Searches for supersymmetric particles decaying to W and Higgs bosons in events with an isolated lepton with the ATLAS detector

The Standard Model (SM) of particle physics is a theory that provides a description of all known elementary particles and their interactions except gravitational interactions. Nevertheless, there are a few open issues which cannot be explained by the SM. Supersymmetry (SUSY) is a well motivated extension of the SM which solves some of its shortcomings and predicts the existence of new particles. This thesis presents two searches for supersym- metric particles in proton-proton collisions delivered by the Large Hadron Collider (LHC) and recorded by the ATLAS detector in 2015 and 2016. The first search considers a simplified SUSY model in which squarks and gluinos are pair-produced by the strong interactions in LHC collisions. In- spired by the discovery of the Higgs boson in 2012, this model includes Higgs bosons in the decay chain as well as W and Z bosons. The final state conside- red consists of one lepton (electron or muon), jets and missing transverse momentum. The analysis uses three signal regions which are designed to target the wide range of kinematics expected from squark or gluino decays. The obtained sensitivity has been compared with a reinterpretation of a pub- lished analysis which targets a slightly different supersymmetric model. The second search explores a simplified SUSY model of electroweak pair production of charginos (χ˜i±) and neutralinos (χ˜0j) in which the lightest chargi- no (χ˜1±) and next-to-lightest neutralino (χ˜02) decay to W and Higgs bosons, respectively. A final state with one lepton (electron or muon), two b-jets from the Higgs boson decay, and missing transverse momentum is explored us- ing three statistically orthogonal signal regions. The data are found to be compatible with the SM predictions in all signal regions

Report from Working Group 3: Beyond the standard model physics at the HL-LHC and HE-LHC

This is the third out of five chapters of the final report [1] of the Workshop on Physics at HL-LHC, and perspectives on HE-LHC [2]. It is devoted to the study of the potential, in the search for Beyond the Standard Model (BSM) physics, of the High Luminosity (HL) phase of the LHC, defined as $3$ ab$^{-1}$ of data taken at a centre-of-mass energy of 14 TeV, and of a possible future upgrade, the High Energy (HE) LHC, defined as $15$ ab$^{-1}$ of data at a centre-of-mass energy of 27 TeV. We consider a large variety of new physics models, both in a simplified model fashion and in a more model-dependent one. A long list of contributions from the theory and experimental (ATLAS, CMS, LHCb) communities have been collected and merged together to give a complete, wide, and consistent view of future prospects for BSM physics at the considered colliders. On top of the usual standard candles, such as supersymmetric simplified models and resonances, considered for the evaluation of future collider potentials, this report contains results on dark matter and dark sectors, long lived particles, leptoquarks, sterile neutrinos, axion-like particles, heavy scalars, vector-like quarks, and more. Particular attention is placed, especially in the study of the HL-LHC prospects, to the detector upgrades, the assessment of the future systematic uncertainties, and new experimental techniques. The general conclusion is that the HL-LHC, on top of allowing to extend the present LHC mass and coupling reach by $20-50\%$ on most new physics scenarios, will also be able to constrain, and potentially discover, new physics that is presently unconstrained. Moreover, compared to the HL-LHC, the reach in most observables will, generally more than double at the HE-LHC, which may represent a good candidate future facility for a final test of TeV-scale new physics

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