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A search for the electroweak production of charginos and sleptons decaying into final states with two electrons or muons is presented. The analysis is based on 139 fb −1 of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at √ s = 13 TeV. Three R-parity-conserving scenarios where the lightest neutralino is the lightest supersymmetric particle are considered: the production of chargino pairs with decays via either W bosons or sleptons, and the direct production of slepton pairs. The analysis is optimised for the first of these scenarios, but the results are also interpreted in the others. No significant deviations from the Standard Model expectations are observed and limits at 95% confidence level are set on the masses of relevant supersymmetric particles in each of the scenarios. For a massless lightest neutralino, masses up to 420 GeV are excluded for the production of the lightest-chargino pairs assuming W-boson-mediated decays and up to 1 TeV for slepton-mediated decays, whereas for slepton-pair production masses up to 700 GeV are excluded assuming three generations of mass-degenerate sleptons. Contents

The ATLAS CollaborationDark 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( )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 √ s = 13 TeV at the LHC. In combination with the results at √ s = 7 and 8 TeV, an exclusion limit on the H → invisible branching ratio of 0.26 (0.17 +0.07 −0.05 ) at 95% confidence level is observed (expected). 1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the z-axis along the beam pipe. The x-axis points to the center of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). The distance between two objects in η-φ space is ∆R = (∆η) 2 + (∆φ) 2 . Transverse momentum is defined by p T = p sin θ.

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.

A combination of the searches for pair-produced vector-like partners of the top and bottom quarks in various decay channels (T→Zt/W b/Ht, B→Z b/Wt/Hb) is performed using 36.1 fb −1 of pp collision data at √ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The observed data are found to be in good agreement with the Standard Model background prediction in all individual searches. Therefore, combined 95% confidence-level upper limits are set on the production cross-section for a range of vector-like quark scenarios, significantly improving upon the reach of the individual searches. Model-independent limits are set assuming the vector-like quarks decay to Standard Model particles. A singlet T is excluded for masses below 1.31 TeV and a singlet B is excluded for masses below 1.22 TeV. Assuming a weak isospin (T, B) doublet and |V T b | |V t B |, T and B masses below 1.37 TeV are excluded.