Professor Chris Hays

Abstract:

A measurement of event shape variables is presented for large momentum transfer proton-proton collisions using the ATLAS detector at the Large Hadron Collider. Six event shape variables calculated using hadronic jets are studied in inclusive multi-jet events in 35 pb^-1 of integrated luminosity at a center-of-mass energy of √s = 7 TeV. These measurements are compared to predictions by three Monte Carlo event generators containing leading-logarithmic parton showers matched to leading order matrix elements for 2→2 and 2→n (n = 2, …, 6) scattering. Measurements of the third-jet resolution parameter, aplanarity, thrust, sphericity, and transverse sphericity are generally well described. The mean value of each event shape variable is evaluated as a function of the average momentum of the two leading jets pT,1 and pT,2, with a mean pT approaching 1 TeV.

Abstract:

A search for supersymmetry (SUSY) in events with large missing transverse momentum, jets, and at least one hadronically decaying τ lepton, with zero or one additional light lepton (e/μ), has been performed using 4.7 fb−1of 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 % confidence level visible cross-section upper limit for new phenomena is set. In the framework of gauge-mediated SUSY-breaking models, lower limits on the mass scale Λ are set at 54 TeV in the regions where the τ1is the next-to-lightest SUSY particle (tanβ > 20). These limits provide the most stringent tests to date of GMSB models in a large part of the parameter space considered.

Abstract:

We present the first direct measurement of the top-quark mass using tt̄ events decaying in the hadronic τ+jets decay channel. Using data corresponding to an integrated luminosity of 2.2fb-1 collected by the CDF II detector in pp̄ collisions at √s=1.96TeV at the Fermilab Tevatron, we measure the tt̄ cross section, σtt̄, and the top-quark mass, Mtop. We extract Mtop from a likelihood based on per-event probabilities calculated with leading-order signal and background matrix elements. We measure σtt̄=8. 8±3.3(stat)±2.2(syst)pb and Mtop=172.7±9.3(stat) ±3.7(syst)GeV/c2. © 2012 American Physical Society.

Abstract:

The combination of searches performed by the CDF and D0 collaborations at the Fermilab Tevatron Collider for neutral Higgs bosons produced in association with b quarks is reported. The data, corresponding to 2.6fb -1 of integrated luminosity at CDF and 5.2fb -1 at D0, have been collected in final states containing three or more b jets. Upper limits are set on the cross section multiplied by the branching ratio varying between 44 pb and 0.7 pb in the Higgs boson mass range 90 to 300 GeV, assuming production of a narrow scalar boson. Significant enhancements to the production of Higgs bosons can be found in theories beyond the standard model, for example, in supersymmetry. The results are interpreted as upper limits in the parameter space of the minimal supersymmetric standard model in a benchmark scenario favoring this decay mode. © 2012 American Physical Society.