Accessing the deuteron source with pion-deuteron femtoscopy in Pb-Pb collisions at sqrt(s_NN) = 5.02 TeV

In ultrarelativistic heavy-ion collisions, various particle species emerge from the created fireball. Among them, non- composite light hadrons are produced predominantly in a thermal way during the hadronization phase at the chemical freeze-out, occurring at a system’s temperature of approx- imately 150–160 MeV. The high temperature has sparked debates about whether composite particles, such as (anti-)deuterons, can form directly alongside hadrons during these thermal processes. This question arises because the production and stability of (anti-)deuterons in a hot ther- mal medium are unexpected, given their modest binding energy of only 2.2 MeV between the neutron and proton— approximately 70 times smaller than the system temperature at chemical freeze-out (∼156 MeV). Instead, an alter- native scenario suggests that (anti-)deuterons form through coalescence, where protons and neutrons, close in phase space in their final state, interact via the strong force during the hadronic phase, between chemical and kinetic freeze-out. The production of light ions via the thermal or coalescence processes has been extensively studied experimentally via measurements of particle yield, spectra, elliptic flow, and fluctuations. However, no single production model fully explains all observed trends, as different stud- ies either lack sensitivity or support different mechanisms.