Rapidity distribution of protons from the potential version of UrQMD model and the traditional coalescence afterburner

TL;DR

This study uses a potential version of the UrQMD model with a coalescence afterburner to accurately describe proton rapidity distributions across various energies, revealing insights into cluster formation and relativistic effects.

Contribution

It introduces a combined approach of potential UrQMD and a specific coalescence parameter set to match experimental data across multiple energy regimes.

Findings

UrQMD with potentials fits proton rapidity data well.

Cluster proton fractions decrease with increasing energy.

Relativistic effects on free protons are minimal due to cancellation.

Abstract

Rapidity distributions of both E895 proton data at AGS energies and NA49 net proton data at SPS energies can be described reasonably well with a potential version of the UrQMD in which mean-field potentials for both pre-formed hadrons and confined baryons are considered, with the help of a traditional coalescence afterburner in which one parameter set for both relative distance $R_0$ and relative momentum $P_0$, (3.8 fm, 0.3 GeV$/$c), is used. Because of the large cancellation between the expansion in $R_0$ and the shrinkage in $P_0$ through the Lorentz transformation, the relativistic effect in clusters has little effect on the rapidity distribution of free (net) protons. Using a Woods-Saxon-like function instead of a pure logarithmic function as seen by FOPI collaboration at SIS energies, one can fit well both the data at SIS energies and the UrQMD calculation results at AGS and SPS energies. Further, it is found that for central Au+Au or Pb+Pb collisions at top SIS, SPS and RHIC energies, the proton fractions in clusters are about 33$\%$, 10$\%$, and 0.7$\%$, respectively.