通过对不变质量将手性磁效应与背景隔离资源-CSDN文库

Open

Access

28 浏览量 2020-03-26 08:29:09 上传评论收藏 517KB PDF 举报

资源推荐

资源详情

资源评论

Eur. Phys. J. C (2019) 79:168

https://doi.org/10.1140/epjc/s10052-019-6671-1

Regular Article - Theoretical Physics

Isolating the chiral magnetic effect from backgrounds by pair

invariant mass

Jie Zhao

1,a

, Hanlin Li

1,2

, Fuqiang Wang

1,3,b

Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA

College of Science, Wuhan University of Science and Technology, Wuhan 430065, Hubei, China

School of Science, Huzhou University, Huzhou 313000, Zhejiang, China

Received: 1 October 2018 / Accepted: 8 February 2019 / Published online: 25 February 2019

Abstract Topological gluon conﬁgurations in quantum

chromodynamics induce quark chirality imbalance in local

domains, which can result in the chiral magnetic effect

(CME) – an electric charge separation along a strong mag-

netic ﬁeld. Experimental searches for the CME in relativis-

tic heavy ion collisions via the charge-dependent azimuthal

correlator (γ ) suffer from large backgrounds arising from

particle correlations (e.g. due to resonance decays) coupled

with the elliptic anisotropy. We propose differential mea-

surements of the γ as a function of the pair invariant mass

inv

), by restricting to high m

inv

thus relatively background

free, and by studying the m

inv

dependence to separate the

possible CME signal from backgrounds. We demonstrate by

model studies the feasibility and effectiveness of such mea-

surements for the CME search.

1 Introduction

Vacuum ﬂuctuations in quantum chromodynamics (QCD)

can result in metastable local domains of gluon ﬁelds with

non-vanishing topological charges [1–3]. Interactions with

those gluon ﬁelds can, under the approximate chiral sym-

metry restoration, change the overall chirality of quarks in

those domains [4,5]. The chirality imbalance yields an elec-

tric charge separation under a strong magnetic ﬁeld, a phe-

nomenon called the chiral magnetic effect (CME) [6–8].

Strong magnetic ﬁelds are generated at early times by the

spectator protons in relativistic heavy ion collisions, raising

the possibility to detect the CME in those collisions [9,10].

An observation of the CME would conﬁrm a fundamental

property of QCD and is therefore of great importance [11].

e-mail: zhao656@purdue.edu

e-mail: fqwang@purdue.edu

CME-like phenomena are not speciﬁc only to QCD and may

have been observed in condense matter physics [12].

A commonly used variable to measure the CME-induced

charge separation in heavy ion collisions is the three-point

correlator [9],

γ ≡cos(α + β −2ψ), (1)

where α and β are the azimuthal angles of two particles and

ψ is that of the reaction plane (span by the beam and impact

parameter directions of the colliding nuclei). Charge sepa-

ration along the magnetic ﬁeld (



B), which is perpendicular

to ψ on average, would yield different values of γ for parti-

cle pairs of same-sign (SS) and opposite-sign (OS) charges:

=−1,γ

=+1. However, there exist background

correlations unrelated to the CME [9,13–20]. For exam-

ple, transverse momentum conservation induces correla-

tions among particles enhancing back-to-back pairs [14–18].

This background is independent of particle charges, affect-

ing SS and OS pairs equally and cancels in the difference,

γ ≡ γ

− γ

. Recent experimental searches have thus

focused on the γ observable [11,21,22]; the CME would

yield γ > 0. There are, however, also mundane physics that

differ between SS and OS pairs. One such physics is reso-

nance/cluster decays [9,13–18], more signiﬁcantly affecting

OS pairs than SS pairs. Backgrounds arise from the cou-

pling of elliptical anisotropy (v

, a common phenomenon

in heavy ion collisions [23]) of resonances/clusters and the

angular correlations between their decay daughters (non-

ﬂow) [9,13,14,17]. Take ρ → π

−

as an example. The

background is (γ )

= r

, where r

= N

/(N

−

)

is the relative abundance of ρ-decay pairs over all OS pairs,

and γ

≡f

2,ρ

=cos(α + β − 2φ

) cos 2(φ

− ψ)

quantiﬁes the ρ decay angular correlations coupled with its

[9,21,22,24].

123

168 Page 2 of 8 Eur. Phys. J. C (2019) 79 :168

Experimentally, signiﬁcant positive γ values have been

observed at the Relativistic Heavy Ion Collider (RHIC)

and the Large Hadron Collider (LHC) [25–29]. The rela-

tive background and CME contributions are under extensive

debate [30]. The recent observations of comparable γ in

small system collisions [31–36], where any CME signals

would average to zero [31,37], challenge the CME inter-

pretation of the measured γ in heavy ion collisions. The

major difﬁculty in distinguishing CME from backgrounds

with the γ observable is their similar behaviors with respect

to the event multiplicity [34–36]. This is because the mag-

netic ﬁeld strength, to which the CME is sensitive, has a

similar dependence as the v

(backgrounds) on the event mul-

tiplicity [10,38–41]. There have been various proposals and

attempts to reduce or eliminate the backgrounds [24,32,42–

46]. The central idea is to “hold” the magnetic ﬁeld ﬁxed (in a

narrow centrality) and vary the event-by-event v

from statis-

tical and dynamical ﬂuctuations [41,44,47]. The ﬁrst attempt

was carried out by STAR [44] where a charge asymmetry

observable was analyzed as a function of the observed event-

by-event v

. A linear dependence was observed, expected

from background, and the intercept was extracted represent-

ing a background-suppressed signal. ALICE [46] divided

their data in each collision centrality according to v

in one

phase space, and found the γ to be approximately pro-

portional to the v

in the phase space of the γ measure-

ment, consistent with background contributions. However,

as recently pointed out by two of us [24], those methods

suppressing background may not completely eliminate it.

Another way to help search for the CME is to compare iso-

baric collisions [47], where the magnetic ﬁelds differ and

the backgrounds are expected to be the same [48]. However,

these simple expectations may not be correct because of the

non-identical isobaric nuclear structures [49].

A new method to search for the CME, as we demonstrate

in this article, is to eliminate the resonance background con-

tributions using particle pair invariant mass (m

inv

)by(i)

applying a lower cut on the m

inv

, (ii) ﬁtting the low m

inv

region by a two-component (TC) model, and (iii) ﬁtting the

low m

inv

region by a two-component model aided by event-

shape engineering (TC+ESE). We illustrate our method using

the AMPT (A Multi-Phase Transport) model [50] and a toy

Monte Carlo (MC) simulation. In both, the resonance masses

are sampled from Breit-Wigner distributions [51,52]. We

use pions within pseudorapidity |η| < 1 and 0.2 < p

2GeV/c, except otherwise speciﬁed.

It is worthwhile to note here, since the CME is generally

alow p

phenomenon, that large m

inv

region may not be the

best place to search for it. If no CME is found at large m

inv

it does not necessarily mean that CME does not exist at low

inv

. However, if a ﬁnite signal is observed at large m

inv

where background sources diminish, it could be a strong evi-

dence for the possible existence of the CME that would call

for further investigations. It is also worthwhile to note that

our low m

inv

ﬁtting procedure is not without assumptions.

Although the background shape may be assessed by the ESE

method, there is still the question of the CME shape as func-

tion of m

inv

. In principle, if the CME and the background

have the same m

inv

shape, then our invariant mass method

would not be able to distinguish the two contributions. Theo-

retical inputs are much needed in this respect. Nevertheless,

our method provides a potentially powerful way to extract

the CME with reasonable assumptions of the m

inv

depen-

dence of the CME, which can be gauged to some extent by

the ﬁtting quality.

The rest of the paper is organized as follows. Section 2

presents the AMPT and toy-model studies by applying a

lower m

inv

cut to extract the γ in the high m

inv

region. Sec-

tion 3 describes a two-component model ﬁt to the γ (m

inv

Section 4 extends the ﬁtting method further by using ESE to

determine the background γ (m

inv

) shape to help extract the

CME signal. Section 5 puts our methods into experimental

context and summarizes the paper.

2 High-m

inv

region

2.1 A transport model study with null CME

AMPT is a parton transport model [50]. It consists of a ﬂuctu-

ating initial condition, parton elastic scatterings, quark coa-

lescence for hadronization, and hadronic interactions. The

initial condition is taken from HIJING [53]. The string melt-

ing version [54] is used in this study. Two-body elastic par-

ton scatterings are treated with Zhang’s Parton Cascade [55],

where the parton scattering cross section is set to 3 mb. After

partons stop interacting, a simple quark coalescence model

is applied to describe the hadronization process that converts

partons into hadrons [56]. Subsequent interactions of these

formed hadrons are modeled by a hadron cascade [56]. How-

ever, it is known that this version of the hadron cascade does

not conserve charge, which is critical to the charge correlation

study here. The hadronic scatterings, while responsible for

the majority of the v

mass splitting, are unimportant for the

main development of v

[57,58], and thus may not be critical

for the CME backgrounds. We thus turn off hadronic cascade

in AMPT for our study here, as was done in Ref. [59]. AMPT

has been quite successful in describing variety of heavy ion

data [52]. It reproduces approximately the measured particle

yields and distributions, and therefore should approximately

describe those of resonances as well, which is relevant to

the CME background study here. We simulate Au+Au colli-

sions at

√

= 200 GeV of various impact parameter (b)

ranges. For simplicity we use the known reaction plane in

our analysis, ﬁxed at ψ = 0.

123

剩余7页未读，继续阅读

评论收藏

内容反馈

weixin_38635166

粉丝: 8
资源: 876

通过对不变质量将手性磁效应与背景隔离

手性动力学理论的非平衡手性磁效应

具有纵向增强不变性和手性磁效应的异常磁流体动力学

手性磁效应及其对后期磁场强度的实验约束

旋转标量的磁发生：标量手性磁效应

温度依赖的字符串启发性CPT违反背景在瘦素形成和手性磁效应中的作用

通过引力异常和全息图淬灭手性磁效应

手性磁效应会改变QCD中的动态通用性类别吗？

轴向病房识别和史维杰机制：实时手性磁效应和冷凝物的应用

粘性流体中的非平衡手性磁/涡效应

存在外部轴向矢量场时的手性磁效应

手性涡旋效应对早期宇宙中超磁场和物质-反物质不对称性演化的贡献

二维Weyl-Dirac物质中拟手性磁效应的各个方面

手性介质中漩涡的动力学：手性推进效应

手性旋涡效应的解剖

磁性背景中的手性介子

手性动力学方程的质量校正

通过磁螺旋的热波动消除手性电荷

手性介质中的手性热波和磁，涡流和热波的混合

扭转麦克斯韦-切尔·西蒙斯电动力学诱导的手性动力学和磁化作用

压实的D4膜和D0膜涂有电荷的手性旋涡效应

强磁场下手性等离子体的维格纳函数和动力学现象

带电磁性黑铜和手性磁性传输的准正规模式

具有涡旋的磁化手性等离子体中的流体力学模式

奇偶违反相互作用驱动粒子质量在磁场产生中的作用

最新资源