Free spectator nucleons in ultracentral relativistic heavy-ion collisions as a probe of neutron skin

Abstract

Besides the yield ratio of free spectator neutrons produced in ultracentral 96 Zr+ 96 Zr to 96 Ru+ 96 Ru collisions [1], we propose that the yield ratio Nn Np of free spectator neutrons to protons in a single collision system at RHIC and LHC can be a more sensitive probe of the neutron-skin thickness ∆rnp and the slope parameter L of the symmetry energy. The idea is demonstrated based on the proton and neutron density distributions of colliding nuclei obtained from Skyrme-Hartree-Fock-Bogolyubov calculations, and a Glauber model that provides information of spectator matter. The final spectator particles are produced from direct emission, clusterization by a minimum spanning tree algorithm or a Wigner function approach, and deexcitation of heavy clusters by GEMINI. A larger ∆rnp associated with a larger L value increases the isospin asymmetry of spectator matter and thus leads to a larger Nn Np, especially in ultracentral collisions where the multiplicity of free nucleons are free from the uncertainties of cluster formation and deexcitation. We have further shown that the double ratio of Nn Np in isobaric collision systems or in collisions by isotopes helps to cancel the detecting efficiency for protons. Effects from nuclear deformation and electromagnetic excitation are studied, and they are found to be subdominant compared to the expected sensitivity to ∆rnp.

Measuring deformed neutron skin with free spectator nucleons in relativistic heavy-ion collisions

“…For L = 30 MeV, the overall N n N p ratio is smaller as already observed in Ref. [42], while the difference between N n N p in central tip-tip and body-body 96 Ru+ 96 Ru collisions as well as in 197 Au+ 197 Au collisions remains qualitatively similar although the magnitude becomes smaller for W 0 = 80 MeVfm 5 . For W 0 = 133 MeVfm 5 or larger, the N n N p ratios are seen to be similar or slightly larger in central tip-tip 96 Ru+ 96 Ru and 197 Au+ 197 Au collisions compared to those in the corresponding body-body collisions.…”

“…We have further proposed in Ref. [42] that the yield ratio N n N p of free spectator neutrons to protons in a single collision system can be a more sensitive probe of the ∆r np , if spectator protons can also be measured by instrumenting the forward region with dedicated detectors [43]. In the previous studies, we consider 96 Zr+ 96 Zr and 96 Ru+ 96 Ru collisions with random orientations.…”

“…For more details of the analysis procedure, we refer the reader to Refs. [41,42]. The numbers N + Z of total spectator nucleons in the four collision systems for different collision configurations and based on density distributions by using different W 0 are compared in Fig.…”

“…With the information of spectator nucleons obtained through a Monte-Carlo Glauber model, free spectator neutrons and protons are further generated through a multifragmentation process as detailed in Refs. [41,42]. By investigating the dependencies of the total and free spectator nucleon numbers as well as the corresponding neutron-proton asymmetries on the collision configuration and the deformed neutron skin in four typical collision systems, we found that although 96 Zr+ 96 Zr and 238 U+ 238 U collisions are not suitable for probing the deformed neutron skin by selecting special collision configurations, 96 Ru+ 96 Ru and 197 Au+ 197 Au collisions are suitable systems for probing the angular distribution of the neutron skin by comparing the yield ratio N n N p in central tip-tip and body-body collisions.…”

Evidence of Hexadecapole Deformation in Uranium-238 at the Relativistic Heavy Ion Collider

“…In the future, one should move away from WS densities and directly input the results of EDF calculations in hydrodynamic simulations, taking into account the difference between spatial distributions of protons and neutrons. For 238 U, this should be especially important due to the strong polarization of its neutron skin across the surface [53,54], whose effect in high-energy collisions may be similar to an overall broadening of the skin thickness. A recent STAR analysis of the structure of 238 U from ultraperipheral collisions suggests, for instance, a larger skin than reported in common WS parametrizations [55].…”