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大核的密度分布通常以伍兹-撒克逊分布为特征,其半径为R0,表皮深度为a。 然后引入变形参数β,以使用球谐函数R0(1 +β2Y20+β4Y40)的展开来描述非球核。 但是,当原子核为非球形时,R0和通过电子散射实验推断出的,在所有核取向上积分的结果都不能直接用作Woods-Saxon分布的参数。 另外,通常从减小的四极电子跃迁几率B(E2)↑得到的β2值与球形谐波膨胀中使用的β2值不直接相关。 B(E2)↑与本征四极矩Q0的关系比与β2的关系更准确。 但是,可以为给定的β2计算Q0,然后从Q0导出B(E2)↑。 在本文中,我们计算并制表了R0,a和β2值,这些值在Woods-Saxon分布中使用时,将得出与电子散射数据一致的结果。 然后,我们介绍使用新参数和旧参数计算的二次谐波和三次谐波参与者偏心率(ε2和ε3)。 我们证明ε3对a尤其敏感,并指出使用a的不正确值对于从重离子碰撞中产生的QGP提取粘度与熵之比(η/ s)具有重要意义。
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Physics Letters B 749 (2015) 215–220
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Parameterization of deformed nuclei for Glauber modeling
in relativistic heavy ion collisions
Q.Y. Shou
a,b,∗
, Y.G. Ma
a
, P. Sorensen
c
, A.H. Tang
c
, F. Videbæk
c
, H. Wang
c
a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
b
Key Laboratory of Quark and Lepton Physics (MOE) and Institute of Particle Physics, Central China Normal University, Wuhan 430079, China
c
Brookhaven National Laboratory, Upton, NY 11973, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
26 March 2015
Received
in revised form 30 July 2015
Accepted
30 July 2015
Available
online 4 August 2015
Editor:
V. Metag
The density distributions of large nuclei are typically modeled with a Woods–Saxon distribution
characterized by a radius R
0
and skin depth a. Deformation parameters β are then introduced to
describe non-spherical nuclei using an expansion in spherical harmonics R
0
(1 +β
2
Y
0
2
+β
4
Y
0
4
). But when
a nucleus is non-spherical, the R
0
and a inferred from electron scattering experiments that integrate
over all nuclear orientations cannot be used directly as the parameters in the Woods–Saxon distribution.
In addition, the β
2
values typically derived from the reduced electric quadrupole transition probability
B(E2)↑ are not directly related to the β
2
values used in the spherical harmonic expansion. B(E2)↑ is
more accurately related to the intrinsic quadrupole moment Q
0
than to β
2
. One can however calculate
Q
0
for a given β
2
and then derive B(E2)↑ from Q
0
. In this paper we calculate and tabulate the R
0
, a,
and β
2
values that when used in a Woods–Saxon distribution, will give results consistent with electron
scattering data. We then present calculations of the second and third harmonic participant eccentricity
(ε
2
and ε
3
) with the new and old parameters. We demonstrate that ε
3
is particularly sensitive to a and
argue that using the incorrect value of a has important implications for the extraction of viscosity to
entropy ratio (η/s ) from the QGP created in Heavy Ion collisions.
© 2015 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
In relativistic nucleus–nucleus collisions, the geometry of the
initial overlap region is reflected in the final momentum space
distributions of produced particles [1–3]. How much that geom-
etry
is translated into the final state distributions is used to infer
information about the properties of the matter created in the col-
lision
fireball like its viscosity [4]. The initial geometry plays a
particularly important role in interpreting the data and in extract-
ing
the viscosity to entropy ratio η/s. It is important therefore to
understand the initial conditions including the exact shape of the
colliding nuclei. The inference of the properties of the fireball from
data is hindered by uncertainties in the characteristics of the ini-
tial
state [5]. Recently collisions between Uranium nuclei (
238
U)
which have an intrinsic prolate shape [6], have been used as a
way to manipulate this initial geometry in order better test our
understanding of the initial state of heavy ion collisions and the
subsequent fireball [7,8].
*
Corresponding author.
E-mail
address: qiye.shou@cern.ch (Q.Y. Shou).
An important part of describing the initial conditions is to cor-
rectly
model the geometry of the incoming nuclei. For many years
in simulations for heavy ion collisions, nuclei were approximated
as smooth density distributions and the only anisotropies consid-
ered
in the initial state were the intrinsic almond shape caused by
the overlap of two spherical nuclei. As the accumulation of RHIC
data gradually demonstrated that final state anisotropies were sen-
sitive
to the initial geometry and its fluctuations, it became nec-
essary
to take into account the lumpiness of the colliding nuclei
[9–14]. This is done through Monte-Carlo simulations (M-C) where
each nucleus is generated with a finite number of nucleons dis-
tributed
with a density ρ described by a Woods–Saxon distribu-
tion [15]:
ρ(r) =
ρ
0
1 +e
(r−R
0
)/a
, (1)
where ρ
0
is the density at the center of the nucleus. The nuclear
radius R
0
and skin depth a are commonly taken from high-energy
electron scattering measurements [16]. For non-spherical nuclei,
this description was extended by introducing spherical harmon-
ics
in the Woods–Saxon distribution to describe the modulation of
http://dx.doi.org/10.1016/j.physletb.2015.07.078
0370-2693/
© 2015 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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