"parton_abs_ZPt" of "Inclusive and differential cross section measurements of single top quark production in association with a Z boson in proton-proton collisions at $\sqrt{s}$ = 13 TeV"

Inclusive and differential cross sections of single top quark production in association with a Z boson are measured in proton-proton collisions at a center-of-mass energy of 13 TeV with an integrated luminosity of $138\ \mathrm{fb}^{-1}$ recorded by the CMS experiment. Events are selected based on the presence of three leptons, electrons or muons, associated with leptonic Z boson and top quark decays. The measurement yields an inclusive cross section of $87.9\text{ }_{-7.3}^{+7.5}\text{ (stat) }_{-6.0}^{+7.3}\text{ (syst) fb}$ for a dilepton invariant mass greater than 30 GeV, in agreement with standard model (SM) calculations. The ratio between the cross sections for the top quark and the top antiquark production in association with a Z boson is measured as $2.37\text{ }_{-0.42}^{+0.56}\text{ (stat) }_{-0.13}^{+0.27}\text{ (syst)}$. Differential measurements at parton and particle levels are performed for the first time. Several kinematic observables are considered to study the modeling of the process. Results are compared to theoretical predictions with different assumptions on the source of the initial-state b quark and found to be in agreement, within the uncertainties. Additionally, the spin asymmetry, which is sensitive to the top quark polarization, is determined from the differential distribution of the polarization angle at parton level to be $0.58\text{ }^{+0.15}_{-0.16} \text{ (stat) } {\pm 0.06}\text{ (syst)}$, in agreement with SM predictions using the MGvATNLO event generator at next-to-leading order.

本研究利用CMS实验采集的质心系能量为13 TeV、积分亮度为138 fb⁻¹的质子-质子对撞数据，测量了伴随Z玻色子（Z boson）的单顶夸克（single top quark）产生过程的总截面与微分截面。实验依据三个轻子（电子或缪子）的存在筛选事例，这些轻子对应Z玻色子与顶夸克的轻子衰变模式。针对双轻子不变质量大于30 GeV的事例样本，本测量得到总截面为$87.9_{-7.3}^{+7.5}$(统计误差)$_{-6.0}^{+7.3}$(系统误差) fb，该结果与标准模型（Standard Model，SM）的理论计算结果相符。同时测得伴随Z玻色子的顶夸克与反顶夸克产生截面之比为$2.37_{-0.42}^{+0.56}$(统计误差)$_{-0.13}^{+0.27}$(系统误差)。本研究首次开展了部分子层次与粒子层次的微分截面测量，选取多项运动学可观测量以研究该产生过程的理论建模细节。将实验结果与针对初始态b夸克来源采用不同假设的理论预言进行对比，结果在误差范围内与理论预言一致。此外，通过部分子层次极化角的微分分布，确定了对顶夸克极化敏感的自旋不对称度为$0.58_{-0.16}^{+0.15}$(统计误差)±0.06(系统误差)，该结果与采用MGvATNLO事件产生器在次领头阶（next-to-leading order，NLO）下的标准模型预言相符。