Reconstruction and selection of $Z \to \tau\tau \to e+\tau$-jet decays at the Compact Muon Solenoid experiment

Tau leptons are important in Higgs searches, particularly within an MSSM scenario, where decays of electrically neutral (MSSM) Higgs bosons to two τ leptons have a significant branching ratio. The CMS detector is ideally placed to detect such signals both due its good tracking performance and the fine granularity of the electromagnetic calorimeter. However before such searches take place, it is important to ensure that τ selections are understood. In this context Z→ττ decays are important, as they provide a test bench for analogous H→ττ and can allow for the measurement of τ-jet selection efficiencies which is vital in extracting the Higgs production cross section and branching ratio and therefore essential in determining potential MSSM parameters. This thesis contains simulation studies on the development of a combined e+τ-jet trigger tuned for the 1032 cm−2s−1 instantaneous luminosity regime giving an efficiency for a pre-selected Z→ττ→e+τ-jet+X sample of (44.6±1.6)% with a QCD rate of (0.7±0.2)Hz. Commissioning the L1 τ-jet algorithm on cosmic muon triggered data during the Cosmic Run at Zero Tesla of CMS showed that the current isolation and activity parameters are robust against HCAL noise showing a <5% drop in efficiency. Simulated data was used to perform a selection and reconstruction of Z→ττ→e+τ-jet+X events for the purpose of performing a first measurement of the τ-jet “core track isolation efficiency using 100 pb−1 of the first LHC data. An efficiency of 85.5%±4.0%(stat.)±5.7%(syst.) was obtained.