Inclusive nonresonant multilepton probes of new phenomena

In this thesis, I have presented an inclusive search for new phenomena in the nonresonant mul- tilepton final states. The search targets three different models: vector-like lepton in the doublet and singlet scenario, type-III seesaw mechanism, and scalar leptoquarks with the top-philic couplings. These three models target different open questions of the SM, such as the existence of vector-like leptons may provide a dark matter candidate and also account for the mass hierarchy between the different generations of matter particles in the SM, the origin and smallness of the neutrino masses can be explained by the production of heavy seesaw fermions, and scalar leptoquarks could provide an explanation for the observed b-anomalies. The primary reason behind this particular selection of BSM phenomena is that they are generators of complementary nonresonant multilepton signatures. The search is designed with multiple electrons, muons, and hadronically decaying tau leptons, utilizing the combined proton-proton collisions data set collected by the CMS experiment at the LHC between 2016–2018, corresponding to an integrated luminosity of L = 138 ifb . With a total of seven orthogonal final states covering almost the entire multilepton landscape, this analysis is a benchmark result with a huge sensitivity for a variety of BSM signals. The model-dependent part of the analysis employs the boosted decision trees algorithm to enhance the sensitivity to the probed BSM scenarios. No significant deviations from the background expectations are observed. Lower limits are set at 95% confidence level on the mass of the vector-like τ lepton in the doublet and singlet extensions of the SM, and are excluded for masses below 1045 GeV and in the mass range 125–150 GeV, respectively. Type-III seesaw heavy fermions are excluded in the mass range 845–1065 GeV for various decay branching fraction combinations to SM leptons. Scalar lepto- quarks decaying exclusively to a top quark and a lepton are excluded for masses below 1.12–1.42 TeV, depending on the lepton flavor. For the vector-like lepton doublet as well as the type-III seesaw model, these constraints are the most stringent to date. For the vector-like lepton singlet model, these are the first constraints from the LHC experiments. To ensure the longevity of this multilepton analysis, a model-independent component based purely on the expected SM predictions and observations is also designed, allowing the results to be reinterpretable for other BSM theories. Detailed results are also provided to facilitate these alternative theoretical interpretations.