Observation of the $W \to \tau\nu$ process in the ATLAS Experiment

The ATLAS Experiment (A Toroidal LHC ApparatuS) [1] is a multi-purpose detector, operating at the LHC (Large Hadron Collider) [2]. In this thesis I present the first observation of W → τντ decays in ATLAS with pp collisions at 7 TeV centre-of-mass energy. The amount of data used corresponds to an integrated luminosity of 2.46±0.27 pb−1. The W → τντ decays are searched in the channel in which the tau decays into a ντ and a hadronic system. These events are characterized by the presence of a hadronic jet (τ-jet) and missing energy, due to the undetected neutrinos from W and tau decays. The W → τντ channel, with a cross section 10 times greater than the Z → ττ, is the most abundant source of taus in first data and therefore provides the first opportunity to see real hadronically decaying taus. These decays are difficult to identify in hadronic collisions, where QCD jets are produced with a much higher cross section. A tau-jet can be distinguished from QCD jets by exploiting its characteristics, such as its narrowness and the low number of associated tracks. The main backgrounds to W → τhντ decays are leptonic decays of W and Z bosons (EW background) and QCD jets. As the uncertainties connected with the QCD jets production (both on the shape and on the cross section of the process) are big, this background has been estimated directly from data, without relying on the simulation. As the main aspects of the analysis concern the identification of hadronically decaying taus, the measurement of the missing energy and the trigger, they are separately discussed in three chapters. Chapter 2 describes how the identification of hadronically decaying taus is performed in ATLAS. My contribution concerns the study of the performance of τh identification for events characterised by the presence of a hadronic tau candidate and missing energy, which is the signature of W → τhντ decays. Chapter 3 explains how the amount of the missing energy is measured. The performance on its reconstruction are shown and the possible sources of fake missing energy explained. As missing energy and related quantities play a fundamental role in the W → τhντ analysis, these quantities have been carefully checked. In Chapter 4 the triggering aspects are described. In particular, I evaluated the efficiency of the trigger chosen for the analysis on the selected events. Finally, in Chapter 5 the analysis that brought to the observation of W → τhντ decays and first hadronically decaying taus in ATLAS is explained.