The dependencies of different particle ratios on the nucleus-nucleus center-of-mass energy,
which can be related to the chemical potential, are calculated by using hadron resonance gas
(HRG) and Ultra-relativistic Quantum Molecular Dynamic (UrQMD) models. For UrQMD
two different types of phase transitions are taken into consideration, namely crossover and
first order, while the HRG model implements fully statistical aspects in describing the
particle production and their correlations, especially in the hadron phase. The freezeout
parameters, temperature (T ) and baryon chemical potential (μ), are deduced by fitting
various particle ratios estimated from UrQMD with HRG calculations at 7.7, 11.5, 19.6,
62.4 GeV. The results agree with the parameters which are independently determined
from statistical fit of the measured particle ratios and the thermal-statistical approaches.
Furthermore, the net-charge fluctuations for K/π, p/π and K/p determined UrQMD, and
HRG are compared with the available STAR and NA49 measurements. The future NICA
facility covers a wide range of low beam energies. The excellent agreement justifies the
conclusion that both UrQMD and HRG are suitable approaches to explain both freezeout
parameters and the dynamical net-charge fluctuations. |
