Application of the coalescence model to deuterons in central⁵⁸Ni + ⁵⁸Ni collisions between 1A and 2A GeV

In the context of the coalescence model and the density-matrix formalism, the radii of the excited volume at chemical freeze-out for central ⁵⁸Ni + ⁵⁸Ni collisions are extracted by using the published transverse momentum spectra of deuterons at incident beam energies from 1A to 2A GeV. The freeze-out radii from the coalescence model are in fair agreement with those from the pion Hanbury-Brown-Twiss analysis for the same collsion system in the same beam energy range. Furthermore, the radii are much larger than the sizes of the projectile and the target nuclei, supporting a significant expansion of the participant fireball before freeze-out. The freeze-out baryon densities are estimated to be about one third of the normal nuclear-matter density.