Top-quark pair production in association with a $Z$ boson in the 4$\ell$ channel with the ATLAS experiment

TL;DR

This paper reports a measurement of the top-quark pair production in association with a Z boson in the four-lepton channel using ATLAS data, confirming consistency with Standard Model predictions.

Contribution

First measurement of the $t\bar{t}Z$ cross section in the 4-lepton channel with ATLAS at 13 TeV, providing a precise test of the Standard Model.

Findings

Measured $t\bar{t}Z$ cross section: 1.07 ± 0.26 pb

Results are consistent with Standard Model predictions

Performed simultaneous fit for $t\bar{t}Z$ and $t\bar{t}W$ processes

Abstract

The cross section of the $t\bar{t}Z$~and $t\bar{t}W$~processes are measured in a simultaneous fit using 36.1 $\text{fb}^{-1}$ of of proton--proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded by the ATLAS experiment at the LHC. In addition, a fit is performed in the 4$\ell$ channel only, resulting in a cross section of $\sigma_{t\bar{t}Z} = 1.07 \pm 0.26 \,\text{pb}$. This result is consistent with the combined fit and agrees with the prediction by the Standard Model.

Figures

Tables

Table 1. Table 1: Definitions of the four signal regions in the 4 ℓ ℓ \ell channel targeting the t ​ t ¯ ​ Z 𝑡 ¯ 𝑡 𝑍 t\overline{t}Z  process  [ 5 ] .

Region	$Z_2$ leptons	$p_{\text{T}4}$	$p_{\text{T}34}$		$\left|m_{Z_2}-m_Z\right|$	$E_{\text{T}}^{\text{miss}}$		$N_{b\text{-tagged jets}}$
4$\ell$-DF-1b	$e^{\pm }{\mu }^{\mp }$	-	$>$ 35 GeV		-	-		1
4$\ell$-DF-2b	$e^{\pm }{\mu }^{\mp }$	$>$ 10 GeV	-		-	-		$\ge 2$
4$\ell$-SF-1b	$e^{\pm }{e}^{\mp },{\mu }^{\pm }{\mu }^{\mp }$	-	$>\text{25 }\text{GeV}$	\ldelim{22ex	$>\text{10 }\text{GeV}$	$>\text{40 }\text{GeV}$ $>\text{80 }\text{GeV}$	\rdelim}22ex	1
4$\ell$-SF-2b	$e^{\pm }{e}^{\mp },{\mu }^{\pm }{\mu }^{\mp }$	$>\text{10 }\text{GeV}$	-	\ldelim{22ex	$>\text{10 }\text{GeV}$	- $>\text{40 }\text{GeV}$	\rdelim}22ex	$\ge 2$

Equations

$${\mu }_{t\overline{t}Z}=\frac{{\sigma }_{t\overline{t}Z}}{{\sigma }_{t\overline{t}Z}^{\text{SM}}}.$$

$${\mu }_{t\overline{t}Z}=1.21\pm 0.29$$

Full text

ATL-PHYS-PROC-2019-015

Top-quark pair production in association with a $Z$ boson in the 4$\ell$ channel with the ATLAS experiment

Sebastian Heer111Supported by European Research Council grant ERC-CoG-617185., on behalf of the ATLAS Collaboration11footnotetext: Copyright 2019 CERN for the benefit of the ATLAS Collaboration. CC-BY-4.0 license.

*Physikalisches Institut

Universität Bonn, 53115 Bonn, Germany*

The cross section of the $t\overline{t}Z$ and $t\overline{t}W$ processes are measured in a simultaneous fit using 36.1 $\text{fb}^{-1}$ of of proton–proton collisions at a centre-of-mass energy of $\sqrt{s}=13\ \text{Te\kern-1.00006ptV}$ recorded by the ATLAS experiment at the LHC. In addition, a fit is performed in the 4$\ell$ channel only, resulting in a cross section of $\sigma_{t\overline{t}Z}=1.07\pm 0.26\,\text{pb}$. This result is consistent with the combined fit and agrees with the prediction by the Standard Model.

PRESENTED AT

$11^{\mathrm{th}}$ International Workshop on Top Quark Physics

Bad Neuenahr, Germany, September 16–21, 2018

1 Introduction

The $t\overline{t}Z$ processes provides direct access to the weak coupling of the top quark to the $Z$ boson. It is an important background in searches involving final states with multiple leptons and $b$-quarks. Previous measurements by the ATLAS and CMS experiments [1, 2] indicate agreement with the Standard Model [3, 4].

Typically the $t\overline{t}Z$ process is measured in a two-dimensional fit together with the $t\overline{t}W$ process. One of the analysis channels targeting the $t\overline{t}Z$ process has four isolated prompt leptons. This final state provides the best signal-to-background ratio among all analysis channels. Due to the branching ratios of the top quark and the $Z$ boson, the number of expected events is small in this analysis channel.

The dataset of 36.1 $\text{fb}^{-1}$ collected in 2015 and 2016 by the ATLAS detector is considered for the presented analysis. More details about the analysis can be found in Reference [5].

2 Tetralepton analysis

Separate analysis channels are defined to target the $t\overline{t}Z$ and $t\overline{t}W$ processes. This document focuses on the $4\ell$ channel, which is sensitive to the $t\overline{t}Z$ process. Both $W$ bosons, resulting from the top-quark decays, and the $Z$ boson are required to decay leptonically. The most dominant background processes arise from the diboson production, the production of a single top quark in association with a $W$ and a $Z$ boson as well as fake leptons. A fit is performed in the 4$\ell$ channel to extract the signal strength as well as the background normalization of the $ZZ$ process.

Events with two pairs of opposite-sign leptons are selected, and at least one pair must have the same flavour. Among the lepton pairs with opposite charge and same flavour (OSSF), the pair with reconstructed invariant mass closest to $m_{Z}$ is attributed to the $Z$ boson decay and denoted in the following by $Z_{1}$. The two remaining leptons are used to define $Z_{2}$. Four signal regions are defined according to the relative flavour of the two $Z_{2}$ leptons, different flavour (DF) or same flavour (SF), and the number of $b$-tagged jets: one, or at least two ($1b$, $2b$). The four signal regions are denoted as 4$\ell$-SF-1b, 4$\ell$-SF-2b, 4$\ell$-DF-1b and 4$\ell$-DF-2b.

In the same-flavour regions, requirements on $E_{\text{T}}^{\text{miss}}$ are applied in order to suppress the $ZZ$ background. This requirement depends on whether the invariant mass of the $Z_{2}$ is close to the mass of the $Z$ boson. To suppress events with fake leptons in the 1 $b$-tag multiplicity regions, additional requirements on the scalar sum of the transverse momenta of the third and fourth leptons ($p_{\text{T}34}$) are imposed. In other regions a requirement on the transverse momentum of all leptons needs to be satisfied, instead. The definitions of all signal regions are given in Table 1.

A control region used to determine the $ZZ$ normalization, referred to as $4\ell$-ZZ-CR is defined to have exactly four reconstructed leptons, a $Z_{2}$ pair with OSSF leptons, the value of both $m_{Z_{1}}$ and $m_{Z_{2}}$ within $10\,\text{Ge\kern-1.00006ptV}$ of the mass of the $Z$ boson, and $20\,\text{Ge\kern-1.00006ptV}<E_{\text{T}}^{\text{miss}}<40\,\text{Ge\kern-1.00006ptV}$. The normalization of the $ZZ$ background is a free parameter in the fit.

Figure 1 shows the data compared to the expected distributions for all four signal regions combined, as well as for the control region, showing good agreement between data and expectation.

The contribution from backgrounds containing fake leptons is estimated from simulation and corrected with scale factors determined in two control regions: one region enriched in $t\overline{t}$ events and one region enriched in $Z$+jets events. The scale factors are calibrated separately for electron and muon fake-lepton candidates, and separately for leptons arising from heavy flavour hadrons and other sources. Therefore, four scale factors are determined in total. The scale factors are applied to all simulated events with fewer than four prompt leptons and depend on the number, flavour and origin of the fake leptons. It is verified that the scale factors for different generators used in the simulation are consistent with each other.

3 Results

A binned maximum-likelihood fit is performed, which includes the signal regions and the control region of the 4$\ell$ channel to extract the signal strength

[TABLE]

In all regions, good agreement between observed values and the expectation is observed. Figure 2 shows all regions included in the fit after the fit has been performed.

The normalization correction for the $ZZ$ background with respect to the prediction is obtained from the fit and found to be compatible with unity: $\mu_{ZZ}=0.94\pm 0.18$. The obtained signal strength of the $t\overline{t}Z$ process is

[TABLE]

and both the observed and expected significances are found to be larger than 5 standard deviations. Using the theoretical prediction of $\sigma_{t\overline{t}Z}=0.88^{+0.09}_{-0.11}$ pb [6], the extracted cross section from the fit in the 4$\ell$ channel is $\sigma_{t\overline{t}Z}^{4\ell}=1.07\pm 0.26,$ pb demonstrating good agreement between the measured and predicted cross sections. The dominant systematic uncertainties are related to the modeling of the signal and flavour tagging.

ACKNOWLEDGEMENTS

The work of the author is currently funded by the European Research Council under the European Union’s Seventh Framework Programme ERC Grant Agreement n. 617185.