indicated by exponential fit with T_LHC=304 MeV

No low pT photon excess in pp and peripheral Pb-Pb collisions

Abstract

The measurement of the direct photon transverse momentum spectrum in Pb-Pb collisions at \unit[$\sqrt{s_{_{NN}}}=2.76]{TeV}$ with data taken by the ALICE experiment is presented. The measurement shows a clear direct-photon signal for 0-40% most central collisions below \unit[4]{GeV/$\mathrm{c}$} that can not be described by next-to-leading-order perturbative QCD (NLO pQCD) calculations. Above this value of $p_{\mathrm{T}}$ the result is in agreement with pQCD predictions. The low $p_{\mathrm{T}}$ signal is expected to have thermal photon contributions. The inverse slope parameter of an exponential fit is extracted as $T_{\mathrm{LHC}}=\unit[304\pm 51^{\mathrm{syst+stat}}]{MeV}$. For a baseline measurement the analysis is performed for proton-proton collisions at \unit[$\sqrt{s}=7]{TeV}$ and for peripheral (40-80%) Pb-Pb collisions. Both results show no low $p_\mathrm{T}$ direct-photon signal and are in agreement with pQCD calculations.