#  12th Workshop on the CKM Unitarity Triangle
Santiago de Compostela, 18-22 September 2023
tt+X measurements by CMS and ATLAS

Bárbara Álvarez González1 on behalf of the CMS and ATLAS
Collaborations111”Copyright 2024 CERN for the benefit of the ATLAS and CMS
Collaborations. Reproduction of this article or parts of it is allowed as
specified in the CC-BY-4.0 license”  1Universidad de Oviedo, Instituto
Universitario de Ciencias y Tecnologías Espaciales de Asturias (ICTEA),
Oviedo, Spain

###### Abstract

These proceedings present the observation of the 4-top production process and
the latest results of the production of top-quark pairs associated with W and
$\gamma$ bosons ($t\bar{t}W$ and $t\bar{t}\gamma$) and b jets
($t\bar{t}b\bar{b}$) at a collision energy of 13 TeV carried out by the CMS
and ATLAS Collaborations.

##  1 Introduction

The top quark is a unique particle. It is the most massive of all observed
elementary particles and the only quark that decays before hadronisation,
which allows direct access to its properties. Top quark properties are studied
extensively in the CMS [1] and ATLAS [2] Collaborations. These studies provide
important input to test theoretical calculations and have the potential to
reveal deviations from the standard model (SM) predictions. The large amount
of LHC data recorded during Run 2 allows probing SM processes with small
production cross sections which become fully accessible inclusively and
differentially.

These proceedings present the observation of 4-top production with more than
5$\sigma$ significance, the inclusive and first differential $t\bar{t}W$ cross
sections, the most precise measurements of $t\bar{t}b\bar{b}$ production up to
date and the first measurement of the charge asymmetry of top-quark pairs in
production in $t\bar{t}\gamma$.

##  2 Observation of the 4-top production process

The 4-top ($t\bar{t}t\bar{t}$) production is a very rare process sensitive to
Higgs boson properties and beyond SM particles. The $t\bar{t}t\bar{t}$
production process is observed by both collaborations, CMS [3] and ATLAS [4],
using proton-proton collisions at 13 TeV with the full run 2 data. The
measurements are based on multilepton final states, two same-sign, three and
four leptons plus additional jets. The main background contributions are
processes of top-quark pairs associated with a W, Z or Higgs boson (known as
$t\bar{t}X$). The leading sources of systematic uncertainties are the signal
MC generator choice and parton shower modelling, the $t\bar{t}X$ background
theory modelling, and the jet and b-jet related sources of uncertainty. The
results of the 4-top production cross sections are shown in Fig. 1.

Figure 1: Summary of ATLAS and CMS measurements of the $t\bar{t}t\bar{t}$
production cross section at 13 TeV in various channels [5].

##  3 Inclusive and differential $t\bar{t}W$ cross sections

Measurements of both the inclusive production cross sections of a top-antitop
quark pair in association with a W boson ($t\bar{t}W$) by the CMS [6] and
ATLAS [7] Collaborations and the differential production cross sections by the
ATLAS [7] Collaboration are presented. The measurements are performed using
the full run 2 data targeting final states with two same-sign or three
isolated electrons and muons. The preliminary ATLAS results are superseded in
Ref. [8]. The $t\bar{t}W$ cross sections have been consistently measured above
expectation. These results, shown in Fig. 2, are compatible at the 1.5 and 2
$\sigma$ level for ATLAS and CMS, respectively. The leading uncertainties are
coming from signal modelling and background normalizations mainly $t\bar{t}Z$,
$WZ$ and misidentified leptons.

Figure 2: Comparison of the measured inclusive $t\bar{t}W$ cross sections to
the theoretical predictions from Sherpa, the MGNLOPY FxFx prescription
including electroweak corrections and the NLO+NNLL prediction [7].

ATLAS absolute and normalised differential cross section measurements
characterise this process in detail for the first time. Several particle-level
observables are compared to theoretical predictions from different MC
generators which are in good agreement with the normalised differential cross
section results.

##  4 Inclusive and differential $t\bar{t}b\bar{b}$ cross sections

The measurement by the CMS Collaboration presents the most precise inclusive
production cross sections of $t\bar{t}b\bar{b}$ up to date [9] using
proton–proton collisions at a centre-of-mass-energy of 13 TeV and an
integrated luminosity of 138 fb-1. The measurements are performed in the
lepton+jets decay channel of the top-quark pair, using events containing
exactly one isolated lepton (muon or electron) and at least five jets. The
cross sections are measured in four fiducial phase space regions requiring
different jet, b-jet, and light-jet multiplicities, targeting different
aspects of the $t\bar{t}b\bar{b}$ process. The uncertainties are dominated by
systematic sources with the leading sources originating from the calibration
of the b tagging and of the jet energy scale, and from the choice of
renormalization scale in the signal $t\bar{t}b\bar{b}$ and background
$t\bar{t}$ processes. The considered event generators predictions are 10-50%
lower than the measured values in the different phase space regions as shown
in Fig. 3.

Figure 3: Measured inclusive cross sections for each considered phase space,
compared to predictions from different $t\bar{t}b\bar{b}$ simulation
approaches shown as coloured symbols [9].

Differential cross section measurements are performed as a function of several
observables compared with predictions from several event generators. The
normalized differential cross sections show varying degrees of compatibility
with the theoretical predictions, however it is found that none generator
simultaneously describes all the measured distributions. These preliminary
results are superseded in the paper publication [10].

##  5 Charge asymmetry in $t\bar{t}\gamma$

A measurement of the top-quark pair charge asymmetry in $t\bar{t}\gamma$ is
presented [11]. It is performed using proton–proton collision data collected
with the ATLAS detector at a centre-of-mass-energy of 13 TeV during the years
2015–2018, corresponding to an integrated luminosity of 139 fb-1. The selected
events have exactly one photon, one lepton, and at least four jets, of which
at least one is b-tagged. The separation between signal ($t\bar{t}\gamma$
production where the $\gamma$ comes only from $t\bar{t}$ production) and
background is enhanced using a Neural Network (NN) approach. The charge
asymmetry is obtained from the distribution of the difference of the absolute
rapidities of the top quark and antiquark ($|y_{t}|-|y_{\bar{t}}|$), as shown
in Fig. 4, using a profile likelihood unfolding approach. It is measured to be
AC = -0.003 $\pm$ 0.024(stat) $\pm$ 0.017(syst) in agreement with the standard
model expectation.

Figure 4: The distributions of $|y_{t}|-|y_{\bar{t}}|$ after the fit in the
two regions defined by the NN output [11].

##  6 Summary

Low cross section top quark processes like $t\bar{t}X$ and $t\bar{t}t\bar{t}$
processes are already accessible at the LHC given the large amount of data
recorded during run 2. Inclusive and differential cross sections are measured
by the CMS and ATLAS Collaborations probing the top quark properties. All
results described in these proceedings are in good agreement with SM
predictions within the limited systematic uncertainties.

## References

  * [1] CMS Collaboration, The CMS experiment at the CERN LHC, JINST 3 (2008) S08004. https://doi.org/10.1088/1748-0221/3/08/S08004
  * [2] ATLAS Collaboration, The ATLAS Experiment at the CERN LHC, JINST 3 (2008) S08003. https://doi.org/10.1088/1748-0221/3/08/S08003
  * [3] CMS Collaboration, Observation of four top quark production in proton-proton collisions at s=13TeV, Phys. Lett. B 847 (2023) 138290. https://doi.org/10.1016/j.physletb.2023.138290
  * [4] ATLAS Collaboration, Observation of four-top-quark production in the multilepton final state with the ATLAS detector, Eur.Phys.J.C 83, 496 (2023), Erratum: Eur.Phys.J.C 84, 156 (2024). https://doi.org/10.1140/epjc/s10052-023-11573-0
  * [5] TopLHCWG twiki page: https://twiki.cern.ch/twiki/bin/view/LHCPhysics/LHCTopWGSummaryPlots#Four_Top_Quark_Production
  * [6] CMS Collaboration, Measurement of the cross section of top quark-antiquark pair production in association with a W boson in proton-proton collisions at $\sqrt{s}$ = 13 TeV, JHEP 07 (2023) 219. https://link.springer.com/article/10.1007/JHEP07(2023)219
  * [7] ATLAS Collaboration, Measurement of the total and differential cross-sections of $t\bar{t}W$ production in pp collisions at 13 TeV with the ATLAS detector, ATLAS-CONF-2023-019. http://cds.cern.ch/record/2855337/files/ATLAS-CONF-2023-019.pdf
  * [8] ATLAS Collaboration, Measurement of the total and differential cross-sections of $t\bar{t}W$ production in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector. https://doi.org/10.48550/arXiv.2401.05299.
  * [9] CMS Collaboration, Inclusive and differential cross section measurements of $t\bar{t}b\bar{b}$ production in the lepton+jets channel at $\sqrt{s}$ = 13 TeV with the CMS detector, CMS-PAS-TOP-22-009. http://cds.cern.ch/record/2852880/files/TOP-22-009-pas.pdf
  * [10] CMS Collaboration, Inclusive and differential cross section measurements of $\mathrm{t\bar{t}b\bar{b}}$ production in the lepton+jets channel at $\sqrt{s}$ = 13 TeV. https://arxiv.org/abs/2309.14442
  * [11] ATLAS Collaboration, Measurement of the charge asymmetry in top-quark pair production in association with a photon with the ATLAS experiment, Phys. Lett. B 843 (2023) 137848. https://doi.org/10.1016/j.physletb.2023.137848