Abstract: This paper describes a search for the single production of an up-type vector-like quark (T) decaying as T -> Ht or T -> Zt. The search utilises a dataset of pp collisions at root s = 13 TeV collected with the ATLAS detector during the 2015-2018 data-taking period of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb(-1). Data are analysed in final states containing a single lepton with multiple jets and b-jets. The presence of boosted heavy resonances in the event is exploited to discriminate the signal from the Standard Model background. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross section of T quarks in different decay channels. The results are interpreted in several benchmark scenarios to set limits on the mass and universal coupling strength (kappa) of the vector-like quark. For singlet T quarks, kappa values above 0.53 are excluded for all masses below 2.3 TeV. At a mass of 1.6 TeV, kappa values as low as 0.35 are excluded. For T quarks in the doublet scenario, where the production cross section is much lower, kappa values above 0.72 are excluded for all masses below 1.7 TeV, and this exclusion is extended to kappa above 0.55 for low masses around 1.0 TeV.

Abstract: A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H -> tau tau decay channel. The analysis is performed using 36.1 fb(-1) of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of root s = 13 TeV. All combinations of leptonic (tau -> l v (v) over bar with l = e, mu) and hadronic (tau -> hadrons v) tau decays arc considered. The H -> tau tau signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H -> tau tau decays. Using the data taken at root s = 13 TeV, the total cross section in the H -> tau tau decay channel is measured to be 3.77(-0.59)(+0.60)(stat)(-0.74)(+0.87) (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H -> tau tau decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be sigma(VBF)(H -> tau tau) = 0.28 +/- 0.09 (stat)(-0.09)(+0.11) (syst) pb and sigma(ggF)(H -> tau tau) = 3.1 +/- 1.0 (stat)(-1.3)(+1.6) (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.

Abstract: A search for a narrow scalar resonance decaying into an opposite-sign muon pair produced in events with and without b-tagged jets is presented in this paper. The search uses 36.1 fb(-1) of =13 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. No significant excess of events above the expected Standard Model background is observed in the investigated mass range of 0.2 to 1.0 TeV. The observed upper limits at 95% confidence level on the cross section times branching ratio for b-quark associated production and gluon-gluon fusion are between 1.9 and 41 fb and 1.6 and 44 fb respectively, which is consistent with expectations.