Search for charged Higgs Bosons in the [tau]+lepton final state with 36.1 fb⁻¹ of pp collision data recorded at [square root]s = 13 TeV with the ATLAS experiment

This dissertation describes a search for charged Higgs bosons decaying to a tau and a neutrino (H⁺ [right arrow] [tau]⁺[nu][sub t] or the charge-conjugate process) in association with a leptonically decaying top quark, using 36.1 fb⁻¹ of pp collision data collected with [square root]s = 13 TeV by the ATLAS detector. The theoretical motivation, including a review of the Standard Model, is given, along with a description of the ATLAS detector and particle reconstruction. A multi-variate analysis approach uses stochastic gradient boosted decision trees to improve the separation between H^[+/-] signal and background. The semi-leptonic channel described in this thesis is used in combination with a fully hadronic channel to search for charged Higgs bosons in a mass range of 90 GeV [less than or equal to] m[sub H^[+/-]] [less than or equal to] 2000 GeV. Finding no significant excess, limits are set at the 95% confidence level on the charged Higgs production cross section times the branching fraction into [tau][nu] ranging from 4.2 pb to 2.5 fb. These limits are interpreted in the hMSSM benchmark scenario as an exclusion at 95% confidence on tan[beta] as a function of m[sub H^[+/-]]. In this scenario, for tan[beta] = 60, the H^[+/-] mass range up to 1100 GeV is excluded, with all values of tan[beta] exclude for m[sub H^[+/-]] [precedes or equivalent to] 160 GeV

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements