Research Article
Energy Dependence of Particle Ratios in High Energy
Nucleus-Nucleus Collisions: A USTFM Approach
Inam-ul Bashir, Rameez Ahmad Parra , Hamid Nanda, and Saeed Uddin
Department of Physics, Jamia Millia Islamia (Central University), New Delhi, India
Correspondence should be addressed to Rameez Ahmad Parra; rameezparra@gmail.com
Received 7 February 2018; Accepted 1 April 2018; Published 13 May 2018
A
c
ademic Editor: Chun-Sheng Jia
Copyright © Inam-ul Bashir et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. e
publication of this article was funded by SCOAP
.
We study the identied particle ratios produced at mid-rapidity (||<0.5) in heavy-ion collisions, along with their correlations with
the collision energy. We employ our earlier proposed unied statistical thermal freeze-out model (USTFM), which incorporates
the eects of both longitudinal and transverse hydrodynamic ow in the hot hadronic system. A fair agreement seen between
the experimental data and our model results conrms that the particle production in these collisions is of statistical nature. e
variation of the chemical freeze-out temperature and the baryon chemical potential with respect to collision energies is studied.
e chemical freeze-out temperature is found to be almost constant beyond the RHIC energy and is found to be close to the QCD
predicted phase-transition temperature suggesting that the chemical freeze-out occurs soon aer the hadronization takes place.
e vanishing value of chemical potential at LHC indicates very high degree of nuclear transparency in the collision.
1. Introduction
Relative hadron yields and their correlations are observable
which can provide information on the nature, composition,
and size of the medium from which they originate in high
energy heavy-ion collisions where a strongly interacting
nuclear matter at high energy density and temperatures is
formed. Within the framework of the statistical model, it
is assumed that a hot and dense reball is formed over an
extended region for a brief period of time (∼afewfm/c)aer
the initial collision which undergoes collective expansion
leading to a decrease in its temperature and nally to the
hadronization. Aer the hadronization of the hot reball, the
hadrons keep interacting with each other and the particle
number changing (inelastic) reaction processes continue to
take place till the temperature drops to a certain value
where a given reaction process almost comes to a stop.
ose particle number changing reaction processes (e.g.,
strangeness exchange process) stop earlier for which the
threshold energy is larger. e temperature at which the
particle number changing process for a given hadron almost
stops is called the “chemical freeze-out” temperature of that
hadronic specie. However, the (elastic) rescattering still takes
place and continues to build up the collective (hydrody-
namic) expansion. Consequently, the matter becomes dilute
and the mean free path for the elastic reaction processes of
given hadronic species becomes comparable with the system
size. At this stage the scattering processes stop and the
given hadron decouples from the rest of the system. is
is called the “kinetic or thermal freeze-out” aer which the
hadron’s energy/momentum spectrum is frozen in time [].
As the inelastic cross sections are only a small fraction of the
total cross section at lower (thermal) energies, the inelastic
processes stop well before the elastic ones. us chemical
freeze-out precedes kinetic or thermal freeze-out [].
Statistical thermal models have successfully reproduced
the essential features of particle production in heavy-ion
collisions [] as well as in many types of elementary collisions
[–] at LHC energies suggesting a statistical nature of
particle production in these collisions. Systematic studies of
particle yields using experimental results at dierent beam
energies have revealed a clear underlying freeze-out pattern
for particle yields in heavy-ion collisions [, ]. e success
of the statistical (thermal) models in describing the ratios of
Hindawi
Advances in High Energy Physics
Volume 2018, Article ID 9285759, 9 pages
https://doi.org/10.1155/2018/9285759