Abstract
We review recent ALICE results on direct photon production in pp, p–Pb and Pb–Pb collisions at LHC energies. In light systems, pp and p–Pb, no signal of direct photons at low pT < 3-5 GeV/c is observed within uncertainties, while at high pT our measurements are consistent with NLO pQCD calculations. In central and mid-central Pb–Pb collisions a thermal photon contribution is observed at low pT < 3 - 4 GeV/c with slopes Teff = (304 ± 11stat ± 40syst) MeV and (407 ± 61stat ± 96syst) MeV respectively. A collective elliptic flow of direct photons is measured and appeared to be close to the decay photon flow.
Highlights
Direct photons are the photons created in interactions of charged particles in course of pp, p-A or AA collision and not produced in decays of final-state hadrons which are copiously produced in these collisions
There is no possibility to separate photons according to production mechanisms event by event, but one can estimate and subtract decay photons contribution, thermal and prompt contributions can be separated on the statistical basis using the shapes of their spectra
Photons in ALICE are reconstructed via several complementary methods: using electromagnetic calorimeters PHOS [2] and EMCAL [3] and by the Photon Conversion Method (PCM) [4] which identifies photons converted to e+e− pairs and reconstructed with the Inner Tracking System (ITS) [5] and the Time Projection Chamber (TPC) [6]
Summary
Direct photons are the photons created in interactions of charged particles in course of pp, p-A or AA collision and not produced in decays of final-state hadrons which are copiously produced in these collisions. Prompt direct photons are emitted from scattering of initial-state partons of colliding nuclei This is the same mechanism as in pp collisions, but in AA case it should be scaled with the number of binary nucleon-nucleon collisions, corrected for the isospin difference and possible modification of structure functions in nuclei. Hot QCD matter created in collisions, similar to any hot matter, emits thermal electromagnetic radiation These thermal direct photons have approximately exponential spectrum and can show up in the direct photon spectrum at low pT. There is no possibility to separate photons according to production mechanisms event by event, but one can estimate and subtract decay photons contribution, thermal and prompt contributions can be separated on the statistical basis using the shapes of their spectra
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