Search for heavy neutral leptons in decays of W bosons produced in 13 TeV p p collisions using prompt and displaced signatures with the ATLAS detectorThe ATLAS CollaborationThe problems of neutrino masses, matter-antimatter asymmetry, and dark matter could be successfully addressed by postulating right-handed neutrinos with Majorana masses below the electroweak scale. In this work, leptonic decays of W bosons extracted from 32.9 fb −1 to 36.1 fb −1 of 13 TeV proton-proton collisions at the LHC are used to search for heavy neutral leptons (HNLs) that are produced through mixing with muon or electron neutrinos. The search is conducted using the ATLAS detector in both prompt and displaced leptonic decay signatures. The prompt signature requires three leptons produced at the interaction point (either µµe or eeµ) with a veto on same-flavour opposite-charge topologies. The displaced signature comprises a prompt muon from the W boson decay and the requirement of a dilepton vertex (either µµ or µe) displaced in the transverse plane by 4-300 mm from the interaction point. The search sets constraints on the HNL mixing to muon and electron neutrinos for HNL masses in the range 4.5-50 GeV.

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A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb −1 of proton-proton collision data collected at a centre-of-mass energy of √ s = 8 TeV. The lepton-plus-jets final state is used, where the top pair decays to W + bW −b , with one W boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow Z boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour Z boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with Γ/m = 15% decaying to tt. These range from 4.8 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.

This article presents measurements of the t-channel single top-quark (t) and top-antiquark (t) total production cross sections σðtqÞ and σðtqÞ, their ratio R t ¼ σðtqÞ=σðtqÞ, and a measurement of the inclusive production cross section σðtq þtqÞ in proton-proton collisions at ffiffi ffi s p ¼ 7 TeV at the LHC. Differential cross sections for the tq andtq processes are measured as a function of the transverse momentum and the absolute value of the rapidity of t andt, respectively. The analyzed data set was recorded with the ATLAS detector and corresponds to an integrated luminosity of 4.59 fb −1 . Selected events contain one charged lepton, large missing transverse momentum, and two or three jets. The cross sections are measured by performing a binned maximum-likelihood fit to the output distributions of neural networks. The resulting measurements are σðtqÞ ¼ 46 AE 1ðstatÞ AE 6ðsystÞ pb, σðtqÞ ¼ 23 AE 1ðstatÞ AE 3ðsystÞ pb, R t ¼ 2.04 AE 0.13ðstatÞ AE 0.12ðsystÞ, and σðtq þtqÞ ¼ 68 AE 2ðstatÞ AE 8ðsystÞ pb, consistent with the Standard Model expectation. The uncertainty on the measured cross sections is dominated by systematic uncertainties, while the uncertainty on R t is mainly statistical. Using the ratio of σðtq þtqÞ to its theoretical prediction, and assuming that the top-quark-related CKM matrix elements obey the relation jV tb j ≫ jV ts j; jV td j, we determine jV tb j ¼ 1.02 AE 0.07.

A search for charged Higgs bosons heavier than the top quark and decaying via H ± → tb is presented. The data analysed corresponds to 36.1 fb −1 of pp collisions at √ s = 13 TeV and was recorded with the ATLAS detector at the LHC in 2015 and 2016. The production of a charged Higgs boson in association with a top quark and a bottom quark, pp → tbH ± , is explored in the mass range from m H ± = 200 to 2000 GeV using multi-jet final states with one or two electrons or muons. Events are categorised according to the multiplicity of jets and how likely these are to have originated from hadronisation of a bottom quark. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass, which range from 2.9 pb at m H ± = 200 GeV to 0.070 pb at m H ± = 2000 GeV. The results are interpreted in two benchmark scenarios of the Minimal Supersymmetric Standard Model.