Performance of the Reconstruction and Identification of Hadronic Tau Decays in ATLAS with 2011 Data

This note describes updates to the reconstruction and identification algorithms developed to efficiently select hadronically decaying τ leptons (τ_had-vis) in ATLAS. These updates are with respect to the baseline algorithms applied to the 2010 data. Two independent multivariate methods to discriminate τ_had-vis from QCD jets have been re-optimised. Another identification method to reject electrons mis-identified as τ_had-vis has also been re-optimised. For a signal identification efficiency of approximately 50%, rejection factors in the range 20–130 against QCD jets and approximately 300 against electrons are achieved. A new procedure has also been developed to reject mis-identified muons. For a signal identification efficiency of 96%, a muon rejection factor of approximately 2 is achieved. Four different “tag-and-probe” measurements of the τ_had-vis identification efficiency and the electron to τ_had-vis mis-identification probability based on 2011 data are presented. The measured τ_had-vis identification efficiencies are in agreement with those estimated using Monte Carlo simulation, within uncertainties of 4-5% for τhad-vis pT > 22 GeV.