Abstract
We review ALICE results on direct photon production in pp, pA and AA collisions and compare them to predictions of NLO pQCD calculations, hydrodynamic, and transport models. In pp and pA collisions the measured direct photon yield is consistent with NLO pQCD calculations, scaled with the number of binary collisions. In Pb-Pb collisions an additional direct photon contribution is observed at low pT < 4 GeV/c which can be interpreted as a thermal direct photon contribution. The measured yield is larger than predictions of hydrodynamic models, but still consistent with them within uncertainties. The elliptic collective flow of direct photons was measured in central and mid-central Pb–Pb collisions and compared with hydrodynamic model predictions.
Highlights
Direct photons are the photons created in interactions of charged particles in course of pp, pA or AA collision and not in decays of final hadrons
In pp and in minimum bias and most central p–Pb collisions no significant direct photon excess was observed in thermal photon region pT < 5 GeV/c
Within present uncertainties data are consistent with predictions of hydrodynamics models which predict some thermal photon emission in this region
Summary
Direct photons are the photons created in interactions of charged particles in course of pp, pA or AA collision and not in decays of final hadrons. Right: combined Rγ as a function of pT measured in pp collisions at s = 8 TeV compared to NLO pQCD predictions. The direct photon double ratio Rγ and spectra were measured in four multiplicity classes in p–Pb collisions.
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