#  $\mathrm{t\bar{t}+X}$ production at ATLAS and CMS

Nicolas Tonon on behalf of the ATLAS and CMS Collaborations  Deutsches
Elektronen-Synchrotron (DESY)
Notkestraße 85, 22607 Hamburg

###### Abstract

Recent inclusive and differential cross section measurements of the associated
production of top quark pairs with gauge bosons or heavy-flavor jets are
reported. A search for physics beyond the standard model in the top quark
sector is also presented. All measurements are based on data samples of
proton-proton collisions at $\sqrt{s}=13$ TeV collected by the ATLAS and CMS
experiments at the CERN LHC. No significant deviation from the standard model
predictions is observed.

##  1 Introduction

The standard model (SM) of particle physics is supported by a large amount of
experimental results covering a wide energy range up to the TeV scale
accessible at the CERN LHC. However, it does not provide explanations for
several key observations such as the existence of dark matter and dark energy,
or the masses of neutrinos. More generally, there exist a number of
indications that the SM only corresponds to a low-energy approximation to a
more fundamental theory.

The large top quark mass of about 173 GeV corresponds to a Yukawa coupling to
the Higgs boson close to unity. This suggests that the top quark may play a
special role within the SM, and that its closer study may shed light on the
electroweak symmetry breaking mechanism. Most canonical top quark processes
have now entered a regime of precision differential measurements at the LHC,
and their uncertainties are systematics-dominated.

Many theories beyond the SM predict sizable deviations in the electroweak
couplings of the top quark with respect to SM predictions. These couplings can
be probed at tree level using the associated production of top quark pairs
with vector bosons. In addition, the study of the associated production of top
quark pairs with heavy-flavor jets probes several QCD predictions and provides
crucial insights on the modeling of the $\mathrm{t\bar{t}}$ system, and can be
used to compare different MC generators. It also constitutes a major
background to analyses such as $\mathrm{t\bar{t}H\to b\bar{b}}$, which
motivates further the precise characterization of these processes.

##  2 $\mathrm{t\bar{t}Z}$ cross section measurements at ATLAS and CMS

Inclusive and differential measurements of the $\mathrm{t\bar{t}Z}$ cross
section were performed by ATLAS [1] and CMS [2] in $3\ell$
($\ell=\textrm{e,}\mu$) and $4\ell$ final states using data samples with
integrated luminosities of $139\leavevmode\nobreak\ \mathrm{fb}^{-1}$ and
$77.5\leavevmode\nobreak\ \mathrm{fb}^{-1}$, respectively.

Both analyses fit the data simultaneously in multiple event categories that
are defined based on the multiplicities and properties of leptons and jets in
the events. Control regions enriched in WZ and ZZ events are included, and the
ATLAS analysis treats the normalizations of these processes as free
parameters. The inclusive CMS result $\sigma(\mathrm{t\bar{t}Z})=0.95\pm
0.05\leavevmode\nobreak\ \mathrm{(stat.)\pm 0.06\leavevmode\nobreak\
\mathrm{(syst.)\leavevmode\nobreak\ pb}}$ and ATLAS result
$\sigma(\mathrm{t\bar{t}Z})=1.05\pm 0.05\leavevmode\nobreak\
\mathrm{(stat.)}\pm 0.09\leavevmode\nobreak\
\mathrm{(syst.)\leavevmode\nobreak\ pb}$ agree with each other, and reach a
better precision ($8-10\%$) than that of the best available SM prediction at
NLO+NNLL accuracy [3].

The ATLAS analysis performs differential cross section measurements at the
parton- and particle-level as a function of nine observables related to the Z
boson, the $\mathrm{t\bar{t}}$ system, and the jet multiplicity. The CMS
analysis provides parton-level differential measurements as a function of
$\mathrm{p_{T}(Z)}$, and of the cosine of the angle between the direction of
the Z boson in the detector reference frame and that of the negatively-charged
lepton from the Z boson decay in the Z boson rest frame. These two observables
are also used to constrain anomalous $tZ$ couplings and Wilson coefficients
(WCs) describing the interaction strengths of four relevant effective field
theory (EFT) operators. The normalized differential cross section measurements
of $\mathrm{p_{T}(Z)}$ from both analyses are shown in Fig. 1. Good overall
data-to-MC agreement is observed in differential distributions.

Figure 1: Normalized $\mathrm{t\bar{t}Z}$ cross section measurements as a
function of $\mathrm{p_{T}(Z)}$ by CMS [2] (left) and ATLAS [1] (right).

##  3 $\mathrm{t\bar{t}\gamma}$ cross section measurements at ATLAS and CMS

Inclusive and differential measurements of the $\mathrm{t\bar{t}\gamma}$ cross
section were performed by ATLAS [4] and CMS [5] using the full Run 2 dataset.
The ATLAS analysis targets the high-purity $\mathrm{e}\mu$ channel and
considers both the $\mathrm{t\bar{t}\gamma}$ and $\mathrm{tW\gamma}$ signals.
The simulation of the signal samples includes resonant, non-resonant,
interference, and off-shell effects, which allows for a direct comparison with
NLO predictions for the $\mathrm{pp\to bWbW\gamma}$ process [6, 7]. A fit to
the data is performed using the distribution of the scalar $\mathrm{p_{T}}$
sum of all leptons, photons, jets, and missing transverse momentum in the
events. The CMS analysis targets the $\ell\mathrm{+jets}$ channel, constrains
the major backgrounds in-situ using dedicated sidebands, and performs a
simultaneous fit in 46 event categories. Both inclusive results are consistent
with SM predictions, and reach a comparable precision of about $6\%$. Both
analyses provide differential measurements as a function of several
observables, including $\mathrm{p_{T}(\gamma)}$ as shown in Fig. 2, and good
overall data-to-MC agreement is observed. In addition, the CMS analysis sets
constraints on two EFT operators impacting the $t-\gamma$ coupling.

Figure 2: Absolute $\mathrm{t\bar{t}\gamma}$ cross section measurements as a
function of $\mathrm{p_{T}(\gamma)}$ by CMS [5] (left) and ATLAS [4] (right).

##  4 Search for physics beyond the standard model using the associated top
quark production at CMS

A CMS analysis [8] using a novel approach to parameterize EFT effects at the
detector-level constrained simultaneously 16 WCs using
$41.5\leavevmode\nobreak\ \mathrm{fb}^{-1}$ of data. It considered EFT effects
in five associated production modes of the top quark with gauge and Higgs
bosons ($\mathrm{t\bar{t}Z}$, $\mathrm{t\bar{t}W}$, $\mathrm{tZq}$,
$\mathrm{t\bar{t}H}$, $\mathrm{tHq}$) in multilepton final states, and
performed counting experiments in event categories enriched in different
processes. Two standard deviation confidence intervals were computed for each
WC while either fixing other WCs to zero or profiling them. This represents an
important step towards direct measurements including EFT effects
simultaneously in all relevant processes.

##  5 Associated $\mathrm{t\bar{t}}$ production with heavy-flavor jets at
ATLAS and CMS

The $\mathrm{t\bar{t}b\bar{b}}$ cross section was measured using
$36\leavevmode\nobreak\ \mathrm{fb}^{-1}$ of data by ATLAS in the
$\mathrm{e}\mu$ and $\ell\mathrm{+jets}$ channels, and by CMS in the $2\ell$,
$\ell\mathrm{+jets}$, and all-hadronic channels. All these analyses extract
results by performing a fit to the b-tagging discriminants of the jets
identified as originating from the additional $\mathrm{b\bar{b}}$ pair.

In the ATLAS analysis [9], the contributions from the non-$\mathrm{t\bar{t}}$
and $\mathrm{t\bar{t}H/Z}\to\mathrm{t\bar{t}b\bar{b}}$ backgrounds are
substracted based on MC simulations. Cross sections are computed in different
fiducial phase spaces as illustrated in Fig. 3 (left). Relative precisions of
$17\%$ and $13\%$ are achieved in the $\ell\mathrm{+jets}$ and $\mathrm{e}\mu$
channel, respectively, and several differential measurements are unfolded at
particle-level. The CMS leptonic analysis [10] measures
$\sigma(\mathrm{t\bar{t}b\bar{b}})$, $\sigma(\mathrm{t\bar{t}j\bar{j}})$, and
their ratio. Fiducial cross sections extracted in the $\ell\mathrm{+jets}$ and
$2\ell$ channels reach precisions of $12\%$ and $13\%$, respectively. The CMS
all-hadronic analysis [11] benefits from a large branching ratio and a fully-
reconstructible final state, but suffers from huge QCD backgrounds. The
contribution from the latter is strongly reduced by using multivariate
analysis techniques (quark-gluon likelihood discriminant, jet-assignment BDT,
and weakly-supervised BDT to separate $\mathrm{t\bar{t}}$ and QCD processes).
Although no significant deviation is observed with respect to SM predictions,
all three analyses consistently report a trend of under-prediction of
$\sigma(\mathrm{t\bar{t}b\bar{b}})$ by all MC generators, which motivates
further differential studies based on the full Run 2 data sample.

Finally, CMS reported the first-ever measurement of the
$\mathrm{t\bar{t}c\bar{c}}$ cross section [12], which is particularly
challenging due to the difficulty to identify c-jets. This analysis exploits a
novel c-tagger algorithm calibrated in a dedicated control region [13]. Along
with the response of a neural network (NN) trained to correctly assign jets in
the events, c-tagging jet discriminants are provided as input to a multiclass
NN classifier tasked with separating the $\mathrm{t\bar{t}c\bar{c}}$ and
$\mathrm{t\bar{t}b\bar{b}}$ processes. A fit to the response of this NN is
performed to measure $\sigma(\mathrm{t\bar{t}c\bar{c}})$,
$\sigma(\mathrm{t\bar{t}b\bar{b}})$,
$\sigma(\mathrm{t\bar{t}+light\leavevmode\nobreak\ jets})$, as well as their
ratios to the inclusive $\sigma(\mathrm{t\bar{t}jj})$ value. All results are
consistent with POWHEG predictions within two standard deviations. The
measured value for $\sigma(\mathrm{t\bar{t}c\bar{c}})$ reaches a precision of
$20\%$ and is slightly over-predicted, while that for
$\sigma(\mathrm{t\bar{t}b\bar{b}})$ slightly exceeds the prediction as shown
in Fig. 3 (right).

Figure 3: Left: comparison of fiducial $\sigma(\mathrm{t\bar{t}b\bar{b}})$
measurements by ATLAS with predictions from several generators [9]. Right:
result of a two-dimensional scan of $\sigma(\mathrm{t\bar{t}c\bar{c}})$ and
$\sigma(\mathrm{t\bar{t}b\bar{b}})$ by CMS [12].

##  6 Summary

Recent inclusive and differential cross section measurements of the associated
production of top quark pairs with gauge bosons or heavy-flavor jets were
reported. While no significant deviation from the standard model predictions
is observed, three analyses based on data collected in 2016 by ATLAS and CMS
consistently report a trend of under-prediction of
$\sigma(\mathrm{t\bar{t}b\bar{b}})$ by all MC generators. A search for physics
beyond the standard model targeting five associated production modes of the
top quark was also presented.

## References

## References

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