Reexamination of the neutron-to-proton-ratio puzzle in intermediate-energy heavy-ion collisions

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-to-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)* <= m(p)* leads to a higher neutron-to-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 National Superconducting Cyclotron Laboratory double neutron-to-proton-ratio data from central Sn-124+Sn-124 and Sn-112+Sn-112 collisions at 50 and 120 MeV/u, thus calling for new mechanisms to explain the puzzlingly high neutron-to-proton ratio observed in the experiments.