利用旁观者探测Pb + Pb碰撞中的几何波动与量子波动

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"这篇论文探讨了在Pb + Pb碰撞中使用旁观者探测方法来研究高能重离子碰撞的初始条件。通过引入观众中子的分箱技术，研究人员能够探测到更广泛的初始条件，这使得对那些罕见的初始状态事件的研究成为可能，这些事件在传统的集中度合并分析中可能会被掩盖。作者发现，观众分档可以独立改变ε2和ε3，即椭圆流与初始状态不规则性的关系。同时，观测到的v2 / ε2与1SdNchdη之间的标准比例关系和ln(vn / εn)与横向系统尺寸的声标度关系被显著破坏，破坏程度与观众中子的分档过程有关。这种新的分析方法允许研究者在保持最终状态多样性相似的情况下，探索更广泛的粘度驱动效应的初始状态变化。" 这篇发表在《Physics Letters B》上的开放访问文章，由Vipul Bairathi、Sandeep Chatterjee、Md. Rihan Haque和Bedangadas Mohanty共同撰写，详细讨论了在SNN = 2760 GeV的Pb + Pb碰撞中，利用旁观者探测技术如何揭示重离子碰撞的复杂性。研究中提到，由于初始条件中的几何和量子波动，传统的方法往往无法捕捉到所有可能的事件构型。通过对观众中子的细分，研究者能够更好地分离和分析不同初始条件下的事件特性。文章指出，观众中子的分档策略不仅打破了关于ε2和ε3的传统理解，而且也影响了声波相关的动态，这表明在不同初始条件下的物理过程可能与预期的简单比例关系有所不同。此外，这一发现对于理解和模拟强相互作用的量子色动力学(QCD)介质的性质具有重要意义，因为它允许研究者在保持整体事件特征不变的情况下，探究更广泛初始条件下的动力学行为。这项工作强调了旁观者探测技术在高能核碰撞研究中的重要性，并为理解和探索这些极端环境下的物质行为提供了新的视角和工具。通过这种方法，物理学家能够更深入地了解重离子碰撞过程中产生的夸克-胶子等离子体的性质及其对初始条件的敏感性。

Physics Letters B 754 (2016) 144–150

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Probing Pb +Pb collisions at

√

= 2760 GeV with spectators

Vipul Bairathi

, Sandeep Chatterjee

b,∗

, Md. Rihan Haque

, Bedangadas Mohanty

School of Physical Sciences, National Institute of Science Education and Research, Jatni, 752050, India

Theoretical Physics Division, Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Kolkata, 700064, India

a r t i c l e i n f o a b s t r a c t

Article history:

Received

30 August 2015

Received

in revised form 16 December 2015

Accepted

15 January 2016

Available

online 18 January 2016

Editor:

J.-P. Blaizot

There is event by event geometric as well as quantum ﬂuctuations in the initial condition of heavy-

ion

collisions. The standard technique of analysing heavy-ion collisions in bins of centrality obtained

from ﬁnal state multiplicity averages out the various initial conﬁgurations and thus restricts the study

to only a limited range of initial conditions. In this paper, we propose an additional binning in terms

of total spectator neutrons in an event. This offers us a key control parameter to probe events with

broader range of initial conditions providing us an opportunity to peep into events with rarer initial

conditions which otherwise get masked when analysed by centrality binning alone. We ﬁnd that the

inclusion of spectator binning allows one to vary ε

and ε

independently. We observe that the standard

scaling relation between v

/ε

and

dη

exhibited by centrality bins is strongly broken by the spectator

neutron bins. Further, the acoustic scaling relation between ln

(

/ε

)

and transverse system size is also

broken- the strength of the breaking being sensitive to the binning procedure. The introduction of the

spectator binning allows us to tune over a wide range viscosity driven effects for events with varying

initial states but similar ﬁnal state multiplicity.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

Of all the stages in a heavy-ion collision (HIC), the initial stage

is the least understood. However, in order to perform a sensitive

test of the theoretical framework, e.g. relativistic viscous hydro-

dynamics,

that correctly describes the evolution of the strongly

interacting matter produced in HIC experiments and thereof allows

unambiguous extraction of the medium properties, e.g. values of

the transport coeﬃcients, require a precise knowledge of the ini-

tial

state (IS). It has been shown that depending on the choice

of the initial condition that one chooses to evolve the relativistic

hydrodynamic equations, the value of the extracted shear viscosity

to entropy density ratio at the RHIC 200 GeV can vary by a factor

of 2 [1–6].

The

nuclei used in HIC experiments are extended objects. This

results in event by event (E/E) geometrical ﬂuctuations in addition

to the intrinsic quantum ﬂuctuations of the nuclear wave function.

The geometry of the nucleus ensures that various characteristics of

the IS in HICs like the number of wounded nucleons N

part

, number

Corresponding author.

E-mail

addresses: vipul.bairathi@niser.ac.in (V. Bairathi), sandeepc@vecc.gov.in

(S. Chatterjee),

rihan.h@niser.ac.in (Md. Rihan Haque), bedanga@niser.ac.in

(B. Mohanty).

of binary collisions N

coll

, shape of the overlap region, say the ellip-

ticity

, are all correlated with the impact parameter b. However,

none of the above IS collision attributes is directly observed in ex-

periments.

This makes the job to constrain the IS very challenging.

The standard method uses the ﬁnal state (FS) charged particle mul-

tiplicity

to characterise the events into different centrality classes

corresponding to different ISs. However, the geometric and quan-

tum

E/E ﬂuctuations in the IS result in appreciable variation of b,

part

, N

coll

, ε

etc., even within the same centrality bin. Thus, a

lack of proper knowledge of the IS is a major hindrance towards

carrying out precise comparisons between theory and experiments.

In this work, we focus on the spectators (those nucleons which do

not participate in the collision) and show that it is possible to ex-

tract

vital information of the IS by analysing them. The signiﬁcant

role played by spectator asymmetry in the various experimental

observables and the possibility of selecting special initial conﬁg-

urations

in HIC using deformed U nuclei has been pointed out

recently [7,8]. In a study based on Monte Carlo Glauber model sim-

ulations

it was suggested that spectator asymmetry could be used

to trigger speciﬁc collision conﬁgurations called Body-Tip with suf-

ﬁcient

magnetic ﬁeld and much lower ellipticity which can lead to

the disentanglement of chiral magnetic effect from its dominant

background anisotropic ﬂow in U + U collisions [7]. In Ref. [8] it

was demonstrated based on A Multi Phase Trasport (AMPT) model

simulations that spectator asymmetry could be utilised to identify

http://dx.doi.org/10.1016/j.physletb.2016.01.025

0370-2693/

SCOAP

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