# Direct photon yield in pp and in Pb-Pb collisions measured with the ALICE experiment

## TL;DR

This paper reports measurements of direct photon production in Pb-Pb and pp collisions at LHC energies, revealing a significant signal in central Pb-Pb collisions below 3 GeV/c and extending the photon spectrum analysis down to lower transverse momenta in pp collisions.

## Contribution

It provides the first detailed measurement of direct photons in Pb-Pb collisions at 2.76 TeV and extends the isolated photon spectrum in pp collisions down to lower transverse momenta.

## Key findings

- Direct photon signal observed in central Pb-Pb collisions below 3 GeV/c.
- No excess of direct photons in semi-central/peripheral Pb-Pb and pp collisions.
- Measurements agree with NLO pQCD calculations and other LHC experiments.

## Abstract

The measurement of direct photon production in Pb-Pb collisions at $ \sqrt{s_{\rm{NN}}} = 2.76 $ TeV and in pp collisions at $ \sqrt{s}= 7 $ TeV with the ALICE experiment is presented. In Pb-Pb collisions a clear direct photon signal below 3 GeV/$c$ is observed only for the $ 0-20 \mbox{ }\%$ most central collisions. No excess is observed for semi-central and peripheral Pb-Pb collisions and in pp collisions. Furthermore, in pp collision the measurement of direct photon production in the range 10 $\leq p_{\rm{T}} \leq$ 60 GeV/$c$ is reported. The analysis is performed on the EMCal-triggered data taken in $2011$. The two main sources of background, namely photons from fragmentation processes and decay photons, have been subtracted from the inclusive photon spectrum by means of an isolation technique combined with the study of the transverse dispersion of electromagnetic showers. The measurement is in agreement both with NLO pQCD calculations and with those performed by the ATLAS and the CMS collaborations, extending the isolated photon spectrum investigated at LHC towards lower values of $p_{\rm{T}}$.