Search for Supersymmetry in final states with jets, missing transverse momentum and at least one τ lepton with the ATLAS Experiment

Abstract

The first search for Supersymmetry (SUSY) in final states with at least one τ lepton, two or more jets and large missing transverse energy based on proton-proton collisions recorded with the ATLAS detector at the LHC is presented. The τ leptons are reconstructed in the hadronic decay mode. To search for new physics in final states with hadronic τ-lepton decays a reliable and efficient reconstruction algorithm for hadronic τ decays is needed. As part of this thesis two existing τ-reconstruction algorithms were further developed and integrated into a single algorithm that has by now become the standard algorithm for τ reconstruction in ATLAS. The suppression of jet background, one of the crucial aspects of τ reconstruction and the SUSY analysis in this thesis, was studied and the probabilities of misidentifying quark- or gluon-initiated jets as hadronic τ-lepton decays were measured in both the 2010 and 2011 ATLAS data at a centre-of-mass energy of √s = 7 TeV, using samples of di-jet events. The ATLAS data used for the SUSY search in this thesis was recorded between March and August 2011 and corresponds to an integrated luminosity of 2.05 fb^-1 . Eleven events are observed in data, consistent with the total Standard Model background expectation of 13.2 ± 4.2 events. As no excess of data over the expected backgrounds is observed, 95% confidence level limits are set within the framework of gauge mediated SUSY breaking (GMSB) models as a function of the GMSB parameters Λ and tan β, for fixed values of the other GMSB parameters: <em>M_mess</em> = 250 TeV, <em>N₅</em> = 3, sign(µ) = + and C_grav = 1. In addition to the GMSB interpretation, a model-independent upper limit of 8.5 on the number of events from potential non-Standard Model sources is derived at the 95% confidence level.