Baryon stopping in 40 and 158 GeV/nucleon Pb+Pb collisions

TL;DR

This study measures proton rapidity distributions in Pb+Pb collisions at 40 and 158 GeV/nucleon, finding little variation with collision centrality and similarities to p+p interactions, with models describing the data but showing detailed differences.

Contribution

It provides new measurements of proton rapidity distributions at SPS energies and compares them with transport models, revealing invariance and model discrepancies.

Findings

Proton spectra are similar across centralities and to p+p interactions.

Transport models HSD and UrQMD describe the data qualitatively.

Final state protons with significant slowing are unaffected by multiple nucleon interactions.

Abstract

Proton rapidity distributions have been measured by the NA49 collaboration in 40 and 158 GeV/nucleon Pb+Pb collisions as function of collision centrality. We find that the shape and the yield per wounded nucleon in the mid-rapidity region vary little with centrality and are similar to the distributions obtained from inelastic p+p interactions. This observation is satisfactorily described by the transport models HSD and UrQMD, although there are significant differences in the details of the spectral shape between the experimental data and the models as well as between the models. The approximate invariance of the normalized proton spectrum in the vicinity of mid-rapidity suggests that multiple nucleon-nucleon interactions in nuclear collisions at SPS energies have little effect on the spectra of those final state protons which are slowed down the most.