# Measurement of the $t\bar{t}Z$ and $t\bar{t}W$ cross sections in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

## TL;DR

This paper reports precise measurements of the production cross sections of top-quark pairs with W and Z bosons at 13 TeV, using the ATLAS detector, confirming Standard Model predictions and constraining new physics effects.

## Contribution

First simultaneous measurement of $t\bar{t}Z$ and $t\bar{t}W$ cross sections at 13 TeV with detailed event categorization and constraints on effective field theory operators.

## Key findings

- Measured $t\bar{t}Z$ cross section: 0.95 pb with uncertainties.
- Measured $t\bar{t}W$ cross section: 0.87 pb with uncertainties.
- Results agree with Standard Model predictions.

## Abstract

A measurement of the associated production of a top-quark pair ($t\bar{t}$) with a vector boson ($W$, $Z$) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using $36.1$ fb$^{-1}$ of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. Events are selected in channels with two same- or opposite-sign leptons (electrons or muons), three leptons or four leptons, and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The $t\bar{t}Z$ and $t\bar{t}W$ production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are $\sigma_{t\bar{t}Z} = 0.95 \pm 0.08_{\mathrm{stat.}} \pm 0.10_{\mathrm{syst.}}$ pb and $\sigma_{t\bar{t}W} = 0.87 \pm 0.13_{\mathrm{stat.}} \pm 0.14_{\mathrm{syst.}}$ pb in agreement with the Standard Model predictions. The measurement of the $t\bar{t}Z$ cross section is used to set constraints on effective field theory operators which modify the $t\bar{t}Z$ vertex.