Direct-photon measurement in small systems and thermal radiation from QGP with ALICE

Johann Wolfgang Goethe-University Frankfurt am Main, Germany

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Published online: 5 November 2025

Abstract

Electromagnetic probes are a unique tool for studying the space-time evolution of the hot and dense matter created in ultra-relativistic heavy-ion collisions. Dielectrons are emitted during the entire evolution of the medium created in such collisions, allowing the extraction of the real direct photon fraction at vanishing mass and providing access to thermal radiation from the early hot stages of the collision. The measurement of dielectron and direct-photon production in minimum-bias pp collisions serves as a crucial baseline for the studies in heavyion collisions, whereas pp collisions with high charged-particle multiplicities allow the search for the possible presence of QGP in small systems.

An overview of the final results of dielectron production in pp collisions at √s = 13 TeV and in central Pb–Pb collisions at √s_NN = 5.02 TeV measured by ALICE is presented, together with their implications for the production of thermal radiation. Finally, an outlook on future measurements is given with the first performance studies from Run 3.