Physics Letters B 762 (2016) 296–300
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Effects of nuclear potential on the cumulants of net-proton and
net-baryon multiplicity distributions in Au+Au collisions
at
√
s
NN
= 5 GeV
Shu He
a
, Xiaofeng Luo
a,∗
, Yasushi Nara
b
, ShinIchi Esumi
c
, Nu Xu
a,d
a
Key Laboratory of Quark & Lepton Physics (MOE) and Institute of Particle Physics, Central China Normal University, Wuhan 430079, China
b
Akita International University, Yuwa, Akita-city 010-1292, Japan
c
Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
d
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
22 July 2016
Received
in revised form 12 September
2016
Accepted
26 September 2016
Available
online 29 September 2016
Editor:
V. Metag
We analyze the rapidity and transverse momentum dependence for the cumulants of the net-proton
and net-baryon distributions in Au+Au collisions at
√
s
NN
= 5 GeV with a microscopic hadronic transport
(JAM) model. To study the effects of mean field potential and softening of equation of state (EoS) on
the fluctuations of net-proton (baryon) in heavy-ion collisions, the calculations are performed with two
different modes. The softening of EoS is realized in the model by implementing the attractive orbit in
the two-body scattering to introduce a reduction pressure of the system. By comparing the results from
the two modes with the results from default cascade, we find the mean field potential and softening of
EoS have strong impacts on the rapidity distributions (dN/dy) and the shape of the net-proton (baryon)
multiplicity distributions. The net-proton (baryon) cumulants and their ratios calculated from all of the
three modes are with similar trends and show significant suppression with respect to unity, which can
be explained by the presence of baryon number conservations. It indicates that the effects of mean field
potential and softening of EoS might be not the ingredients that are responsible to the observed strong
enhancement in the most central Au+Au collisions at 7.7 GeV measured by the STAR experiment at RHIC.
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
One of the main interests of relativistic heavy ion collision ex-
periments
is to explore the phase structure of the QCD matters.
The QCD phase structure can be displayed in the two dimen-
sional
QCD phase diagram, in which the temperature T is plotted
as a function of the baryon chemical potential μ
B
. Lattice QCD
calculations show that the transition from Quark–Gluon Plasma
(QGP) to hadronic phase is a crossover transition [1] when μ
B
= 0,
and the QCD based model calculations predict the transition at
large μ
B
is of the first order [2]. Thus, the crossover region and
first order region should be connected by a so called QCD critical
point [3–5]. Due to sign problem in Lattice QCD calculations at fi-
nite
baryon density, there are large uncertainties to determine the
location of critical point by theoretical and/or QCD based model
calculations [5,6]. The fluctuation of conserved quantities, such as
*
Corresponding author.
E-mail
address: xfluo@mail.ccnu.edu.cn (X. Luo).
net-baryon number, and its proxy observable net-proton number,
served as the observable sensitive to the correlation length of
nuclear matter [7–10], have been extensively studied experimen-
tally [11–13] and
theoretically [14–27].
In recent experimental search for the critical point by the
STAR experiment at RHIC, the forth order net-proton fluctuations
(κσ
2
) displayed a non-monotonic behavior in the energy depen-
dence [29,28] (see
Fig. 1). One of the most striking observation
is the large deviation above unity of the net-proton κσ
2
in the
most central Au+Au collisions at
√
s
NN
= 7.7 GeV, which is consis-
tent
with receiving strong positive critical contribution predicted
by theoretical calculations. This observation cannot be described by
the UrQMD and AMPT model [30–34], both of which are transport
model without mean field and QCD phase transition. This moti-
vates
us to investigate whether the large increase in the forth order
net-proton fluctuations (κσ
2
) are caused by non-critical contribu-
tions,
such as mean field potentials, which may play an important
role at low collision energies and high baryon density region. Theo-
retical
model estimated that the size of the correlation length near
http://dx.doi.org/10.1016/j.physletb.2016.09.053
0370-2693/
© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.
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