Physics Letters B 786 (2018) 255–259
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Investigating different and
¯
polarizations in relativistic heavy-ion
collisions
Zhang-Zhu Han
a,b
, Jun Xu
a,c,∗
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 Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
a r t i c l e i n f o a b s t r a c t
Article history:
Received
23 July 2017
Received
in revised form 30 September
2018
Accepted
1 October 2018
Available
online 4 October 2018
Editor:
W. Haxton
Based on the chiral kinetic equations of motion, spin polarizations of various quarks, due to the
magnetic field induced by spectator protons as well as the quark-antiquark vector interaction, are studied
within a partonic transport approach. Although the magnetic field in QGP enhances the splitting of the
spin polarizations of partons compared to the results under the magnetic field in vacuum, the spin
polarizations of s and
¯
s quarks are also sensitive to the quark-antiquark vector interaction, challenging
that the different and
¯
spin polarization is a good measure of the magnetic field in relativistic heavy-
ion
collisions. It is also found that there is no way to obtain the large splitting of the spin polarization
between and
¯
at
√
s
NN
=7.7 GeV with partonic dynamics.
© 2018 The Author(s). 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
.
Understanding the properties of the quark -gluon plasma (QGP)
is one of the main purposes of relativistic heavy-ion collision ex-
periments.
In noncentral heavy-ion collisions, QGP is expected to
be polarized perpendicular to the reaction plane [1–3]due to the
large angular momentum as well as the strong magnetic field. The-
oretical
studies predict that the strong vorticity and magnetic field
lead to a series of chiral effects (see, e.g., Ref. [4]for a review) as
well as the spin polarizations of hyperons and vector mesons [5–8],
which are experimentally measurable through their decays. On the
experimental side, continuous efforts have been made on measur-
ing
the spin polarization of these particles [9–12]. In the collision
systems at higher energies with nearly zero baryon chemical po-
tential,
shorter duration of the magnetic field, and smaller angular
velocity, the spin polarizations of and
¯
are found to be very
small [9,11]. Recently , the finite spin polarizations of and
¯
at
lower collision energies have been observed experimentally [12],
with the
¯
spin polarization slightly larger than that of . Consid-
erable
efforts have been devoted to understanding the polarization
of [13–17]but few of them try to address the different spin
polarizations of and
¯
.
The
studies in Refs. [13–17] attribute the hyperon polarization
to the coupling to the vorticity field of the QGP, and the spin po-
*
Corresponding author at: Shanghai Advanced Research Institute, Chinese
Academy of Sciences, Shanghai 201210, China.
E-mail
address: xujun@sinap.ac.cn (J. Xu).
larizations of quarks and antiquarks are affected in a similar way.
On the other hand, the vector potentials, including those from the
quark-antiquark vector interaction and the electromagnetic field,
are expected to be responsible for the different polarizations for
and
¯
at lower collision energies. Due to the finite bar yon
chemical potential, quarks and antiquarks are affected by differ-
ent
spin-dependent interactions in the baryon-rich matter. It was
also proposed that the difference of the spin polarization between
and
¯
can be used as a measure of the magnetic field in rela-
tivistic
heavy-ion collisions (see, e.g., Ref. [18]), with the strength
of the later suffering from the uncertainty of the electrical conduc-
tivity
of the QGP. The strength of the vector potentials, especially
the magnetic field, is responsible for the occurrence of the chiral
magnetic effect and the chiral magnetic wave.
In the present study, we investigate the different spin polariza-
tions
of and
¯
in Au +Au collisions at
√
s
NN
=39 and 7.7 GeV
as a result of the vector potentials with partonic transport simu-
lations
based on the chiral kinetic equations of motion. The vector
potentials include the dominating magnetic field contribution from
the spectator protons in the QGP with a temperature-dependent
electrical conductivity, and the space component of the quark-
antiquark
vector potential related to the net quark flux. We found
that the s and
¯
s quark spin polarizations, which are responsible
for the and
¯
spin polarizations via the coalescence model,
are sensitive to the strength of both the magnetic field and the
quark-antiquark vector potential. In addition, there is no way to
https://doi.org/10.1016/j.physletb.2018.10.001
0370-2693/
© 2018 The Author(s). 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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