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: A search is performed for a heavy particle decaying into different-flavor, dilepton pairs (e mu, e tau or μtau), using 36.1 fb(-1) of proton-proton collision data at root s = 13 TeV collected in 2015-2016 by the ATLAS detector at the Large Hadron Collider. No excesses over the Standard Model predictions are observed. Bayesian lower limits at the 95% credibility level are placed on the mass of a Z' boson, the mass of a supersymmetric tau-sneutrino, and on the threshold mass for quantum black-hole production. For the Z' and sneutrino models, upper cross-section limits are converted to upper limits on couplings, which are compared with similar limits from low-energy experiments and which are more stringent for the e tau and μtau modes.

Abstract: This paper describes a search for events with one top-quark and large missing transverse momentum in the final state. Data collected during 2015 and 2016 by the ATLAS experiment from 13 TeV proton-proton collisions at the LHC corresponding to an integrated luminosity of 36.1 fb(-1) are used. Two channels are considered, depending on the leptonic or the hadronic decays of the W boson from the top quark. The obtained results are interpreted in the context of simplified models for dark-matter production and for the single production of a vector-like T quark. In the absence of significant deviations from the Standard Model background expectation, 95% confidence-level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the models considered.

Abstract: A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb(-1) of pp collision data at a centre-of-mass energy of root s =13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV.

Abstract: A search for Higgs boson decays to invisible particles is performed using 20,3 of fb(-1) collision data at a centre-of-mass energy of 8 TeV recorded by the ArL As detector at the Large IHIadron Collider. The process considered is Higgs boson production in association with a vector boson (V = W or Z) that decays hadronically, resulting in events with two or more jets and large missing transverse momentum. No excess of candidates is observed in the data over the background expectation. The results are used to constrain V H production followed by H decaying to invisible particles for the Higgs boson mass range 115 < m(H) < 300 GeV. The 95 % confidence-level observed upper limit on sigma vH x BR(H -> inv.) varies from 1.6 pb at 115 GeV to 0.13 ph at 300 GeV. Assuming Standard Model production and including the gg -> H contribution as signal, the results also lead to an observed upper limit of 78 c/c at 95 % confidence level on the branching ratio of Higgs bosons decays to invisible particles at a mass of 125 GeV.