"Normalized PbPb Jet girth for $x_J$>0.8" 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.本物理快报报道了大型强子对撞机（Large Hadron Collider，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，包含孤立光子及其反冲喷注的事件内，修剪后喷注半径$R_{mathrm{g}}$与喷注展度$g$的首次测量结果。可观测量$R_{mathrm{g}}$与$g$可定量表征喷注的宽窄程度。本分析采用紧凑型μ子螺线管实验（CMS）于2018年与2017年采集的PbPb与pp数据样本，二者的积分亮度分别为1.7 nb$^{-1}$与301 pb$^{-1}$。实验要求事件需满足：存在横向动量$p_{mathrm{T}}^{gamma} > 100$ 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$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.4$。我们将测得的$R_{mathrm{g}}$与$g$分布解卷至粒子层次，以便于开展PbPb与pp碰撞结果的相互比对，以及与理论预言的对照。研究发现，当$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.8$（即喷注横向动量与光子横向动量近乎平衡）时，PbPb碰撞中的喷注较pp碰撞中的更窄。若将选择条件放宽至包含$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.4$的喷注，则相较于pp参考样本，PbPb中喷注角结构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，在喷注子结构测量结果的诠释中扮演着重要角色。