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

A search for Supersymmetry (SUSY) in final states with at least one lepton and large missing transverse energy based on proton-proton collisions recorded with the ATLAS detector at the LHC is presented. The leptons are reconstructed in the hadronic decay mode. Final states with leptons o er a good sensitivity for SUSY models where the coupling to the third generation of fermions is enhanced, e.g. gauge mediated SUSY breaking (GMSB) models, for which the supersymmetric partner of the lepton is the next-to-lightest SUSY particle (NLSP) and its decay to electrons or muons is strongly suppressed. To search for new physics in final states with hadronic -lepton decays a reliable and e cient re- construction algorithm for hadronic decays is needed to separate real decays and backgrounds from quark- or gluon-initiated jets and electrons. As part of this thesis two existing -reconstruction al- gorithms were further developed and integrated into a single algorithm that has by now become the standard algorithm for reconstruction in ATLAS. The suppression of jet background, one of the cru- cial aspects of reconstruction and the SUSY analysis in this thesis, was studied and the probabilities of misidentifying quark- or gluon-initiated jets as hadronic -lepton decays were measured in both the 2010 and 2011 ATLAS data at a centre-of-mass energy of p 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 . Events are selected that contain at least two jets with high transverse momentum, one identified hadronic -lepton decay and large missing transverse energy. The Standard Model background processes that contribute most to the selected event sample are W - and Z -boson production in association with jets as well as top quark production. 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 GMSB models as a function of the GMSB parameters and tan , for fixed values of the other GMSB parameters: M mess = 250 TeV, N 5 = 3, sign( ) = + 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. This limit corresponds to an upper limit on the visible cross section of 4 : 0 fb, where the visible cross section is defined as the product of production cross section of the non-Standard Model process, its branching fraction to at least one lepton, acceptance, and e ciency of the event selection applied. These limits are the first results of SUSY searches at the LHC that include events with one hadronic -lepton decay and significantly extend those placed by previous experiments at LEP and the Tevatron.