An improved tau-Identification for the ATLAS experiment

This note documents the work done to study the tau-jets separation in ATLAS using the large event samples full simulated in the DC1 context. The results obtained complete and improve the results shown in the ATLAS Detector and Physics Performance Technical Design Report (TDR). The criteria used to separate taus from jets are based on information both from the calorimeters and from the Inner detector. The tau-identification and the corresponding jet-rejection depend on the jet ET, on the eta position, on the jet-type and also on the event type. Significant quantities to distinguish between taus and jets were studied and the separation cuts were optimised to give the best tau-identification/jet rejection ratio. A Likelihood function was also built starting from those significant quantities and again the best cut on that was studied. The cuts were tuned both for events without and with electronic noise added in calorimeter cells. A parametrisation of the jet-rejection vs the tau-identification, based on the studies done in full simulation was prepared for the fast simulation program in ATLAS (ATLFAST), trying to take into account the dependence on the quantities listed above.