Search for third-generation scalar leptoquarks decaying into a top quark and a tau lepton with the CMS detector

In this thesis a search for pair production of third-generation scalar leptoquarks is presented.Leptoquarks are new bosons predicted by many theories beyond the Standard Model. Theycouple to a quark and a lepton. Due to constraints from flavor changing neutral currents andother rare processes it is predicted that leptoquarks decay only into particles of the same Standard Model generation. In this thesis the decay channel into a top quark and a tau lepton isstudied. For the analysis the complete data sample from the year 2012 recorded by the CMSexperiment at the LHC at a centre-of-mass energy of 8 TeV is used, corresponding to a totalintegrated luminosity of 19.6 fb−1 .Since a search in this channel has never been done before different selections based on at leastone muon candidate and one hadronically decaying tau lepton are studied in order to define asignal region with high sensitivity. After maximizing the sensitivity by calculating expectedlimits on the production cross section times branching ratio, it is found that the distribution ofthe transverse momentum of the leading tau lepton shows high sensitivity for different leptoquark masses and this distribution is used for the final interpretation of the result.No excess over the Standard Model expectation is observed and exclusion limits on the crosssection times branching ratio are set. Third-generation leptoquarks decaying to a top quark anda tau lepton are excluded for masses up to 582 GeV/c2 at 95% C.L.