Abstract: Direct searches for lepton flavor violation in decays of the Z boson with the ATLAS detector at the LHC are presented. Decays of the Z boson into an electron or muon and a hadronically decaying r lepton are considered. The searches are based on a data sample of proton-proton collisions collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb(-1) at a center-of-mass energy of root s = 13 TeV. No statistically significant excess of events above the expected background is observed, and upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95% confidence level: B(Z -> e tau) < 5.8 x 10(-5) and B(Z -> μtau) < 2.4 x 10(-5). This is the first limit on B(Z -> e tau) with ATLAS data. The upper limit on 13(Z -> μtau) is combined with a previous ATLAS result based on 20.3 fb(-1) of proton protoncollision data at a center-of-mass energy of root s = 8 TeV and the combined upper limit at 95% confidence level is B(Z -> μtau) < 1.3 x 10(-5).

Abstract: Differential cross-sections are measured for top-quark pair production in the all-hadronic decay mode, using proton-proton collision events collected by the ATLAS experiment in which all six decay jets are separately resolved. Absolute and normalised single- and double-differential cross-sections are measured at particle and parton level as a function of various kinematic variables. Emphasis is placed on well-measured observables in fully reconstructed final states, as well as on the study of correlations between the top-quark pair system and additional jet radiation identified in the event. The study is performed using data from proton-proton collisions at root s = 13 TeV collected by the ATLAS detector at CERN's Large Hadron Collider in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb(-1). The rapidities of the individual top quarks and of the top-quark pair are well modelled by several independent event generators. Significant mismodelling is observed in the transverse momenta of the leading three jet emissions, while the leading top-quark transverse momentum and top-quark pair transverse momentum are both found to be incompatible with several theoretical predictions.

Abstract: Differential cross-sectionmeasurements are presented for the electroweak production of two jets in association with a Z boson. These measurements are sensitive to the vector-boson fusion production mechanism and provide a fundamental test of the gauge structure of the Standard Model. The analysis is performed using proton-proton collision data collected by ATLAS at root s = 13 TeV and with an integrated luminosity of 139 fb(-1). The differential cross-sections are measured in the Z -> l(+)l(-) decay channel (l = e, mu) as a function of four observables: the dijet invariant mass, the rapidity interval spanned by the two jets, the signed azimuthal angle between the two jets, and the transverse momentum of the dilepton pair. The data are corrected for the effects of detector inefficiency and resolution and are sufficiently precise to distinguish between different state-of-the-art theoretical predictions calculated using Powheg+Pythia8, Herwig7+Vbfnlo and Sherpa 2.2. The differential cross-sections are used to search for anomalous weak-boson self-interactions using a dimension-six effective field theory. Themeasurement of the signed azimuthal angle between the two jets is found to be particularly sensitive to the interference between the Standard-Model and dimension-six scattering amplitudes and provides a direct test of charge-conjugation and parity invariance in the weak-boson self-interactions.

Abstract: Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs (t (t) over bar) are measured using the full LHC Run 2 data sample collected with the ATLAS detector. The data were collected at a pp collision energy of root s = 13TeV and correspond to an integrated luminosity of 140 fb(-1). The measurements use events containing an oppositely charged e μpair and b-tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton p(T) distributions is obtained by reweighting the t (t) over bar sample so as to reproduce the top-quark p(T) distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to be sigma(t (t) over bar) = 829 +/- 1 (stat) +/- 13 (syst) +/- 8 (lumi) +/- 2 (beam) pb, where the uncertainties are due to statistics, systematic effects, the integrated luminosity and the beam energy. This is in excellent agreement with the theoretical expectation.

Abstract: Dark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for H -> invisible decays where H is produced according to the standard model via vector boson fusion, Z(ll)H, and W/Z(had)H, all performed with the ATLAS detector using 36.1 fb(-1) of pp collisions at a center-of-mass energy of root s = 13 TeV at the LHC. In combination with the results at root s = 7 and 8 TeV, an exclusion limit on the H -> invisible branching ratio of 0.26(0.17(-0.05)(+0.07)) at 95% confidence level is observed (expected).