Search for Direct Stau Pair Production at 8 TeV with the CMS Detector

In the Standard Model (SM) of Particle Physics, the particle masses are generated through theHiggs mechanism through the breaking of the electroweak symmetry. The Higgs mechanismpredicts the existance of the Higgs boson. Recent results from both the ATLAS and CMS experiments report the discovery of a new particle consistent with the Higgs boson. However, theHiggs sector of the SM suffers from the so-called hierarchy problem to which Supersymmetryis a possible solution. Supersymmetry postulates the existence of new particles, partners to theknown particles, with spin differing by half a unit. The search for these new particles is, in thisway, an important endeavour in the search for a deeper understanding of the fundamental theoryof elementary particles.This thesis describes a search for the supersymmetric partner of the tau lepton (stau). Thestaus subsequently decay to a regular tau lepton and a neutralino. The thesis considers the semihadronic final state, where one of the taus decay to an electron or muon and the other decayshadronically. The challenge in this analysis rests with the identification of the tau lepton, wherethe fake tau background from W+Jets events proved of particular importance.Results from CMS proton-proton data recorded in 2012 at 8 TeV are presented, with anintegrated luminosity of 19.7𝑓 𝑏−1 . The events were required to have an opposite sign pair ofan electron or muon and a tau in association with missing transverse energy and no b-jets. Themain background, fake taus, was measured from data. The analysis employs a simplified modelspectra simulation to generate the signal simulation, allowing the final result to be presentedin an as model independent way as possible. No significant evidence of signal was observedand limits were established on the cross section times branching ratio. The results were alsointerpreted under the MSSM scenario where the stau is the NLSP and the neutralino the LSP.