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

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 $E_{sym}(\ddot{I})$ and the neutron-proton effective mass splitting $m_n^{\ast }-m_p^{\ast }$ on the neutron/proton ratio of free nucleons and those in light clusters. It is found that the $m_n^{\ast }-m_p^{\ast }$ has a relatively stronger effect than the $E_{sym}(\ddot{I})$ and the assumption of $m_n^{\ast }\le m_p^{\ast }$ 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 $E_{sym}(\ddot{I})$ and $m_n^{\ast }-m_p^{\ast }$ 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.

6 pages, 2 figures, discussions added