Prof Dr Armin Reichold

Abstract:

We report a measurement of the cross section of single top-quark production in the t-channel using 1.04 fb-1 of pp collision data at s=7 TeV recorded with the ATLAS detector at the LHC. Selected events feature one electron or muon, missing transverse momentum, and two or three jets, exactly one of them identified as originating from a b quark. The cross section is measured by fitting the distribution of a multivariate discriminant constructed with a neural network, yielding σt=83±4(stat.)-19 +20(syst.) pb, which is in good agreement with the prediction of the Standard Model. Using the ratio of the measured to the theoretically predicted cross section and assuming that the top-quark-related CKM matrix elements obey the relation |Vtb|≫|Vts|, |Vtd|, the coupling strength at the W-t-b vertex is determined to be |Vtb|=1.13-0.13 +0.14. If it is assumed that |Vtb|≤1 a lower limit of |Vtb|>0.75 is obtained at the 95% confidence level. © 2012 CERN.

Abstract:

The results of a search for the production of second generation scalar leptoquarks are presented for final states consisting of either two muons and at least two jets or a muon plus missing transverse momentum and at least two jets. A total of 1.03 fb⁻¹ integrated luminosity of protonproton collision data produced by the Large Hadron Collider at √s = 7 TeV and recorded by the ATLAS detector is used for the search. The event yields in the signal regions are found to be consistent with the Standard Model background expectations. The production of second generation leptoquarks is excluded for a leptoquark mass mLQ < 594 (685) GeV at 95 % confidence level, for a branching ratio of 0.5 (1.0) for leptoquark decay to a muon and a quark.

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 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.