Baryon stopping as a probe of deconfinement onset in relativistic heavy-ion collisions

It is argued that an irregularity in the baryon stopping is a natural consequence of onset of deconfinement occurring in the compression stage of a nuclear collision. It is a combined effect of the softest point inherent in an equation of state (EoS) with a deconfinement transition and a change in the nonequilibrium dynamics from hadronic to partonic transport. Thus, this irregularity is a signal from a hot and dense stage of the nuclear collision. In order to illustrate this proposition, calculations within the three-fluid model were performed with three different EoSʼs: a purely hadronic EoS, an EoS with a first-order phase transition and that with a smooth crossover transition. It is found that predictions within the first-order-transition scenario indeed reveal a strong irregularity in the incident energy dependence of the form of the net-proton rapidity distributions in central collisions. This behavior is in contrast to that for the hadronic scenario, where the distribution form gradually evolves, displaying no irregularity. The case of the crossover EoS is intermediate. Only a weak irregularity takes place. Experimental data also exhibit a trend of similar irregularity, which is however based on still preliminary data at energies of 20A GeV and 30A GeV.