Thermal Radiation and Direct Photon Production in Pb–Pb and \(pp\) Collisions with Dielectrons in ALICE

Dielectrons are a unique tool to study the space-time evolution of the hot and dense matter created in ultrarelativistic heavy-ion collisions. They are produced by a variety of processes during all stages of the collision with negligible final-state interactions. At low invariant mass (mee), thermal radiation from the hot hadron gas (HG) contributes to the spectrum while at larger mee, thermal radiation from the quark–gluon plasma (QGP) carries information about the early temperature of the medium. The latter is nevertheless dominated by a large background of correlated heavy-flavour (HF) hadron decays affected by energy loss and flow in the medium. At very low mee (mee → 0), the fraction of direct photons, including thermal contributions, can be extracted from the dielectron spectrum as a function of transverse momentum (pT,ee). In proton–proton (pp) collisions, such measurement serves as a fundamental test for perturbative QCD calculations and a baseline for the studies in heavy-ion collisions. We report on the latest ALICE results on dielectron studies in Pb–Pb and pp collisions at √ sNN = 5.02 TeV and √ s = 13 TeV, respectively. The results are compared to the expected dielectron yield from known hadronic sources and predictions for thermal radiation from the medium. The production of direct photons in the different colliding systems including high-multiplicity (HM) pp collisions is discussed.