"Normalized PbPb Jet girth for $x_J$>0.4" of "Girth and groomed radius of jets recoiling against isolated photons in lead-lead and proton-proton collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV"

This Letter presents the first measurements of the groomed jet radius $R_{\mathrm{g}}$ and the jet girth $g$ in events with an isolated photon recoiling against a jet in lead-lead (PbPb) and proton-proton (pp) collisions at the LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The observables $R_{\mathrm{g}}$ and $g$ provide a quantitative measure of how narrow or broad a jet is. The analysis uses PbPb and pp data samples with integrated luminosities of 1.7~nb$^{-1}$ and 301~pb$^{-1}$, respectively, collected with the CMS experiment in 2018 and 2017. Events are required to have a photon with transverse momentum $p_{\mathrm{T}}^{\gamma} > 100$~GeV and at least one jet back-to-back in azimuth with respect to the photon and with transverse momentum $p_{\mathrm{T}}^{\text{jet}}$ such that $p_{\mathrm{T}}^{\text{jet}}/p_{\mathrm{T}}^{\gamma} > 0.4$. The measured $R_{\mathrm{g}}$ and $g$ distributions are unfolded to the particle level, which facilitates the comparison between the PbPb and pp results and with theoretical predictions. It is found that jets with $p_{\mathrm{T}}^{\text{jet}}/p_{\mathrm{T}}^{\gamma} > 0.8$, i.e, those that closely balance the photon $p_{\mathrm{T}}^{\gamma}$, are narrower in PbPb than in pp collisions. Relaxing the selection to include jets with $p_{\mathrm{T}}^{\text{jet}}/p_{\mathrm{T}}^{\gamma} > 0.4$ reduces the narrowing of the angular structure of jets in PbPb relative to the pp reference. This shows that selection bias effects associated with jet energy loss play an important role in the interpretation of jet substructure measurements.

本快报首次报道了大型强子对撞机(LHC)在5.02 TeV核子-核子质心系能量下，铅-铅(PbPb)与质子-质子(pp)碰撞中，伴随与喷注反冲的孤立光子的事件内，修匀喷注半径$R_{mathrm{g}}$与喷注展宽$g$的测量结果。可观测量$R_{mathrm{g}}$与$g$可定量表征喷注的宽窄程度。本分析使用紧凑型缪子线圈(CMS)实验采集的PbPb与pp数据样本，二者的积分亮度分别为$1.7 mathrm{nb^{-1}}$与$301 mathrm{pb^{-1}}$，采集年份分别为2018年与2017年。分析要求事件需满足：存在横向动量$p_{mathrm{T}}^{gamma} > 100 mathrm{GeV}$的光子，且至少存在一个与该光子方位角背对背的喷注，其横向动量$p_{mathrm{T}}^{ ext{jet}}$满足$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.4$。将测得的$R_{mathrm{g}}$与$g$分布开展粒子层级展开，以方便对比PbPb与pp碰撞的实验结果，并与理论预言进行比对。研究发现，当满足$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.8$（即喷注横向动量与光子横向动量近乎平衡）时，PbPb碰撞中的喷注要比pp碰撞中的更窄。若放宽选择条件至$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.4$，则PbPb碰撞中喷注角结构相较于pp参考样本的收窄效应会被削弱。这表明，与喷注能量损失相关的选择偏倚效应，在喷注子结构测量的诠释中扮演着重要角色。

搜集汇总

背景与挑战

背景概述

该数据集来自CMS合作组，提供了在5.02 TeV铅-铅对撞中，喷注与孤立光子反冲且x_J > 0.4条件下的归一化喷注宽度（girth）测量结果，对应论文图4（左）。数据包括喷注宽度分布、统计和系统误差，用于研究喷注子结构在核介质中的变化，特别是比较PbPb和质子-质子对撞中喷注窄化效应，以揭示喷注能量损失的影响。