Jeff Tseng

Abstract:

The results of a search for pair production of the lighter scalar partners of top quarks (t~1) in 2.05 fb^-1 of pp collisions at s=7 TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e, μ) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking 'natural' scenarios, where the neutralino (χ~10) is the next-to-lightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115GeVmZ. © 2012 CERN.

Abstract:

This Letter presents a search for tb resonances in 1.04fb-1 of LHC proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 7 TeV. Events with a lepton, missing transverse momentum, and two jets are selected and the invariant mass of the corresponding final state is reconstructed. The search exploits the shape of the tb invariant mass distribution compared to the expected standard model backgrounds. The model of a right-handed WR′ with standard model-like couplings is chosen as the benchmark model for this search. No statistically significant excess of events is observed in the data, and upper limits on the cross section times the branching ratio of WR′ resonances at 95% C.L. lie in the range of 6.1-1.0 pb for WR′ masses ranging from 0.5 to 2.0 TeV. These limits are translated into a lower bound on the allowed right-handed WR′ mass, giving m WR′>1.13TeV at 95% C.L. © 2012 CERN.

Abstract:

A search is reported for the pair production of a new quark b′ with at least one b′ decaying to a Z boson and a bottom quark. The data, corresponding to 2.0fb-1 of integrated luminosity, were collected from pp collisions at √s=7TeV with the ATLAS detector at the CERN Large Hadron Collider. Using events with a b-tagged jet and a Z boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum b′ candidates is tested for an enhancement. No evidence for a b′ signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of b′ quarks with masses m b′<400GeV that decay entirely via b′→Z+b. In the case of a vectorlike singlet b′ mixing solely with the third standard model generation, masses mb′<358GeV are excluded. © 2012 CERN.

Abstract:

A search for events with large missing transverse momentum, jets, and at least two tau leptons has been performed using 2 fb-1 of proton-proton collision data at s=7 TeV recorded with the ATLAS detector at the Large Hadron Collider. No excess above the Standard Model background expectation is observed and a 95% CL upper limit on the visible cross section for new phenomena is set, where the visible cross section is defined by the product of cross section, branching fraction, detector acceptance and event selection efficiency. A 95% CL lower limit of 32 TeV is set on the gauge-mediated supersymmetry breaking (GMSB) scale Λ independent of tanβ. These limits provide the most stringent tests to date in a large part of the considered parameter space. © 2012.

Abstract:

A search for production of supersymmetric particles in final states containing jets, missing transverse momentum, and at least one hadronically decaying τ lepton is presented. The data were recorded by the ATLAS experiment in s=7TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 2.05fb-1 of data. The results are interpreted in the context of gauge mediated supersymmetry breaking models with Mmess=250TeV, N5=3, μ>0, and Cgrav=1. The production of supersymmetric particles is excluded at 95% C.L. up to a supersymmetry breaking scale Λ=30TeV, independent of tanβ, and up to Λ=43TeV for large tanβ. © 2012.