Search for Leptoquarks and Dark Matter in final states with top quarks at the CMS experiment

This thesis presents two searches for physics beyond the Standard Model (SM). A searchfor pair production of third-generation leptoquarks decaying into a top quark and a taulepton using pp collision data recorded at a center-of-mass energy of 8TeV with theCMS experiment is presented. The search is based on an event selection requiring anisolated muon or electron candidate, one hadronically decaying tau lepton candidate andat least three jets. The main reducible background originates from processes, where ajet is misidentied as a tau lepton candidate. A measurement of the tau lepton misidentication rate is performed in a sideband enriched in W + jets events. The transversemomentum distribution of the leading tau lepton candidate is used for the statisticalinterpretation of the result. No excess over the SM expectation is observed. Uppercross section limits on the pair production cross section of leptoquarks decaying into atop quark and a tau lepton are set. By combining the presented search with an analysisrequiring same-sign muon-tau lepton-pairs, leptoquarks with masses below 685 GeV(695 GeV expected) are excluded at 95% C.L., assuming a branching ratio of 100% intoa top quark and a tau lepton.The second analysis presented in this thesis is a search for Dark Matter (DM) producedin association with a top quark pair using data collected at a center-of-mass energy of13TeV by the CMS experiment. The analysis targets nal states in which the top quarksreceive large transverse momenta by recoiling against the DM particles. Therefore, theapplied event selection allows for non-isolated leptons and uses top tagging techniquesto identify merged top quark decays. The normalizations of the main background processes,tt + jets and W + jets, are determined in data using control regions enriched inthe respective process. For the nal statistical interpretation the spectra of the missingtransverse momentum in two signal regions are studied. Data and the SM backgroundexpectation agree within the uncertainties. Upper cross section limits are set on theproduction cross section of DM produced with a top quark pair assuming dierent DMparticle and mediator masses, as well as scalar and pseudoscalar couplings of the mediators.Scalar mediators below 36 GeV are excluded at 95% C.L. if DM particle massesof 1 GeV and coupling strengths of one are assumed. In addition, a novel reconstructionof the neutrino four-momentum based on a likelihood is presented, which aims at discriminatingbetween tt + jets in the lepton+jets decay channel and signal events.Furthermore, this thesis presents studies on the impact of dierent pileup mitigationtechniques on top tagging algorithms using simulated events at a center-of-mass energyof 13TeV. Optimal parameter choices for top tagging algorithms using these techniquesare derived.