质子-质子与质子-铅碰撞中dijet方位角相关性研究

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"p-p和p-Pb碰撞中dijet方位角相关性的展宽和饱和效应" 这篇在《Physics Letters B》上发表的文章探讨了在质子-质子(p-p)和质子-铅(p-Pb)碰撞中，双喷流(dijet)的方位角相关性所表现出的展宽和饱和效应。dijet是指两个高能量的喷流，它们在高能粒子碰撞中产生，通常由一对硬散射夸克或胶子产生。这些现象在理解强相互作用，尤其是量子色动力学(QCD)的基本原理方面具有重要意义。文章指出，ATLAS实验合作组在5.02TeV的能量下对质子-质子和质子-铅碰撞进行了前向dijet相关性的测量，这些测量结果与胶子饱和度的理论预测相吻合。胶子饱和度是QCD中的一个关键概念，它描述了在高能量密度条件下，胶子密度增长到一定程度后，进一步增加的能量将不会显著增加胶子密度的现象。这在高能核物理中是一个重要的考虑因素，因为它有助于解释重离子碰撞中的集体效应。同时，文章还讨论了硬对数的Sudakov效应。Sudakov效应是指在高能过程中的软散射导致的大对数项的积累，这些项可能对计算结果产生显著影响。在dijet相关性的研究中，Sudakov对数的恢复意味着这些软散射效应需要被正确地重新组合和重排，以得到准确的预测。作者们通过分析发现，胶子饱和和Sudakov效应两者的结合是解释实验观测的关键。两者都对描述dijet方位角的相关性展宽起着不可或缺的作用。这一发现加强了对QCD在极端条件下的理解，并可能对未来更精确的理论计算和实验测量提供指导。这项研究揭示了在高能碰撞中，dijet相关性不仅受到胶子饱和度的影响，还受到Sudakov对数效应的显著影响。这些发现对于深化我们对强相互作用的理解，特别是在高密度环境下的QCD动力学，具有深远的意义。同时，由于文章是开放获取的，这使得全球的研究人员都能自由地访问和使用这些成果，推动相关领域的科学研究。

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Physics Letters B 795 (2019) 511–515

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Broadening and saturation effects in dijet azimuthal correlations in p-p

and p-Pb collisions at

√

=5.02 TeV

Andreas van Hameren, Piotr Kotko, Krzysztof Kutak

∗

, Sebastian Sapeta

Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland

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

Article history:

Received

26 April 2019

Received

in revised form 27 June 2019

Accepted

28 June 2019

Available

online 2 July 2019

Editor:

A. Ringwald

We demonstrate that the recent forward-forward dijet correlation data measured by the ATLAS

collaboration for proton-proton and proton-lead collisions are consistent with the broadening effects

due to both the gluon saturation and the resummation of large logarithms of a hard scale (the so-called

Sudakov logarithms). We ﬁnd that both effects are necessary to describe the experimental results.

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

1. Introduction

High energy collisions of protons and heavy nuclei at the Large

Hadron Collider (LHC) provide a unique tool to probe dense sys-

tems

of partons – quarks and gluons – bounded in a highly

Lorentz-contracted, large nucleus. From this point of view, espe-

cially

interesting are processes where well-deﬁned jets of hadrons

are produced in the forward direction with respect to the incom-

ing

proton. Kinematically, such jets have large rapidities. Thus, they

trigger events in which the partons extracted from the nucleus

carry small longitudinal momentum fraction x. Due to the well-

known

rise of gluon distributions at small x, this kinematic setup

is perfectly suited to investigate the phenomenon of gluon satura-

tion,

which should occur at some value of x to prevent violation

of the unitarity bound (for a review of this subject see Ref. [1]).

In recent years there has been growing evidence in favour of the

occurrence of saturation in nature [2].

Recently , the ATLAS collaboration studied azimuthal correla-

tions

of dijets in proton-lead (p-Pb) and proton-proton (p-p) col-

lisions

at the center-of-mass energy

√

=5.02 TeV, covering, in

particular, the forward rapidity region between 2.7 −4.0units[3].

The measurement indicates sizable nuclear effects at small val-

ues

of x. In this region, the description of the data using con-

ventional

methods – collinear factorization and nuclear parton

distribution functions (PDFs) – is challenging: the nuclear PDFs

are burdened with large uncertainty at small x and they do not

Corresponding author.

E-mail

addresses: hameren@ifj.edu.pl (A. van Hameren), piotr.kotko@ifj.edu.pl

(P. Kotko),

krzysztof.kutak@ifj.edu.pl (K. Kutak), sebastian.sapeta@ifj.edu.pl

(S. Sapeta).

account properly for large density effects [4]. The behaviour of

dense systems of partons when x becomes small is predicted by

Quantum Chromodynamics (QCD) and leads to non-linear ev olu-

tion

equations known as Balitsky-Jalilian-Marian-Iancu-McLerran-

Weigert-Leonidov-Kovner

(B-JIMWLK) equations [5,6], which, in

particular, can be derived within the Color Glass Condensate (CGC)

theory [7]. In CGC, the calculation of forward jet production in

proton-nucleus collisions relies on the so-called hybrid approach

[8], where the large-x projectile is described simply by the con-

ventional

collinear PDFs, while the nucleus must be treated with

nonlinear equations. However, description of multi-jet production

is rather complicated even in this simpliﬁed framework [9,10]. A

novel approach to such processes was initiated in Ref. [11]for di-

jets

in the back-to-back correlation regime and in Ref. [12]for a

more general situation. The latter is known in the literature as

the small-x improved, transverse momentum dependent (ITMD)

factorization. It provides a momentum-space factorization formula

which has the form of a generalized factorization, i.e. it involves

several transverse momentum dependent (TMD) gluon distribu-

tions

characterizing partons in the dense target. The ITMD formula

accounts for: (i) complete kinematics of the scattering process with

off-shell gluons, (ii) gauge invariant deﬁnitions of the TMD gluon

densities, (iii) gauge invariant expressions for the off-shell hard

matrix elements, (iv) it also recovers the high energy factorization

(aka k

-factorization) [13–15]in the limit of large off-shellness of

the initial-state gluon from the nucleus. Recently, it was proved

that the ITMD formalism corresponds to resummation of all kine-

matic

twists in the CGC theory [16]. Steps in further extension of

the formalism to three and more jets were undertaken in Ref. [17]

and

in [18]in the correlation limit. The ITMD formalism has been

applied so far to double-inclusive jet production [19]in p-Pb and

p-p collisions, jet production in ultra-peripheral heavy ion colli-

https://doi.org/10.1016/j.physletb.2019.06.055

0370-2693/

SCOAP

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