Top quark physics with the ATLAS detector: recent highlights

The top quark is the heaviest known fundamental particle. As it is the only quark that decays without hadronisation, it provides the unique opportunity to probe the properties of bare quarks at the Large Hadron Collider (LHC). This article will present highlights of a few recent precision measurements of the top quark using 13 TeV collision data with the ATLAS experiment. Top-quark pair and single top production cross sections, including differential distributions and production in association with bosons, will be presented alongside top quark properties measurements. Measurements of the top-quark mass and searches for rare top decays are also presented.


Introduction
The large mass of the top quark, which is close to the electroweak symmetry breaking scale, indicates that this particle could play a special role in the Standard Model (SM) as well as in beyond the Standard Model (BSM) theories. Moreover, the top quark has a very short lifetime (τ=0.5×10 −25 s) and decays before hadronisation (τ had ∼10 −23 s) or spin decorrelation take place (τ spindec. ∼10 −22 s). Therefore several properties of the top quark may be measured precisely from its decay products.
Due to the high top-pair production (t t ) cross section for 13 TeV proton-proton (pp) collisions, the Large Hadron Collider (LHC) experiments collect an unprecedented number of top-quark events. The copious amount of detected events allows for high precision measurements in order to probe predictions of quantum chromodynamics (QCD), which provides the largest contribution tot t production. This process may also reveal BSM particles with a large coupling to the top quark.
This note describes the most recent results in the topquark sector by the ATLAS [1] Collaboration. It consists of four sections: • latest cross-section measurements; • state-of-the-art measurement of the top mass; • measurements of the properties of the top quark (spin correlation, charge asymmetry, | | V tb , ...); • latest results on the associated production of top-quark pairs with additional heavy-flavour jets; • search for new physics produced in association with top quarks; • measurements of strange hadron production fromt t pair decays.
2. Cross section measurements at ffiffiffi s p ¼ 13 TeV 2.1. Measurements of top-quark pair differential and doubledifferential cross-sections in the ℓ+jets channel The measurement of single-and double-differential crosssections at = s 13 TeV [2], performed by the ATLAS Collaboration, probe the top-quark kinematic properties at this centre-of-mass energy and complement recent measurements involving leptonic final states by ATLAS and CMS. All the measurements are performed in the selected fiducial phase-spaces and the results extrapolated to the full phasespace at the parton level.
In the SM, the top quark decays almost exclusively into a W boson and a b quark. The signature of at t decay is therefore determined by the W boson decay modes. This analysis makes use of the semileptonic decay ℓ+jets of thet t pair. In this case one W boson decays into an electron or a muon and a neutrino, and the other W boson decays into a quark-antiquark pair, with the two decay modes referred to as the e+jets and μ+jets channels, respectively. Events in which the W boson decays into an electron or muon through a τlepton decay may also meet the selection criteria. Since the reconstruction of the top quark depends on its decay products, two top quarks are referred to as 'hadronically (or leptonically) decaying top quarks' (or alternatively 'hadronic/leptonic top'), depending on the W boson decay mode.
Two complementary topologies of thet t final state in the ℓ+jets channel are exploited, referred to as 'resolved' and 'boosted', where the decay products of the hadronically decaying top quark are either angularly well separated or collimated into a single large-radius jet reconstructed in the calorimeter, respectively. As the jet selection efficiency of the resolved analysis decreases with increasing top-quark transverse momentum, the boosted selection allows events with higher-momentum hadronically decaying top quarks to be efficiently selected.
Since thet t production differential cross-sections are measured as a function of observables involving the top quark and thet t system, a dedicated event reconstruction is performed in each topology. The underlying differential crosssection distributions are obtained from the detector-level events using an unfolding technique that corrects for detector effects. The iterative Bayesian method [3] as implemented in RooUnfold [4] is used.
The differential cross-sections fort t production are measured as a function of a large number of variables (>60 spectra in total) including, for the first time in this channel in ATLAS, double-differential distributions. Moreover, the amount of data and the reduced detector uncertainties compared to previous publications also allows double differential measurements in the boosted topology to be made for the first time.
The analysis investigates a list of variables that characterise various aspects of thet t system production. In particular, the variables selected are sensitive to the kinematics of the top and anti-top quarks and of thet t system or are sensitive to initial-and final-state radiation effects. Furthermore, the variables are sensitive to the differences among parton density functions (PDFs) and possible effects beyond the SM. Both normalised and absolute differential cross-sections are measured, with more emphasis given to the discussion of the normalised results.
The particle-level measurements are compared with nextto-leading order plus parton shower (NLO+PS) Monte Carlo (MC) predictions as implemented in state-of-the-art MC generators. At the particle level, the predictions agree with the single-differential measurements over a wide kinematic region for both the resolved and boosted topologies, although poorer modelling is observed in specific regions of the probed phase-space, as shown in figure 1.
In the boosted topology, which is focused in the region where the hadronic top quark is produced with high p T , a disagreement between the measured inclusive cross-section and several predictions is observed, as shown in figure 2. Overall, the NLO+PS MC generators showt t poorer modelling of the double-differential distributions and no combination that includes p T can be described by the generators in the resolved topology.
Overall, the POWHEG [6]+PYTHIA8 [7] generator and, in the boosted topology, POWHEG+HERWIG7 [8] are the two generators able to give a good prediction of the largest fraction of the probed variables. The measurements show high sensitivity to the different aspects of the predictions of the MC generators. They are hence relevant for the tuning of the MC generators and will contribute to an improved description of thet t final state and to reducing the systematic uncertainties related to top-quark modelling.
The measured parton-level differential cross-sections are compared with state-of-the-art fixed-order next-to-next-toleading order (NNLO) QCD predictions [9]. A general improvement relative to the NLO+PS MC generators is found in the level of agreement of the single-and doubledifferential cross-sections in both the resolved and boosted regimes. The comparison of double-differential distributions with NNLO predictions provides a very stringent test of the SM description oft t production. The comparison with the NNLO pQCD predictions including EW corrections [10], due to the still rather limited range probed for the measured transverse momenta of the top and anti-top quarks, does not yet allow the impact of the EW corrections in the production of top-quark pairs to be quantified. The measured differential cross-sections at the parton level will be able to be used in detailed phenomenological studies and in particular to improve the determination of the gluon density in the proton and of the top-quark pole mass.

Measurement of the t t production cross-section and lepton differential distributions in eμ dilepton events
The opposite-sign eμ plus b-tagged jets signature allows precise measurements of the differential distributions of the leptons produced int t events to be made.
The cross-section is measured, at 13 TeV with an integrated luminosity of 36.1 fb −1 , to bes tt = 826.4±3.6 (stat) ± 11.5 (syst)±15.7 (lumi) ( )  1.9 beam pb [11], where the four uncertainties are due to data statistics, experimental and theoretical systematic effects, integrated luminosity, and LHC beam energy, giving a total uncertainty of 2.4%. The result is consistent with state-of-the-art theoretical QCD calculations and represents the most precise cross section measurement at the moment. The measurement is used to determine the top quark pole mass via the dependence of the predicted crosssection on m t pole resulting in = -+ . It is also combined with previous measurements at = s 7 TeV and = s 8 TeV, as shown in figure 3, to derive ratios and double ratios oft t and Z cross-sections at different energies.
This paper also presents the measurements of the absolute and normalised differential cross-sections as functions of the transverse momentum p ℓ T and absolute pseudorapidity | | h ℓ of the single leptons, the p T , invariant mass and absolute rapidity of the m e system, the absolute azimuthal angle | | f D between the two leptons in the transverse plane, and the scalar sums of the momenta and energies of the two leptons. These distributions are found to be generally well described by predictions from a variety of leading order (LO) multileg and next-to-leading order (NLO)t t matrix-element event generators interfaced to parton showers, and by NLO fixed-order QCD calculations. The sensitivity of the data to the gluon PDF and to the top quark pole mass is also shown. Two- e as functions of m m e are also reported. The data are again compared to the predictions of various NLOt t matrix-element event generators, but the interpretations in terms of PDF constraints and m pole are left for future work.

Top-quark mass measurements
The measurement takes advantage of the sensitivity to the top-quark mass of the differential cross section oft t production in association with at least one energetic jet. The presence of the additional jet enhances the sensitivity to the top-quark mass in comparison with similar observables defined for thet t system only. In particular, the observable used to extract the top-quark mass, R, is defined as the nor-   fixed to 170 GeV and¯+ m tt 1 jet representing the invariant mass of the¯+ tt 1 jet system. The measurement [14] is performed using 20.2 fb −1 collision data produced by the LHC and collected by the ATLAS experiment. The normalised differential cross section of top-quark pair production in association with an energetic jet is measured in the lepton+jets final state and unfolded to parton and particle levels. The unfolded distribution at parton level can be described using NLO QCD predictions in terms of either the top-quark pole mass or the running mass as defined in the (modified) minimal subtraction scheme (MS). A comparison between the experimental distribution and the theoretical prediction allows the top-quark mass to be extracted in the two schemes. The value obtained for the polemass scheme is . The extracted value in the running-mass scheme is . The results for the top-quark mass using the two schemes are consistent, when translated from one scheme to the other. Figure 4 shows the summary of top-quark pole mass measurements: it represents the best individual differential measurement, with a relative uncertainty of 0.7%, mainly due to jet-energy-scale and MC modelling uncertainties. The top quark mass is measured using a template method in the¯ tt lepton+jets channel (lepton is e or μ) using ATLAS data recorded in 2012 at the LHC, at a proton-proton centreof-mass energy of = s 8 TeV and corresponding to an integrated luminosity of 20.2 fb −1 [15].
The¯ tt lepton+jets channel is characterized by the presence of a charged lepton, a neutrino and four jets, two of which originate from bottom quarks (b). Exploiting a threedimensional template technique, the top quark mass is determined together with a global jet energy scale factor and a relative b-to-light-jet energy scale factor. , which corresponds to a relative uncertainty of 0.29%. Figure 5 compares the top masses from different experiments. 8 TeV results are taken from [12], with the LHC beam energy uncertainties reduced according to [13].
[11] Copyright 2020 CERN for the benefit of the ATLAS Collaboration. CC-BY-4.0 license.

Top properties measurements
4.1. Inclusive and differential measurements of the charge asymmetry in t t events at 13 TeV Production of top quark pairs is symmetric at leading-order (LO) under charge conjugation. The asymmetry between the t andt originates from interference of the higher-order amplitudes in theqq and qg initial states, with theqq annihilation contribution dominating. The contribution from electro-weak corrections is about 13% for the inclusive asymmetry. Thē  qg tt g production process is also asymmetric, but its cross section is much smaller thanqq. Gluon fusion production is symmetric to all orders. As a consequence of these asymmetries, the top quark is preferentially produced in the direction of the incoming quark.
At app collider, where the preferential direction of the incoming quark (antiquark) always almost coincides with that of the proton (anti-proton), a forward-backward asymmetry A FB can be measured directly. At the LHC pp collider, since the colliding beams are symmetric, it is not possible to measure A FB as there is no preferential direction of either the top quark or the top antiquark. However, due to the difference in the proton parton distribution functions, on average the valence quarks carry a larger fraction of the proton momentum than the sea antiquarks. This results in more forward top quarks and more central top antiquarks.
A central-forward charge asymmetry for thet t production, referred to as the charge asymmetry (A C ), is defined as y t y t is the difference between the absolute value of the top-quark rapidity | | y t and the absolute value of the top-antiquark rapidity | | y t . The measurement of thet t charge asymmetry is performed using data corresponding to an integrated luminosity of 139 fb −1 from the ATLAS experiment [16]. It is performed in the single-lepton channel combining both the resolved and boosted topologies of top quark decays. A Bayesian unfolding procedure is used to infer the asymmetry at parton level, correcting for detector resolution and acceptance effects.
The inclusivet t charge asymmetry is measured as 0.0060 0.0015 stat syst. C , which differs from zero by 4 standard deviations. It corresponds to the first evidence for charge asymmetry in pp collisions. Differential measurements are performed as a function of the invariant mass and longitudinal boost of thet t system. Both inclusive and differential measurements are found to be compatible with the SM predictions, at NNLO in perturbation theory with NLO electroweak corrections, and are shown in figure 6. In the SM, at leading order (LO) in perturbation theory, the top-quark decay width, Γ t , depends on the top-quark mass m t , the Fermi coupling constant G F and the CKM matrix element | | V tb . The most precise theoretical predictions at NNLO yield Γ t =1.322 GeV for a top-quark mass of 172.5 GeV [17]. The uncertainty on the theoretical prediction of the top-quark decay width from variations of the renormalisation scale is <1%. Propagating the experimental uncertainties to the parameters that enter the calculation increases the uncertainty up to ∼6%. Possible deviations from the expected value of the top-quark decay width could hint at non-SM decay channels of the top quark or modification of top-quark couplings (e.g., existence of a charged Higgs bosons or Flavour Changing Neutral Current (FCNC) processes). Γ t can be measured either directly or indirectly. Indirect measurements rely on some specific predictions of the SM, e.g. the theoretical SM calculation of the single-top-quark cross-section and theoretical calculations of the partial topquark decay width. This analysis [18], instead uses a direct approach to measure Γ t . It is based on data collected at a centre-of-mass energy of = s 13 TeV, recorded with the ATLAS detector, and focuses ont t events in the dilepton decay channel. This topology results in a signature of two bjets, two charged leptons and large missing transverse momentum, due to the two neutrinos. The invariant mass of a charged lepton and a b-jet, m ℓb , from eμ events is reconstructed and used in a profile-likelihood fit with simulated templates for various top-quark decay widths to extract the width in data. Systematic uncertainties are incorporated into the fit and are constrained by simultaneously fitting the invariant mass of the two b-jets,m bb , from combined ee and μμevents. The measured top-quark decay width is Γ t =1.9±0.5 GeV for a top-quark mass of m t =172.5 GeV, in agreement with the SM prediction. Due to the unstable nature of top quarks, their spin information is accessed through their decay products. However, not all decay particles carry the spin information to the same degree, with charged leptons arising from leptonically decaying W bosons carrying almost the full spin information of the parent top quark. This feature, along with the fact that charged leptons are readily identified and reconstructed by collider experiments, means that observables to study spin correlation int t events are often based on the angular distributions of the charged leptons in events where both W bosons decay leptonically (referred to as the dilepton channel). The simplest observable is the absolute azimuthal opening angle between the two charged leptons, measured in the laboratory frame in the plane transverse to the beam line. This opening angle is denoted by Δf.
The measurement of observables sensitive to spin correlations int t production uses 36.1 fb −1 of pp collision data at = s 13 TeV recorded with the ATLAS detector [19]. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle Δf and the absolute difference in pseudorapidity | | h D between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of thet t system. The measured differential cross-sections, unfolded to fiducial particle level and full phase-space parton level, are compared to predictions by several NLO MC generators and fixed-order calculations.
None of the studied generators are able to reproduce the normalised Δf distribution within the experimental errors. A comparison was made with fixed-order predictions at NNLO in QCD and in an expansion (of the normalised differential distribution in powers of the couplings) at NLO in QCD and EW couplings with a fixed scale choice. The former slightly improves the description of the data, while the latter describes the data but with large scale uncertainties ( figure 7).
The discrepancy remains unresolved, but is stimulating state-of-the-art predictions and development in the theory community [20].
Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into SM top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass. 8 TeV. The square of the magnitude of the CKM matrix element V tb multiplied by a form factor f LV is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation  Two summary plots are shown in figure 8.

5.
Measurements related to the associated production of a t t quark pair with additional heavyflavour jets at ffiffiffi s p ¼ 13 TeV Measurements of the production cross-section oft t pairs with additional jets provide important tests of QCD predictions. Among these, the process oft t produced in association with jets originating from b-quarks (b-jets) is particularly important to measure, as there are many uncertainties in the calculation of the process.
A measurement of such a production process in pp collisions at the LHC at a centre-of-mass energy of = s 13 TeV is performed, using data recorded with the ATLAS detector and corresponding to an integrated luminosity of 36.1 fb −1 [22]. Fiducial cross-section measurements are performed in the dilepton and lepton-plus-jets decay channels and presented at particle level in the form of inclusive cross-sections oft t final states with three and four bjets as well as differential cross-sections as a function of global event properties and properties of b-jet pairs.
As shown in figure 9, the measured inclusive fiducial cross-sections generally exceed the¯tt bb predictions from various NLO matrix element calculations matched to a parton shower but are compatible within the total uncertainties. The experimental uncertainties are smaller than the uncertainties in the predictions. Comparisons of state-of-the-art theoretical predictions with the differential measurements are shown and found in good agreement with data for most of them. In the SM, FCNC decays are suppressed relative to the dominant  t Wb decay mode, since tqH interactions are forbidden at the tree level and suppressed even at higher orders in the perturbative expansion due to the Glashow-Iliopoulos-Maiani (GIM) mechanism. As a result, the SM predictions for the  t Hq branching ratios are exceedingly small, ( ) ~-BR t Hu 10 17 and ( ) ~-BR t Hc 10 15 , making them undetectable in the foreseeable future. In contrast, large enhancements of these branching ratios are possible in some scenarios beyond the SM (e.g., quark-singlet models, two-Higgs-doublet models (2HDM), supersymmetric models with R-parity violation, composite Higgs models). In  decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, is exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production.

Search of new physics involving top quark production
No significant excess of events above the background expectation is found, and 95% CL upper limits on the  t Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the  This paper presents a search for FCNCs in processes with a top quark and a photon based on data collected with the ATLAS experiment at = s 13 TeV [24]. The analysis is most sensitive to the production of a single top quark plus a photon, but also considers the decay of a pair-produced top quark into an up or charm quark plus a photon. Events with a photon, an electron or muon, a b-tagged jet, and missing  transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the tqγ coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (righthanded) tuγ coupling of 36 fb (78 fb) and on the branching ratio for t → γu of 2.8×10 −5 (6.1×10 −5 ). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (right-handed) tcγ coupling of 40 fb (33 fb) and on the branching ratio for t → γc of 22×10 −5 (18×10 −5 ).

Measurement of strange hadron production from t t pair decays
The production of K S 0 and Λ 0 hadrons int t final states has been studied on a data sample with integrated luminosity of 4.6 fb −1 , at a centre-of-mass energy of 7 TeV with the ATLAS detector [25]. Neutral strange particles are separated into three classes, depending on whether they are contained in a jet, with or without a b-tag, or not associated with a selected jet. The aim is to look for differences in their main kinematic distributions. A comparison of data with several MC simulations using different hadronisation and fragmentation schemes, colour reconnection models and different tunes for the underlying event has been made. The production of neutral strange particles int t dileptonic events is found to be well described by current MC models for K S 0 and Λ 0 production within jets, but not for those produced outside jets, as shown in figure 10.

Conclusions and outlook
The latest results in the top-quark sector with the ATLAS experiment at the LHC have been presented. The one-and two-dimensional differential cross-section measurements for the production of top-antitop pairs confirm the SM NLO predictions with high precision, for a large number of observables. A precision of less than 3% has been reached in the measurement of thet t production cross section, better than the NNLO predictions.
Top-quark properties are measured with ever-increasing precision. Different techniques are employed to measure the top-quark mass: an uncertainty of less than 2 GeV (0.7%) has been achieved for direct measurements of the pole mass, making it a very competitive method. The first evidence of non-zero top charge asymmetry at the LHC has been obtained: the inclusivet t charge asymmetry is measured as ( ) =  + A 0.0060 0.0015 stat syst. C , which differs from zero by 4 standard deviations. The first top width measurement at 13 TeV has been performed, with a result that is already competitive with the pole-mass uncertainty. Some tensions between data and MC predictions for thet t spin-correlation measurement triggered lots of stimulating discussions and development in the theory community. Several NLO and NNLO MC models were investigated, but the discrepancy remains unresolved. Single-top quark production measurements in ATLAS and CMS have been combined and the CKM matrix element | | V tb has been determined with an uncertainty of 4%, in agreement with the SM prediction.
The associated production oft t pairs with additional jets provide important tests of QCD predictions: the measured inclusive fiducial cross-sections fort t produced in association with jets originating from b-quarks (b-jets) exceed the predictions from various NLO calculations, but are compatible within the total uncertainties. This has also triggered new developments to reduce the uncertainties in the theory predictions, which is the most limiting factor in such measurement nowadays.
Finally, one order of magnitude improvement with respect to the previous analysis has been obtained in the search for FCNC tγ.