A search for physics beyond the Standard Model (SM) is performed in events with final states including hadronic activity, missing energy, and significant momentum imbalance as measured with the MT2 variable. The results are based on data collected by the Compact Muon Solenoid detector at the Large Hadron Collider, and correspond to a total integrated luminosity of 35.9fb−1 of proton-proton collisions at a center-of-mass energy of 13TeV. No significant excess above the predicted SM background is observed. The results are interpreted as 95% confidence-level exclusion limits on the masses of hypothesized particles in a variety of simplified models of R-parity conserving supersymmetry (SUSY). Additional techniques for extending the search to target final states with low-momentum leptons is discussed, interpreted in in the context of SUSY models with compressed mass spectra, and compared to the exclusion strength of a typical all-hadronic search targeting such models.

This thesis presents the results of a search for new physics in proton-proton collisions at the Large Hadron Collider, running with 13 TeV center of mass energy, using data gathered by the Compact Muon Solenoid (CMS). The search targets TeV mass-scale dark matter candidates and uses final states with two opposite-charge and same-flavor light leptons (electrons or muons) having dilepton mass consistent with the Z boson, at least 2 hadronic jets, and at least 100 GeV of transverse momentum imbalance. The thesis contains a historical introduction to particle physics, brief reviews of the standard model and supersymmetry, an in-depth discussion of the acquisition

of data by the Large Hadron Collider and CMS detector, and a pedagogical overview of the analysis methods used. No statistically significant deviation is found from the expected standard model background. The search results are presented and interpreted in the context of several simplified models of supersymmetry, including a model of Gauge Mediated Supersymmetry- Breaking (GSMB) with gluino production and models with Electroweakino production. The excluded mass ranges for these models are advanced by approximately 50-100% with respect to the best previous searches. This work represents the current state of the art for their exclusion

This dissertation presents the first observation of Higgs boson production in association with a top quark-antiquark pair in the diphoton decay channel, with a significance of 6.6 standard deviations. The measurement is performed with a dataset of 13 TeV proton-proton collisions recorded by the Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 137 \fbinv.

This work presents two searches for new physics characterized by pair-production of strongly interacting particles, each decaying to hadronic jets and a particle that is not detectable. The searches use the full 13 TeV proton-proton collision dataset produced by CERN’s Large Hadron Collider and recorded by the CMS detector from 2016 to 2018, with total integrated luminosity 137 fb−1. The presence of particles interacting too weakly to be detected is inferred using imbalance in the transverse momentum of the collision products, and sensitivity to pair- production is enhanced by requiring large values of the kinematic variable MT2 in events with at least two jets. The first search is inclusive, binning events using the total hadronic transverse energy, the total number of jets, the number of jets reconstructed as originating from a bottom quark, and either the value of MT2 in multijet events, or the transverse momentum of the jet in monojet events. The second search extends the first, by requiring the presence of a disappearing track in the event, and adds binning in the length and transverse momentum of the disappearing track. Both searches are sensitive to a variety of extensions to the Standard Model that include dark matter candidates. Of greatest interest, the results set constraints on pair production of squarks and gluinos as predicted by R-parity conserving supersymmetric extensions of the Standard Model, in which the lightest supersymmetric particle is a neutralino. The first search is sensitive to any decay chain terminating in Standard Model hadrons plus the neutralino, while the second specifically targets, with greatly enhanced sensitivity, decay chains containing an intermediate long-lived chargino. These constraints are the most stringent yet produced by any experiment.

Two related analyses of data from the CMS Experiment are presented. The first is performed using 19.5 inverse femtobarns of proton-proton collision data from CMS Run I. In this analysis, six different asymmetry variables are measured in events with top-antitop quark pairs decaying to final states with two leptons. Unfolding techniques are used to extrapolate these measurements to parton level. No deviations from the predictions of the Standard Model are found, implying the absence of any influence from physics beyond the Standard Model. The second analysis is performed using 35.9 inverse femtobarns of proton-proton collision data collected during CMS Run II. In this analysis, a search is performed for evidence of stop squark pair production and decay to single-lepton final states. Several backgrounds, including dileptonic top-pair production, are estimated using control regions in the data. No excess above the Standard Model backgrounds is found, and exclusion limits are placed on three models of stop squark pair production.