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Abstract: Several models of physics beyond the Standard Model predict neutral particles that decay into final states consisting of collimated jets of light leptons and hadrons (so-called “lepton jets”). These particles can also be long-lived with decay length comparable to, or even larger than, the LHC detectors’ linear dimensions. This paper presents the results of a search for lepton jets in proton-proton collisions at the centre-of-mass energy of √s= 8TeV in a sample of 20.3 fb−1 collected during 2012 with the ATLAS detector at the LHC. Limits on models predicting Higgs boson decays to neutral long-lived lepton jets are derived as a function of the particle’s proper decay length.

A measurement of the t (t) over barZ and t (t) over barW production cross sections in final states with either two same-charge muons, or three or four leptons (electrons or muons) is presented. The analysis uses a data sample of proton-proton collisions at root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015, corresponding to a total integrated luminosity of 3.2 fb(-1). The inclusive cross sections are extracted using likelihood fits to signal and control regions, resulting in sigma(t (t) over barZ) = 0.9 +/- 0.3 pb and sigma(t (t) over barW) = 1.5 +/- 0.8 pb, in agreement with the Standard Model predictions.

Measurements of the electroweak production of aWboson in association with two jets at high dijet invariant mass are performed using√s̅ = 7and8TeV proton-proton collision data produced by the Large Hadron Collider, corresponding respectively to 4.7 and 20.2 fb ⁻¹of integrated luminosity collected by the ATLAS detector. The measurements are sensitive to the production of aWboson via a triple-gauge-boson vertex and include both the fiducial and differential cross sections of the electroweak process.

Measurements of jet activity in top-quark pair events produced in proton–proton collisions are presented, using 3.2 fb $$^{-1}$$ -1 of pp collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Events are chosen by requiring an opposite-charge $$e\\mu $$ eμ pair and two b-tagged jets in the final state. The normalised differential cross-sections of top-quark pair production are presented as functions of additional-jet multiplicity and transverse momentum, $$p_{\\text {T}}$$ pT. The fraction of signal events that do not contain additional jet activity in a given rapidity region, the gap fraction, is measured as a function of the $$p_{\\text {T}}$$ pT threshold for additional jets, and is also presented for different invariant mass regions of the $$e\\mu b\\bar{b}$$ eμbb¯ system. All measurements are corrected for detector effects and presented as particle-level distributions compared to predictions with different theoretical approaches for QCD radiation. While the kinematics of the jets from top-quark decays are described well, the generators show differing levels of agreement with the measurements of observables that depend on the production of additional jets.