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 $E_{sym}(\rho )$ 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 $E_{sym}(\rho )$ 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 $E_{sym}(\rho )$ 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

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• Density-dependent nucleon-nucleon interaction from three-nucleon forces
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• Density matrix expansion for the MDI interaction
• Neutron-proton mass difference in finite nuclei and the Nolen-Schiffer anomaly
• Neutron-proton mass difference in isospin asymmetric nuclear matter
• Neutron-proton mass difference in nuclear matter
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Cited by in corpus (10)

• Dynamics of clusters and fragments in heavy-ion collisions
• Transport approaches for the Description of Intermediate-Energy Heavy-Ion Collisions
• Neutron-proton effective mass splitting in neutron-rich matter
• Influence of the treatment of initialization and mean-field potential on the neutron to proton yield ratios
• Bayesian inference on the isospin splitting of nucleon effective mass from giant resonances in ${}^{208}$Pb
• Isospin dependence of nucleon effective masses in neutron-rich matter
• Effects of the momentum dependence of nuclear symmetry potential on pion observables in Sn + Sn collisions at 270 MeV/nucleon
• Investigating the possibility of extracting neutron-skin thickness in nuclei by their collisions at intermediate energies
• Bayesian Analyses of Proton Multiple Flow Components in Intermediate Heavy Ion Collisions with Momentum-Dependent Interactions
• Transport Model Comparison Studies of Intermediate-Energy Heavy-Ion Collisions