Differential measurement of top quark cross sections in association with a Z boson in CMS

Abstract

The first simultaneous measurement of the Z boson-associated top quark production cross sections, in pair ($\mathrm{t\bar{t}Z}$), or with a single top quark ($\mathrm{tZq}$ and $\mathrm{tWZ}$) is presented. The data were recorded from proton-proton collisions at a center of mass energy of 13 TeV with the CMS Experiment at CERN's Large Hadron Collider, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events with multilepton final states are selected and the signal-background discrimination is optimized with a multiclass deep neural network. Due to the interference between the $\mathrm{t\bar{t}Z}$ and $\mathrm{tWZ}$ processes and their similar experimental signature, their sum is reported in the results. A profile likelihood fit is performed to extract the inclusive and differential cross sections of the two signal categories simultaneously. The inclusive cross sections for a dilepton invariant mass within $70$ and $110~\mathrm{GeV}$ are measured to be $1.14\pm0.07~\mathrm{pb}$ for the sum of $\mathrm{t\bar{t}Z}$ and $\mathrm{tWZ}$, and $0.81\pm 0.10~\mathrm{pb}$ for $\mathrm{tZq}$