ATL-PHYS-PROC-2018-

Top-quark pair production in association with a $W$ or $Z$ boson with the
ATLAS experiment

Sebastian Heer111Supported by European Research Council grant ERC-CoG-617185.,
on behalf of the ATLAS Collaboration11footnotetext: Copyright 2018 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 proton-proton
> collisions at a centre of mass energy of $\sqrt{s}=13\
> \text{Te\kern-1.00006ptV}$ recorded by the ATLAS detector at the LHC. The
> result is found to be $\sigma_{t\overline{t}Z}=0.95\pm 0.08\text{
> (stat.)}\pm 0.10\text{ (syst.)}\,\text{pb}$ and
> $\sigma_{t\overline{t}W}=0.87\pm 0.13\text{ (stat.)}\pm 0.14\text{
> (syst.)}\,\text{pb}$ and compatible with the Standard Model.

> PRESENTED AT
>
>
>
>
> $11^{\mathrm{th}}$ International Workshop on Top Quark Physics
> Bad Neuenahr, Germany, September 16–21, 2018

##  1 Introduction

The $t\overline{t}Z$ and $t\overline{t}W$ processes provide direct access to
the weak couplings of the top quark. In addition, the two processes are often
found to be a relevant background in searches involving final states with
multiple leptons and $b$-quarks. Measurements of $\sigma_{t\overline{t}Z}$ and
$\sigma_{t\overline{t}W}$ can be used to set constraints on the weak couplings
of the quarks, as well as impose limits on possible beyond the Standard Model
(BSM) effects. Previous measurements by the ATLAS and CMS experiments [1, 2]
indicate agreement with the Standard Model [3, 4].

The dataset of 36.1 $\text{fb}^{-1}$ of proton-proton collisions at a centre
of mass energy of $\sqrt{s}=13\ \text{Te\kern-1.00006ptV}$, collected in 2015
and 2016 by the ATLAS detector is analysed. Final states with two, three or
four isolated prompt leptons are considered in the analysis. More details
about the analysis can be found in Reference [5].

##  2 Analysis Overview

Separate analysis channels are defined to target the $t\overline{t}Z$ and
$t\overline{t}W$ processes. Each channel is divided into multiple regions in
order to enhance the sensitivity to the signal. Table 1 lists the analysis
channels and the targeted decay modes of the $t\overline{t}Z$ and
$t\overline{t}W$ processes.

Table 1:  List of $t\overline{t}W$ and $t\overline{t}Z$ decay modes and analysis channels targeting them. The symbols $b$ and $\nu$ denote a $b$-quark or antiquark and neutrino or antineutrino, respectively [5]. Process |  $t\overline{t}$ decay | Boson decay | Channel
---|---|---|---
$t\overline{t}W$ | $(\ell^{\pm}\nu b)(q\overline{q}b)$ | $\ell^{\pm}\nu$ | 2$\ell$ SS
$(\ell^{\pm}\nu b)(\ell^{\mp}\nu b)$ | $\ell^{\pm}\nu$ | Trilepton
$t\overline{t}Z$ | $(q\overline{q}b)(q\overline{q}b)$ | $\ell^{+}\ell^{-}$ |  $2\ell$ OS
$(\ell^{\pm}\nu b)(q\overline{q}b)$ | $\ell^{+}\ell^{-}$ | Trilepton
| $(\ell^{\pm}\nu b)(\ell^{\mp}\nu b)$ | $\ell^{+}\ell^{-}$ | Tetralepton

The 2$\ell$-OS channel targets the $t\overline{t}Z$ process and is affected by
large backgrounds from $Z$+jets or $t\overline{t}$ production. In order to
separate the signal from background in the most efficient way, Boosted
Decision Trees (BDTs) are used. Three BDTs are constructed and trained in
regions requiring at least 5 jets. The number of input variables varies
between 14 and 17. The most important input variables are found to be the
pseudorapidity of the dilepton system and the transverse momenta of all jets
divided by the sum of their energies. Each signal region is divided into 19
equal sized bins of the BDT discriminator. The normalization factors of the
$Z$+jets background with one or two heavy flavour jets, labeled as Z+1HF and
Z+2HF, are determined in the fit to data.

The 2$\ell$-SS channel targets the $t\overline{t}W$ process and uses a total
of 12 signal and control regions, classified by information about the lepton
flavour, total charge and $b$-jet multiplicity. The dominant background arises
from fake leptons, which are estimated by the matrix method [6] and charge-
flip leptons. The control regions are used in a fit to measure fake and real
lepton efficiencies, as well as in the final fit to determine the signal cross
sections, anticorrelating the fake lepton background with the signal process.

The regions in the 3$\ell$ channel are divided into two groups, depending on
whether a pair of OSSF leptons whose invariant mass is within
$10\,\text{Ge\kern-1.00006ptV}$ of the $Z$ boson mass is present. The signal
regions are further categorized based on jet and $b$-jet multiplicities. Four
signal regions for each signal process are defined. The dominant backgrounds
for the regions targeting $t\overline{t}Z$ arise from diboson production and
the production of single top quarks in association with a $Z$ boson. A control
region is used to determine the normalization of the $WZ$ background in the
fit to data. The dominant background for the regions targeting
$t\overline{t}W$ arises from fake leptons, which is estimated by the matrix
method.

The 4$\ell$ channel targets the $t\overline{t}Z$ process and uses four signal
regions depending on the $b$-jet multiplicity and the flavour of the leptons
from the $t\overline{t}$ system. In addition, a control region is used to
determine the normalization of the $ZZ$ background in the fit to data. The
dominant backgrounds 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. The fake lepton background is estimated from simulation, applying
scale factors obtained from a fit in control regions enriched by fake leptons.

##  3 Results

The $t\overline{t}Z$ and $t\overline{t}W$ cross sections are simultaneously
extracted using a binned maximum-likelihood fit to the numbers of events in
the dilepton, trilepton and tetralepton signal and control regions. In total,
the fit includes 11 signal plus 2 control regions from the $t\overline{t}Z$,
and 16 signal plus 12 control regions from the $t\overline{t}W$ analysis. In
all regions, good agreement between observed values and the expectation is
observed. The normalization corrections for the $WZ$ and $ZZ$ backgrounds with
respect to the predictions are obtained from the fit and found to be
compatible with unity: $0.91\pm 0.10$ for the $WZ$ background and $1.11\pm
0.17$ for the $ZZ$ background. The normalizations of the $Z$+1HF and $Z$+2HF
backgrounds are mainly constrained in the low BDT output bins of the opposite-
sign dilepton channel signal regions, where the signal contamination is low.
Their values are found to be $1.19\pm 0.25$ and $1.09\pm 0.13$, respectively.
The measured values of the cross sections from the combined fit are
$\sigma_{t\overline{t}Z}=0.95\pm 0.08\text{ (stat.)}\pm 0.10\text{
(syst.)}\,\text{pb}=0.95\pm 0.13\,\text{pb}$ and
$\sigma_{t\overline{t}W}=0.87\pm 0.13\text{ (stat.)}\pm 0.14\text{
(syst.)}\,\text{pb}=0.87\pm 0.19\,\text{pb}$, assuming the SM value for the
other cross section, respectively. Figure 1 shows a comparison of the fit
results with theoretical expectations,
$\sigma_{t\overline{t}Z}=0.88^{+0.09}_{-0.11}$ pb and
$\sigma_{t\overline{t}W}=0.60^{+0.08}_{-0.07}$ pb [7], demonstrating good
agreement between the measured and predicted cross sections. For the
$t\overline{t}Z$ process, both the observed and the expected significances are
found to be much larger than 5 standard deviations. For the $t\overline{t}W$
process, an excess of events over the expected Standard Model background-only
hypothesis is found with an observed (expected) significance of $4.3$ ($3.4$)
standard deviations. Table 2 shows the uncertainties in the measured
$t\overline{t}Z$ and $t\overline{t}W$ cross sections, grouped in categories,
along with the total uncertainties. For both processes, the precision of the
measurement is affected by statistical and systematic uncertainties in similar
proportions. For the $t\overline{t}Z$ determination, the dominant systematic
uncertainty sources are the modelling of the backgrounds and of the signal.
For the $t\overline{t}W$ determination, the dominant systematic uncertainty
sources are the modelling of the signal and the limited amount of data
available in the control regions and simulated samples.

Figure 1:  The result of the simultaneous fit to the $t\overline{t}Z$ and $t\overline{t}W$ cross sections along with the 68% and 95% confidence level (CL) contours [5]. The cross shows the theoretical uncertainties in the SM predictions, and include renormalization and factorization scale uncertainties, as well as PDF uncertainties including $\alpha_{s}$ variations [7]. Table 2: List of relative uncertainties in the measured cross sections of the $t\overline{t}Z$ and $t\overline{t}W$ processes from the fit, grouped in categories. All uncertainties are symmetrized. The quadratic sum may not be equal to the total due to correlations between uncertainties introduced by the fit [5].  Uncertainty | $\sigma_{t\overline{t}Z}$ | $\sigma_{t\overline{t}W}$
---|---|---
Luminosity | 2.9$\%$ | 4.5$\%$
Simulated sample statistics | 2.0$\%$ | 5.3$\%$
Data-driven background statistics | 2.5$\%$ | 6.3$\%$
JES/JER | 1.9$\%$ | 4.1$\%$
Flavor tagging | 4.2$\%$ | 3.7$\%$
Other object-related | 3.7$\%$ | 2.5$\%$
Data-driven background normalization | 3.2$\%$ | 3.9$\%$
Modeling of backgrounds from simulation | 5.3$\%$ | 2.6$\%$
Background cross sections | 2.3$\%$ | 4.9$\%$
Fake leptons and charge misID | 1.8$\%$ | 5.7$\%$
$t\overline{t}Z$modelling | 4.9$\%$ | 0.7$\%$
$t\overline{t}W$modelling | 0.3$\%$ | 8.5$\%$
Total systematic | 10$\%$ | 16$\%$
Statistical | 8.4$\%$ | 15$\%$
Total | 13$\%$ | 22$\%$

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.

## References

  * [1] ATLAS Collaboration, JINST 3 (2008) S08003.
  * [2] CMS Collaboration, JINST 3 (2008) S08004.
  * [3] ATLAS Collaboration, Eur. Phys. J. C 77 (2017), 40, arXiv:1609.01599 [hep-ex].
  * [4] CMS Collaboration, JHEP 08 (2018) 011, arXiv:1711.02547 [hep-ex].
  * [5] ATLAS Collaboration, ATLAS-CONF-2018-047,
http://cds.cern.ch/record/2639674.

  * [6] ATLAS Collaboration, Eur. Phys. J. C 71 (2011) 1577,
arXiv:1012.1792 [hep-ex].

  * [7] J. Alwall et al., JHEP 07 (2014) 079, arXiv:1405.0301 [hep-ex].