Liu, Lu-Meng; Zhang, Chun-Jian; Zhou, Jia; Xu, Jun; Jia, Jiangyong; Peng, Guang-Xiong

PHYSICS LETTERS B

2022, 834, 0370-2693

Liu, Lu-Meng; Zhang, Chun-Jian; Zhou, Jia; Xu, Jun; Jia, Jiangyong; Peng, Guang-Xiong

We show that the yield ratio of free spectator neutrons produced in high-energy Zr-96+Zr-96 to Ru-96+Ru-96 collisions is a clean probe of the neutron-skin thickness of colliding nuclei and the slope parameter Lof the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions via a state-of-the-art Skyrme-Hartree-Fock-Bogolyubov calculation. Among spectator nucleons given by the Glauber model, free spectator neutrons include those from direct production that survive from clusterization as well as those from deexcitation of heavy clusters described by the popular GEMINI model. More free neutrons are produced in collisions of Zr-96 nucleus due to its larger neutron skin, compared to those produced in collisions of Ru-96 nucleus with a smaller neutron skin. The difference of the free spectator neutron yield is further increased with the increasing difference of the neutronskin thickness between Zr-96 and Ru-96 with a larger Lvalue, and the increase in ultracentral collisions is particularly insensitive to model details and experimental uncertainties. Since the production of free spectator neutrons is not affected by the complicated dynamics in the mid-rapidity region, the ratio of their multiplicities in ultracentral isobaric collisions is a robust observable for constraining the neutron skin and Lvalue. (C) 2022 The Author(s). Published by Elsevier B.V.