Reconstruction and Identification of Hadronic Decays of Tau Leptons in ATLAS

Hadronically decaying tau leptons are of prime importance in numerous physics analyses in ATLAS. The spectrum of the possible applications of hadronically decaying tau leptons reaches from Standard Model measurements, including Higgs searches, to searches for physics beyond the Standard Model. Tau leptons are nowadays at the forefront of searches for new physics in high energy physics, especially after the, recently found, strong evidence for the decay of the Standard Model Higgs boson to pairs of tau leptons by the ATLAS and CMS collaborations. This particular decay mode can provide a direct measurement of the coupling of the Higgs boson to fermions, henceforth testing an important prediction of the theory manifesting that fermions acquire their mass through the Higgs mechanism. Consequently, the reconstruction and identification algorithms of hadronically-decaying tau leptons, have to be continuously estimated (or adjusted) and improved to account for the most up-to-date running conditions and to succeed at overcoming the unprecedented experimental challenges faced by the LHC running at higher than ever energies and luminosities. The basic principles behind the sophisticated tau reconstruction and identification techniques, which are specifically designed to identify hadronically-decaying taus and reject various background processes, are delineated (or described). Special emphasis is placed on the 2012 LHC data-taking period, for which the tau reconstruction and identification schemes have been specifically re-optimized for the high number of simultaneous collisions per proton-proton bunch crossing (pile-up). The resulting performance of the identification algorithms is presented, exhibiting efficiencies which are independent of pile-up. Estimates of the signal efficiency and background rejection based on studies of simulations and data-driven methods are provided. Finally, novel developments are presented, geared towards the next LHC run, which exploit the tau substructure to provide higher identification efficiencies and more accurate four-momentum resolutions.