近日，荷兰格罗宁根大学的Luca Maxia及其研究团队取得一项新进展。经过不懈努力，他们揭示了UPCs中轻子和重夸克成对产生的方位不对称性。相关研究成果已于2025年1月15日在国际知名学术期刊《高能物理杂志》上发表。

本文研究了高电荷离子超边缘碰撞（UPC）中轻子和重夸克成对产生过程中的方位角调制现象。这种调制体现在末态粒子对的横动量之和与差以及横向碰撞参数的方位角上，其起因于非极化和极化光子的碰撞。本文给出了基于光子广义横动量依赖部分子分布（GTMDs）的截面完整描述，并仔细考虑了向碰撞参数空间的傅里叶变换。

特别是，这导致了一种不同阶次谐波之间的馈入机制，该机制原则上能生成所有（偶数）阶次的谐波。在可比的情况下，研究人员关于方位角调制的分析结果与其他相关论文中的结果相一致。

相较于其他研究，该研究区分了源自广义横动量依赖部分子分布（GTMDs）各向异性的效应与非源自此的效应，并保留了与产生粒子质量成正比的项，因为这些项对于缪子、粲夸克和底夸克的产生至关重要。研究表明，归一化微分截面随产生粒子质量的变化显著，这一效应在相对论重离子对撞机（RHIC）和大型强子对撞机（LHC）的超边缘碰撞（UPC）中应是可观测的。

对于数值结果，研究人员采用了几个光子GTMD关联子的模型，并发现它们之间以及它们与STAR实验的UPC数据均相当吻合。他们还呈现了RHIC运动学条件下各种方位角调制的结果，其中比较了电子-正电子成对产生与更重粒子产生的情况，以及LHC运动学条件下以缪子-反缪子成对产生为重点的结果。这些结果展示了不对称性中有趣的与质量相关的特性，可能有助于研究底层光子GTMD描述中所产生的各向异性。

附：英文原文

Title: Azimuthal asymmetries in lepton and heavy-quark pair production in UPCs

Author: Boer, Danil, Maxia, Luca, Pisano, Cristian

Issue&Volume: 2025-01-15

Abstract: Azimuthal modulations in lepton and heavy-quark pair production in ultraperipheral collisions (UPCs) of highly charged ions are investigated. The modulations in the azimuthal angles of the sum and difference of the transverse momenta of the pair of particles in the final state, as well as of the transverse impact parameter, arise from the collisions of unpolarized and polarized photons. A full description of the cross section in terms of Generalized Transverse Momentum Dependent parton distributions (GTMDs) for photons is given including a careful consideration of the Fourier transform to impact parameter space. In particular, this leads to a feed-in mechanism among harmonics of different orders, which in principle generates harmonics of all (even) orders. Wherever comparable, our analytical results for the azimuthal modulations agree with those presented in other papers on this topic. Compared to these other works, we separate effects that arise from the anisotropies of the GTMDs from those that do not and retain terms proportional to the mass of the produced particles, as they are relevant for muon, charm and bottom quark production. We show that the normalized differential cross section changes considerably with the produced particle mass, which should be discernible in UPCs at RHIC and LHC. For the numerical results we adopt several models for the photon GTMD correlator, and find that all of them are in fairly good agreement with each other and with UPC data from STAR. We also present results for various azimuthal modulations for RHIC kinematics, where we compare e⁺e^- production with the production of heavier particles, and for LHC kinematics, focusing on μ⁺μ^-production. These results exhibit interesting mass-dependent features in the asymmetries that may help study the anisotropies arising from the underlying photon GTMD description.

DOI: 10.1007/JHEP01(2025)076

Source: https://link.springer.com/article/10.1007/JHEP01(2025)076