Measurement of the tau misidentification probability from QCD jets with ATLAS data at $\sqrt{s}$ = 7 TeV

In 2010, the ATLAS experiment recorded 45 pb − 1 of data from proton-proton collisions at a centre- of-mass energy of 7TeV at the LHC. In many signatures of the Standard Model and in searches for new physics the τ -lepton plays an important role. It is identified in the detector by the decay products of its hadronic decay channel. However, the signature in the detector is similar to the signature of QCD jets, which are therefore the main background in studies with τ -leptons. In this thesis the probability to reconstruct and identify a QCD jet as a hadronically decaying τ -lepton is studied. A tag-and-probe method is used to determine the misidentification probability directly from data for the current ATLAS τ identification methods: a cut based identification, boosted decision trees and a projective likelihood. The focus lies on the measurement of the misidentification probability as a function of suitable variables and the determination of systematic uncertainties.