Physics Letters B 733 (2014) 105–111
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Nuclear modification factor in intermediate-energy heavy-ion
collisions
M. Lv
a,b
,Y.G.Ma
a,c,∗
, G.Q. Zhang
a
,J.H.Chen
a
,D.Q.Fang
a
a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
b
University of Chinese Academy of Sciences, Beijing 100049, China
c
Shanghai Tech University, Shanghai 200031, China
article info abstract
Article history:
Received 20 December 2013
Received in revised form 14 March 2014
Accepted 10 April 2014
Available online 18 April 2014
Editor: W. Haxton
The transverse momentum dependent nuclear modification factor (NMF), namely R
CP
,isinvestigated
for protons produced in Au
+ Au at 1 A GeV within the framework of the isospin-dependent quantum
molecular dynamics (IQMD) model. It is found that the radial collective motion during the expansion
stage affects the NMF at low transverse momentum a lot. By fitting the transverse mass spectra of protons
with the distribution function from the Blast-Wave model, the magnitude of radial flow can be extracted.
After removing the contribution from radial flow, the R
CP
can be regarded as a thermal one and is
found to keep unitary at transverse momentum lower than 0.6 GeV/c and enhance at higher transverse
momentum, which can be attributed to the Cronin effect.
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/3.0/). Funded by SCOAP
3
.
1. Introduction
Recently, the nuclear modification factor (NMF) has been exten-
sively investigated for different particles at various collision ener-
gies in relativistic heavy-ion collisions (HIC) [1–4].Thesestudies
indicate that the NMF, which can be represented either by the
modification factor between nucleus–nucleus (AA) collisions and
proton–proton (pp) collision R
AA
or the one between the central
collisions and peripheral collisions R
CP
, is very useful for the study
of the quantitative properties of the nuclear medium response
when the high speed jet transverses it. In high transverse mo-
mentum (p
T
) region, NMF is suppressed owing to jet quenching
effect in hot-dense matter and thus has become one of the ro-
bust evidences on the existence of the Quark-Gluon-Plasma [5,6].
In lower p
T
region, radial flow boosts or the Cronin Effect [7]
competes with the quenching effect and enhances the NMF, which
has been also demonstrated by the Relativistic Heavy-Ion Collider
(RHIC) beam energy scan (BES) project [8].
Meanwhile, collective motion plays an important role in the
time
evolution of particles, which has been studied over a wide
range of collision energy in heavy-ion collision. Around 1A GeV
incident energy in central HIC, the colliding nuclei are expected
to be stopped and lead to densities of 2
∼ 3ρ
0
(ρ
0
is the normal
nuclei density) at the largest compression time [9]. At this point,
*
Corresponding author.
E-mail address: y
gma@sinap.ac.cn (Y.G. Ma).
a high excitation energy stage is reached and some parts of the ex-
citation energy are converted into collective motion, such as radial
flow [10,11]. Thus, in the following expansion stage, the products
move outward containing both the collective motion and the ther-
mal motion. It will be of interests to decouple these two parts,
because each of them reflects important information of the HIC
process. Efforts have been made by the Blast-Wave model [12–14],
and the collective motion parameter (radial flow velocity) together
with the thermal motion parameter (temperature) can be extracted
at the same time.
Nevertheless, till now, NMF has not been investigated in inter-
me
diate energy HIC yet to our knowledge. In particular, the R
CP
shape will be strongly affected by the radial flow which plays a sig-
nificant role in the expansion stage especially for the low p
T
parti-
cles. In order to understand the properties of the nuclear medium
response, one may want to know what the behavior of R
CP
without
the contribution from radial flow might be. In the present paper,
we address NMF in intermediate energy HIC to study the prop-
erties of nuclear medium for the first time. After removing the
contribution from the radial flow, the NMF can be regarded as a
thermal one, which reflects the property of thermal medium pro-
duced in intermediate energy HIC. The R
CP
for emitted protons in
Au
+ Au collision at 1A GeV is investigated systematically.
The article is organized as follows: A brief introduction about
the
IQMD model is given in Section 2. In Section 3, we compare
the kinetic energy spectra of light fragments obtained by the IQMD
with the one from the EOS experimental result, then we fit the
proton transverse mass (m
T
) spectra with the Boltzmann function
http://dx.doi.org/10.1016/j.physletb.2014.04.025
0370-2693/
© 2014 The Authors. Publishe d by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). Funded by
SCOAP
3
.