Speaker
Description
The temperature and size of the quark-gluon plasma that is created in heavy-ion collisions are properties that are to be experimentally constrained. This can be done via direct photon Hanbury Brown and Twiss (HBT) measurements that are able to both quantify the excess of thermal photons as well as the size of the emitting source. In contrast to femtoscopic measurements of bulk particles, this measurement is more sensitive to earlier times in the systems evolution.
In the ALICE experiment, photons can be reconstructed either by using the calorimeters or via conversions in the detector material. Conversions benefit from an excellent energy resolution and are able to provide direct photon measurements down to very low momentum. For HBT correlations, the detector setup can be exploited to combine a conversion photon with a calorimeter photon, such that near zero opening angles are measured which is where the main signal is situated.
The poster presents the first results on direct photon HBT correlations in pp and Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV by ALICE. Although this is a challenging measurement with the existing LHC Run 2 data, fits of the signal in Pb--Pb collisions can be used to provide constraints of the direct photon excess and size of the source.
