Highlights of Top Quark Measurements with the ATLAS Experiment

Measurements of processes involving single top quarks or top-quark-antiquark pairs are of major importance to check the predictions of the Standard Model or to provide sensitivity to new physics beyond the Standard Model. In the recent years, the ATLAS experiment at the Large Hadron Collider performed precise measurements of top-quark related properties and also measured more complex Standard Model processes associated with top quarks, which have typically very low cross sections compared to the main production modes at the Large Hadron Collider. On this subject, some of the most relevant results released by the ATLAS collaboration in 2021 are presented: the inclusive and differential cross-section measurement of top-quark-antiquark production in association with a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Z$$\end{document} boson, the cross-section measurement of four top-quark production, the fiducial and differential measurement of the top-quark-antiquark cross section with high transverse momenta of the top quarks, as well as the polarisation measurement of single top quarks (antiquarks) produced in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$t$$\end{document}-channel.


MEASUREMENTS OF THE INCLUSIVE AND DIFFERENTIAL PRODUCTION CROSS SECTIONS OF A tt PAIR IN ASSOCIATION WITH A Z BOSON
AT √ s = 13 TeV WITH THE ATLAS DETECTOR The ttZ process is the production of a top-quarkantiquark (tt) pair in association with a Z boson. As this process provides direct access to the neutral coupling of the top quarks to electroweak gauge bosons and represents an important background for many Standard Model (SM) measurements as well as searches for physics beyond the SM (BSM), a precise knowledge about its cross section is of particular interest. The inclusive ttZ cross section has been measured already by the ATLAS experiment [1] with a subset of the Run 2 data [2]. With the larger amount of data collected during the full Run 2 and improved techniques for background rejection and Monte Carlo (MC) modelling, it is also possible to measure its cross section differentially as a function * E-mail: fabio.cardillo@cern.ch different variables which probe the kinematics of the ttZ system.
The updated measurements are performed by selecting events with three or four leptons (electrons or muons) and are based on √ s = 13 TeV pp collision data recorded from 2015 to 2018, corresponding to an integrated luminosity of 139 fb −1 [3]. Signal regions for three leptons and four leptons enriched with ttZ events are used together with dedicated control regions to address the W Z and ZZ + jets backgrounds, which represent the major background sources in the three-and four-lepton channels, respectively. The inclusive production cross section is obtained by performing a simultaneous maximum-likelihood fit in the signal and control regions with the ttZ normalisation as the parameter of interest.
The inclusive cross section is measured to be σ ttZ = 0.99 ± 0.05 (stat.) ± 0.08 (syst.) pb. The differential cross sections are unfolded to particle-and parton-level to allow comparisons with theoretical predictions. Nine different variables which are sensitive to BSM effects or relevant in the context of MC tuning are considered. Figure 1 shows one example: the transverse momentum (p T ) of the reconstructed Z boson at parton-level.
The compatibility between the data and the different MC-based and theoretical predictions is evaluated based on a χ 2 /ndf and p-value computation. For most of the considered observables, good agreement between data and the predictions is observed. The differences between the various predictions are typically covered by the statistical or systematic uncertainties of the measurements.

MEASUREMENT OF THE tttt
The predicted cross section of the production of four top quarks (tttt) is very low with respect to many other SM processes produced at the Large Hadron Collider and is, therefore, very challenging to investigate. Since the tttt cross section is sensitive to the magnitude, charge-conjugation and parity properties of the Yukawa coupling between the top quark and the Higgs boson, its measurement can be interesting in the context of various BSM scenarios, e.g., effective fields theories (EFT) with four-fermion couplings or supersymmetric models. As all four top quarks can  decay either leptonically or hadronically, the tttt process covers a wide range of possible event topologies with different lepton and jet multiplicities.
A first evidence of tttt production has been found in the multi-lepton topologies targeting either two same-sign leptons (electrons/muons) or at least three leptons [4]. It was later combined with the less sensitive single-lepton and di-lepton (opposite-sign) channels to achieve the best possible precision on the inclusive cross section [5]. Both analyses apply boosted decision trees (BDTs) to effectively separate the tttt process from the other SM backgrounds, as e.g., shown in Fig. 2 for the signal region of the multi-lepton analysis. The full ATLAS Run 2 dataset is used for the measurements. Figure 3 shows the ratio between the measured and SM-expected cross sections (signal strengths) for the individual channels, as well as for the statistical combination of the channels.
The measurements in both channels deliver compatible results. As the measurement in the multilepton channel is the more precise one, it dominates the combined result. The combined cross section is measured to be σ tttt = 24 ± 4 (stat.) +5 −4 (syst.) fb which is around twice as much as theoretically expected, but still consistent within 2.0 standard deviations with the SM prediction.

MEASUREMENTS OF DIFFERENTIAL CROSS-SECTIONS IN tt EVENTS WITH A HIGH p T TOP QUARK AND LIMITS ON BSM CONTRIBUTIONS TO tt PRODUCTION WITH THE ATLAS DETECTOR
The cross section of tt production in a kinematic regime with an high transverse momentum of the top quark can give access to many EFT scenarios, as some of the associated EFT operators are expected to show significant deviations from the SM at large energy scales (for instance large values of p top T or m tt ). Fiducial and differential cross sections in this kinematic phase-space have already been measured by ATLAS, but only with a subset of the Run 2 data (only 2015-2016) and without any EFT interpretations [6].
To probe the predictions of the SM in this region of the phase-space and to set limits on the most relevant EFT operators, differential and fiducial cross sections are measured by the ATLAS experiment with data from the full Run 2 dataset [7]. The measurements are done by selecting events with exactly one lepton (electron/muon) and one hadronically decaying top quark (reconstructed from the jets found in the event) with p top T > 355 GeV. The fiducial cross section in this phase-space region is measured to be σ tt = 1.267 ± 0.005 (stat.) ± 0.053 (syst.) pb, in agreement with the SM expectation. The differential measurements are performed as functions of different kinematic variables which are expected to be sensitive to the considered operators. One example is the cross section depending on the p T of the hadronically decaying top quark, which is shown in Fig. 4. The cross sections are unfolded to particle-level to allow a better comparison to theoretical predictions. The compatibility between data and the predictions from different MC generators and parton-shower algorithms is quantified with a χ 2 /ndf and p-value computation.
No evidence for BSM physics is observed in the fiducial or differential measurements. The differential cross-section distribution as a function of the top-quark p T is used to derive exclusion limits on the Wilson coefficients of two EFT operators which modify the coupling of the top quark to gluons and light-flavour quarks: C tG and C (8) tq [8]. The obtained 95% credibility intervals are C tG ∈ [−0.68, 0.21] and C A measurement of these polarisarion vectors has been performed by the ATLAS experiment using the full Run 2 dataset [9]. Events with exactly one lepton (electron/muon) are selected. Furthermore, specific cuts on the lepton p T , missing transverse energy, transverse mass and angular-separation variables are applied to define signal regions enriched with top quarks produced in the t-channel. A multi-dimensional likelihood minimisation procedure is used to extract the top quark/antiquark angular variables and their components of the polarisation vectors in the rest frame of the quarks [10].
Normalised differential cross-section measurements are performed as functions of the three angular variables: cos(θ x ), cos(θ y ), cos(θ z ). The results obtained in data are unfolded to particlelevel and compared to the predictions of different MC generators and parton-shower algorithms. Figure 5 shows an example for one of these angular variables cos(θ x ). To evaluate the compatibility between data and the different MC generators for the differential  cross sections, χ 2 /ndf and p-value are computed. Limits on two components of the polarisation vectors {P x , P z } of the top quark and antiquark are derived, as depicted in Fig. 6. The differential cross sections and polarisation-vector components measured in data are in good agreement with the predictions from the SM. The systematic uncertainties of the measurements cover in most cases the differences between the alternative MC predictions.
The normalised differential cross sections as functions of the angular variables are used to derive exclusion limits on the Wilson coefficients of two EFT operators which modify the tW b coupling: C tW and C itW [8]. The limits (at 95% confidence level) are: C tW ∈ [−0.7, 1.5] and C itW ∈ [−0.7, 0.2]. Also simultaneous (two-dimensional) limits on C tW and C itW are derived. Both, the individual and the twodimensional limits are in agreement with the SM expectations.

CONCLUSIONS
Four different ATLAS analyses measuring topquark related properties or cross sections of rare processes associated with top quarks have been discussed. Within their uncertainties, all measurements show good agreement with the predictions of the SM. Some processes where measured for the first time, whereas other analyses accomplished to improve previous measurements. In addition to the results of the measurements, some analyses derived exclusion limits on different BSM scenarios which, in many cases, improved the previously existing limits on these models.

CONFLICT OF INTEREST
The author declares that he has no conflicts of interest.