Impact parameter smearing effects on isospin sensitive observables in heavy ion collisions

TL;DR

This study evaluates the reliability of impact parameter estimation from charged particle multiplicity in heavy ion collisions and examines how isospin-sensitive observables respond to impact parameter and effective mass splitting, with implications for future experiments.

Contribution

It demonstrates the limitations of using charged particle multiplicity for impact parameter estimation at low energies and explores the sensitivity of isospin observables to effective mass splitting in heavy ion collisions.

Findings

Charged particle multiplicity poorly estimates impact parameter at energies below 70 MeV/u.

Charge distribution differences are evident when defining central collisions by multiplicity versus impact parameter.

The neutron to proton yield ratio's sensitivity to isospin effects is system-dependent and can be enhanced in certain reactions.

Abstract

The validity of impact parameter estimation from the multiplicity of charged particles at low-intermediate energies is checked within the framework of ImQMD model. The simulations show that the multiplicity of charged particles cannot estimate the impact parameter of heavy ion collisions very well, especially for central collisions at the beam energies lower than $\sim$70 MeV/u due to the large fluctuations of the multiplicity of charged particles. The simulation results for the central collisions defined by the charged particle multiplicity are compared to those by using impact parameter b=2 fm and it shows that the charge distribution for $^{112}$Sn +$^{112}$Sn at 50 MeV/u is different evidently for two cases; and the chosen isospin sensitive observable, the coalescence invariant single neutron to proton yield ratio, reduces less than 15\% for neutron-rich systems $^{124,132}$Sn +$^{124}$Sn at $E_{beam}$=50 MeV/u, while the coalescence invariant double neutron to proton yield ratio does not have obvious difference. The sensitivity of the chosen isospin sensitive observables to effective mass splitting is studied for central collisions defined by the multiplicity of charged particles. Our results show that the sensitivity is enhanced for $^{132}$Sn+$^{124}$Sn relative to that for $^{124}$Sn+$^{124}$Sn, and this reaction system should be measured in future experiments to study the effective mass splitting by heavy ion collisions.