Inclusive nonresonant multilepton probes of new phenomena

Abstract

The Standard Model (SM) of particle physics is a very successful breakthrough in our current understanding of the universe. It is a framework built on quantum field theory approach, unifying the three out of the four fundamental forces of nature: the strong nuclear, the weak nuclear, and the electromagnetic interactions. Within the SM, the matter is composed of fundamental fermions whereas the interactions between them are mediated via force-carrier fundamental bosons. The SM has provided strong reasoning of how the universe evolved ever since the Big Bang. It has also paved the way for future discoveries by predicting the existence of new particles, such as the Higgs boson which was the latest and the final addition to the SM particles’ group. However, it is just a little short of being a complete theory of the universe. The missing explanations behind the matter-antimatter asymmetry, the origin and smallness of the neutrino masses, the observed flavor anomalies in the b-hadron decays, and the particle nature of dark matter are a few shortcomings of the SM. Aside from these, there are also additional questions such as ‘why just three generations of matter, and the mass hierarchy among them?’, ‘why do neutrinos occur as a left-handed singlet, but charged leptons do not?’, ‘why does the Higgs boson have a mass of 125 GeV, despite the radiative corrections from all the particles it couples to?’, and ‘why is gravity not part of the unified quantum field formulation, and is so many orders of magnitude weaker than other fundamental forces?’. There are many proposed theories of beyond-the-Standard Model (BSM) phenomena which attempts to address one or more of these open questions of the universe. This is done with the help of new hypothesized particles interacting with the SM particles. Thus, any unusual signature beyond the expectations of SM in particle physics experiments, consistent with the predictions of a particular theory, can provide strong evidence in its support. The Large Hadron Collider (LHC) at CERN is the world’s largest and highest energy particle accelerator, carrying out proton-proton or proton-lead or lead-lead collisions. Through these high energy collisions, a state equivalent to the universe just after the Big Bang is created for a very brief period of time. During this time, new particles are created from the plasma of quarks and gluons which then decay instantly to SM particles. Dedicated physics searches are designed to perform measurements of the SM free parameters like mass and decay widths of particles, coupling coefficients, and so on. Searches for BSM phenomena are conducted by utilizing the kinematic regions where the theory will manifest itself. If the proposed theory of BSM phenomena holds, then the hope is that with enough data, new hypothesized particles predicted by the theory will also be created similarly to the SM particles, provided the energy scales are the same. Through their unique topogical signatures, one can confirm the theory behind that BSM phenomena. In this thesis, I have presented an inclusive search for new phenomena in the nonresonant multilepton 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 fb −1 . 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 leptoquarks 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.