Abstract
The measurement of direct photon production in Pb-Pb collisions at sNN=2.76 TeV and in pp collisions at 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 % 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≤pT≤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 pT.
Highlights
In pp collisions the measurement of direct photon production can be used to test both the pQCD calculations and the binary scaling behaviour of the initial hard scattering
At high pT, pQCD processes like quarkgluon Compton scattering and quark-antiquark annihilation are the main contribution to direct photon production, and allow us to probe directly the gluons within hadrons
This result extends the range investigated by the ATLAS [10] and the CMS collaborations [11] down to 10 GeV/c. In these proceedings we presented a summary of the direct photon measurements in pp and Pb-Pb collisions performed by the ALICE collaboration
Summary
In pp collisions the measurement of direct photon production can be used to test both the pQCD calculations and the binary scaling behaviour of the initial hard scattering. At high pT, pQCD processes like quarkgluon Compton scattering and quark-antiquark annihilation are the main contribution to direct photon production, and allow us to probe directly the gluons within hadrons. Trum is dominated by thermal production in the quarkgluon plasma and in the hadron-gas phase, giving us access to information on the temperature of the hot and dense medium in which direct photons are produced. For pT greater than 5 GeV/c, direct photons are mainly produced in hard partonic scattering processes in the early stage of the collision, leaving the strongly interacting medium unscathed and provide access to information about the initial dynamics
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