Abstract
We present an overview of the photon analysis in pp and Pb-Pb collisions with data taken by the ALICE experiment at the LHC. The ALICE detectors reconstruct photons by using the two electromagnetic calorimeters (photon spectrometer, sampling calorimeter) and central tracking systems for photon converted e + e pairs in the material of the inner ALICE layers. In Pb-Pb collisions the direct photon calculations under- predict the data below 4 GeV/c where it is expected to have a contribution from thermal radiations. The direct photon measurement also shows evidence for a non-zero elliptic flow for 1< pT < 3 GeV/c. The nuclear modi- fication factor of the 0 production at di erent collision centralities shows a clear pattern of strong suppression in a hot QCD medium with respect to pp collisions. Finally, parton fragmentation following hard collisions is investigated by correlating high momentum direct photons and charged hadrons with the goal of revealing new insights into medium e ects in the QGP.
Highlights
Depending on their energy and their production mechanism, photons probe different properties of the strongly interacting matter produced in heavy ions collisions
The nuclear modification factor RAA is calculated, defined as the ratio of hadron transverse momentum spectra, in our case π0, between Pb-Pb collisions scaled by the number of binary nucleon-nucleon collisions Ncoll and pp collisions, RAA =
This comparison can help to tune the parton distribution function and the fragmentation function involved in the NLO calculations
Summary
Depending on their energy and their production mechanism, photons probe different properties of the strongly interacting matter produced in heavy ions collisions. By correlating the direct photon with hadrons in the opposite direction, one can study jet properties in pp and Pb-Pb collisions in a transverse momentum range below 50 GeV/c where the full jet reconstruction is challenging because of the relatively large contribution from the underlying event. Such processes are tagged experimentally identifying leading isolated photons and their correlated associated hadrons.
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