# Measurement of top quark pair production in association with a Z boson in proton-proton collisions at $\sqrt{s} =$ 13 TeV

## TL;DR

This paper reports a precise measurement of the top quark pair production cross section with an associated Z boson at 13 TeV, including differential distributions and limits on anomalous couplings, using CMS data from 2016-2017.

## Contribution

First measurement of differential cross sections for t-tbar-Z production at 13 TeV, providing new constraints on top-Z couplings within the effective field theory framework.

## Key findings

- Measured cross section: 0.95 ± 0.05 (stat) ± 0.06 (syst) pb.
- First differential distributions of Z boson transverse momentum and lepton angular variables.
- Most stringent limits on top-Z anomalous couplings to date.

## Abstract

A measurement of the inclusive cross section of top quark pair production in association with a Z boson using proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC is performed. The data sample corresponds to an integrated luminosity of 77.5 fb$^{-1}$, collected by the CMS experiment during 2016 and 2017. The measurement is performed using final states containing three or four charged leptons (electrons or muons), and the Z boson is detected through its decay to an oppositely charged lepton pair. The production cross section is measured to be $\sigma(\mathrm{t\bar{t}Z})$ $=$ 0.95 $\pm$ 0.05 (stat) $\pm$ 0.06 (syst) pb. For the first time, differential cross sections are measured as functions of the transverse momentum of the Z boson and the angular distribution of the negatively charged lepton from the Z boson decay. The most stringent direct limits to date on the anomalous couplings of the top quark to the Z boson are presented, including constraints on the Wilson coefficients in the framework of the standard model effective field theory.