"Normalized pp 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.

本快报首次测量了大型强子对撞机（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，孤立光子（isolated photon）反冲喷注事件内的修匀喷注半径$R_{mathrm{g}}$（groomed jet radius）与喷注展宽$g$（jet girth）。可观测量$R_{mathrm{g}}$与$g$可定量表征喷注的宽窄程度。本分析采用紧凑缪子线圈（Compact Muon Solenoid, CMS）实验于2018年与2017年采集的PbPb与pp数据样本，二者的积分亮度（integrated luminosity）分别为1.7 nb⁻¹与301 pb⁻¹。事件筛选条件要求包含横向动量（transverse momentum）$p_{mathrm{T}}^{gamma} > 100 mathrm{GeV}$的光子，且存在至少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$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.4$。将测量得到的$R_{mathrm{g}}$与$g$分布展开至粒子级（particle level），以便于开展PbPb与pp碰撞结果的对比，同时可与理论预言进行比对。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碰撞中的喷注比pp碰撞中的更窄。将筛选条件放宽至包含$p_{mathrm{T}}^{ ext{jet}}/p_{mathrm{T}}^{gamma} > 0.4$的喷注后，相较于pp参照样本，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（jet energy loss）相关的选择偏倚（selection bias）效应，在喷注子结构（jet substructure）测量的诠释中发挥着重要作用。