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 and the neutron-proton effective mass splitting on the neutron-to-proton ratio of free nucleons and those in light clusters. It is found that the has a relatively stronger effect than the and the assumption of 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 and 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 and 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.
- Received 3 February 2015
- Revised 2 March 2015
DOI:https://doi.org/10.1103/PhysRevC.91.047601
©2015 American Physical Society