Measurement of the $t\bar{t}W$ and $t\bar{t}Z$ production cross sections in $pp$ collisions at $\sqrt{s} = 8 \mathrm{\ Te\kern -0.1em V}$ with the ATLAS detector

TL;DR

This paper reports the first measurement of the production cross sections of top-quark pairs with associated W and Z bosons at 8 TeV using ATLAS data, confirming Standard Model predictions.

Contribution

It provides the first simultaneous measurement of $t\bar{t}W$ and $t\bar{t}Z$ cross sections at 8 TeV with significance levels and consistency with theoretical calculations.

Findings

Measured $t\bar{t}W$ cross section: 369^{+100}_{-91} fb

Measured $t\bar{t}Z$ cross section: 176^{+58}_{-52} fb

Significance of $t\bar{t}W$ and $t\bar{t}Z$ signals above background

Abstract

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from $pp$ collisions at $\sqrt{s} = 8 \mathrm{\ Te\kern -0.1em V}$. The dataset corresponds to an integrated luminosity of $20.3\ \mathrm{fb}^{-1}$ collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the $t\bar{t}W$ and $t\bar{t}Z$ processes simultaneously yields a significance of $5.0\sigma$ ($4.2\sigma$) over the background-only hypothesis for $t\bar{t}W$ ($t\bar{t}Z$) production. The measured cross sections are $\sigma_{t\bar{t}W} = 369^{+100}_{-91}$ fb and $\sigma_{t\bar{t}Z} = 176^{+58}_{-52}$ fb. The background-only hypothesis with neither $t\bar{t}W$ nor $t\bar{t}Z$ production is excluded at $7.1\sigma$. All measurements are consistent with next-to-leading-order calculations for the $t\bar{t}W$ and $t\bar{t}Z$ processes.