LHC对撞机双射光产生限制核parton分布的精度提升

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本文主要探讨了大型强子对撞机（Large Hadron Collider, LHC）在铅-铅（Pb-Pb）超外围碰撞中产生的包容性双射光（inclusive dijet photoproduction）如何被用来约束核parton分布（nuclear parton distribution functions, nPDFs）。研究者利用量子色动力学（Quantum Chromodynamics, QCD）的计算结果，将这些理论预测视为伪数据，针对nCTEQ15、nCTEQ15np和EPPS16这三种不同的nPDF进行了细致分析。首先，通过将LHC实验条件下的铅-铅超外围碰撞中双射光的横截面纳入贝叶斯加权（Bayesian reweighting）技术的框架下，作者试图评估这些实际观测结果对nPDF的不确定性产生的影响。这种技术允许他们在统计上调整模型参数，以便更好地匹配实际数据，从而提高对核内夸克和 gluon分布的了解。研究结果显示，对于nCTEQ15和nCTEQ15np这两种nPDF，当假定误差设定在5%时，在较小的动量分数$$ x_A $$（通常代表核内的部分子参与过程的概率）区域，如$$ x_A = 10^{-3}$$，不确定性显著下降。这意味着通过结合实验数据，可以精细化这些nPDF在低$x_A$区域的描述，这是以前理论模型难以准确预测的部分。相比之下，EPPS16 nPDF的不确定性的减少相对较小，因为其公差值较高且参数化形式较为灵活。这表明EPPS16可能在处理这类数据时对模型的调整能力较弱，但仍能提供一定的限制和指导。这项工作的重要性在于它展示了实验数据如何增强我们对核物理中基本粒子性质的理解，特别是核parton分布，这对于理解高能重离子碰撞中的复杂现象至关重要。通过将大型强子对撞机的实验结果与先进的统计分析方法相结合，研究者能够显著缩小理论预测与实际观察之间的差距，推动了核物理领域的精确测量和理论进步。

Eur. Phys. J. C (2019) 79:396

https://doi.org/10.1140/epjc/s10052-019-6905-2

Regular Article - Theoretical Physics

Constraints on nuclear parton distributions from dijet

photoproduction at the LHC

V. Guzey

1,2,3,a

,M.Klasen

4,b

Department of Physics, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland

Helsinki Institute of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland

National Research Center “Kurchatov Institute”, Petersburg Nuclear Physics Institute (PNPI), Gatchina 188300, Russia

Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 9, 48149 Münster, Germany

Received: 19 February 2019 / Accepted: 27 April 2019 / Published online: 8 May 2019

Abstract Using QCD calculations of the cross section of

inclusive dijet photoproduction in Pb–Pb ultraperipheral col-

lisions in the LHC kinematics as pseudo-data, we study the

effect of including these data using the Bayesian reweighting

technique on nCTEQ15, nCTEQ15np, and EPPS16 nuclear

parton distribution functions (nPDFs). We ﬁnd that, depend-

ing on the assumed error of the pseudo-data, it leads to a

signiﬁcant reduction of the nPDF uncertainties at small val-

ues of the momentum fraction x

. Taking the error to be

5%, the uncertainty of nCTEQ15 and nCTEQ15np nPDFs

reduces approximately by a factor of two at x

= 10

−3

.At

the same time, the reweighting effect on EPPS16 nPDFs is

much smaller due to the higher value of the tolerance and a

more ﬂexible parametrization form.

1 Introduction

Collinear nuclear parton distribution functions (nPDFs)

are fundamental quantities of Quantum Chromodynamics

(QCD) encoding information on the one-dimensional distri-

butions of quarks and gluons in nuclei in terms of the light-

cone momentum fraction x

at a given resolution scale μ.

Nuclear PDFs are essential ingredients of QCD calculations

of cross sections at high energies involving charged lepton–

nucleus and neutrino–nucleus deep inelastic scattering (DIS)

with ﬁxed targets and – in the future – in the collider mode and

proton–nucleus and nucleus–nucleus scattering at the Rela-

tivistic Heavy Ion Collider (RHIC) and the Large Hadron

Collider (LHC). While nPDFs are non-perturbative quanti-

ties, which cannot be calculated from ﬁrst principles of QCD,

the QCD collinear factorization for hard processes and the

e-mail: guzey_va@nrcki.pnpi.ru

e-mail: michael.klasen@uni-muenster.de

Dokshitzer–Gribov–Lipatov–Altarelli–Parisi (DGLAP) evo-

lution equations allow one to set up a framework of global

QCD ﬁts, which enables one to extract nPDFs from avail-

able data [1–7]. Different analyses give noticeably differ-

ent predictions for nPDFs and carry signiﬁcant uncertainties

originating mostly from the limiting kinematic coverage of

the available data, indirect determination of the gluon nPDF

from the DIS data using the scaling violations, and different

assumptions about the shape of nPDFs at the input scale. As

a result, quark nPDFs for small x and the gluon nPDFs for

essentially all x are rather poorly known.

Further progress in constraining nPDFs relies on studies of

high-energy hard processes with nuclei at collider energies,

notably, in proton–nucleus ( pA) scattering at the LHC [8–10]

and lepton–nucleus (eA) scattering at the future EIC [11,12]

and LHeC [13]. However, the QCD analyses of the data on

various hard processes in pA scattering at the LHC during

Run1[14–17] showed that the data provide only modest

restrictions on nPDFs at small x. At the same time, it was

proposed [18] that measurements of low-mass lepton pair

production in proton–lead collisions at the LHC has a large

potential to reduce the theoretical uncertainties on nPDFs in

a wide range of x or even rule out some parameterizations.

While the potential of hard pA scattering at the LHC will

certainty continue to be explored, see, e.g. [19], it is topical

to study complementary probes of nPDFs at the LHC.

It has been realized that collisions of ultrarelativistic ions

at large impact parameters, when the strong interaction is

suppressed and the ions interact electromagnetically via the

emission of quasi-real photons in so-called ultraperipheral

collisions (UPCs), give an opportunity to study photon–

photon, photon–proton, and photon–nucleus scattering at

unprecedentedly high energies [20]. This program was real-

ized during Run 1 at the LHC by measuring exclusive pho-

toproduction of charmonia [J/ψ and ψ(2S) vector mesons]

123

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