Fluctuations of the largest fragment charge in projectile fragmentation and its nonequilibrium effect

Background: The projectile fragmentation is associated with the liquid-gas phase transition in the finite nuclear system. Recent experimental data and calculations by the statistical multifragmentation model indicated that the higher-order fluctuations of the largest fragment charge exhibit the signatures of the second-order phase transition [PLB809(2020)135763]. However, these higher-order fluctuations calculated by the dynamical model are still not reported. Purpose: This work is proposed to investigate dynamically this signature and emphasize the nonequilibrium effect. Methods: The isospin-dependent quantum molecular dynamics (IQMD) model is used to study the nonequilibrium thermalization and fragmentation in the Sn-124 + Sn-120 collisions at 600 MeV/nucleon. The minimum spanning tree algorithm is applied to distinguish the hot projectile-like system during the dynamics evolution. Results: The fluctuations of the largest fragment charge Z(max) up to the fourth order by the IQMD model reproduce the experimental data. The pseudocritical point indicated by zero of skewness (third-order fluctuation) together with minimum of kurtosis excess (fourth-order fluctuation) is found at impact parameter b = 8.7 fm, where the multifragmentation and nucleon evaporation are well balanced. Two observables are defined to describe the asymmetry of temperature and density in the projectile-like system. The nonequilibrium of the projectile-like system is verified by the distributions of these two observables. By comparing the distributions of the largest fragment charge for the equilibrium and nonequilibrium systems, it is found that the nonequilibrium of the hot projectile-like system influences the competitive relation between the multifragmentation and nucleon evaporation. Conclusions: It is proposed that the nonequilibrium effect should be considered when study the pseudocritical point from the projectile fragmentation.

Automated summary

This study investigates the high-order fluctuations of the largest fragment charge in the projectile fragmentation process using the IQMD model. The results show that the nonequilibrium effect has an influence on the competition between multifragmentation and nucleon evaporation. Furthermore, a pseudocritical point is identified where the balance between these two processes occurs.