Revisit of the neutron/proton ratio puzzle in intermediate-energy heavy-ion collisions

TL;DR

This study investigates how the density dependence of nuclear symmetry energy and neutron-proton effective mass splitting influence neutron/proton ratios in heavy-ion collisions, revealing the dominant effect of mass splitting and highlighting discrepancies with experimental data.

Contribution

It introduces an improved isospin- and momentum-dependent interaction in the IBUU11 model and analyzes the relative effects of symmetry energy and mass splitting on neutron/proton ratios.

Findings

Mass splitting has a stronger effect than symmetry energy on neutron/proton ratios.

Assuming neutron effective mass less than or equal to proton mass increases neutron/proton ratio.

Current models underestimate the experimental neutron/proton ratios, indicating missing physics.

Abstract

Incorporating a newly improved isospin- and momentum-dependent interaction in the isospin-dependent Boltzmann-Uehling-Uhlenbeck transport model IBUU11, we have investigated relative effects of the density dependence of nuclear symmetry energy $E_{sym}(\rho)$ and the neutron-proton effective mass splitting $m^*_n-m^*_p$ on the neutron/proton ratio of free nucleons and those in light clusters. It is found that the $m^*_n-m^*_p$ has a relatively stronger effect than the $E_{sym}(\rho)$ and the assumption of $m^*_n\leq m^*_p$ leads to a higher neutron/proton ratio. Moreover, this finding is independent of the in-medium nucleon-nucleon cross sections used. However, results of our calculations using the $E_{sym}(\rho)$ and $m^*_n-m^*_p$ both within their current uncertainty ranges are all too low compared to the recent NSCL/MSU double neutron/proton ratio data from central $^{124}$Sn+$^{124}$Sn and $^{112}$Sn+$^{112}$Sn collisions at 50 and 120 MeV/u, thus calling for new mechanisms to explain the puzzlingly high neutron/proton ratio observed in the experiments.