Production of Light Nuclei at Thermal Freezeout in Heavy-Ion Collisions

Abstract: We revisit the problem of the production of light atomic nuclei in ultrarelativistic heavy-ion collisions. While their production systematics is well produced by hadro-chemical freezeout at temperatures near the QCD pseudo-critical temperature, their small binding energies of a few MeV per nucleon suggest that they cannot survive as bound states under these conditions. Here, we adopt the concept of effective chemical potentials in the hadronic evolution from chemical to thermal freezeout (at typically $T_{\rm fo}$$\simeq$100\,MeV), which, despite frequent elastic rescatterings in hadronic matter, conserves the effective numbers of particles which are stable under strong interactions, most notably pions, kaons and nucleons. It turns out that the large chemical potentials that build up for antibaryons result in thermal abundances of light nuclei and antinuclei, formed at thermal freezeout, which essentially agree with the ones evaluated at chemical freezeout. Together with their transverse-momentum spectra, which also indicate a kinetic freezeout near $T_{\rm fo}$, this provides a natural explanation for their production systematics without postulating their survival at high temperatures.