Exploring hot QCD matter via direct photons at ALICE

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 p_T < 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. (paper)