Professor Chris Hays

Abstract:

We report a measurement of the single-top-quark production cross section in 2.2fb-1 of pp̄ collision data collected by the Collider Detector at Fermilab at s=1.96TeV. Candidate events are classified as signal-like by three parallel analyses which use likelihood, matrix element, and neural network discriminants. These results are combined in order to improve the sensitivity. We observe a signal consistent with the standard model prediction, but inconsistent with the background-only model by 3.7 standard deviations with a median expected sensitivity of 4.9 standard deviations. We measure a cross section of 2.2-0.6+0.7(stat+syst)pb, extract the Cabibbo-Kobayashi-Maskawa matrix-element value |Vtb|=0.88-0.12+0.13(stat+syst)±0.07(theory), and set the limit |Vtb|>0.66 at the 95% C.L. © 2008 The American Physical Society.

Abstract:

We use the three lepton and missing energy trilepton signature to search for chargino-neutralino production with 2.0fb-1 of integrated luminosity collected by the CDF II experiment at the Tevatron pp collider. We expect an excess of approximately 11 supersymmetric events for a choice of parameters of the mSUGRA model, but our observation of 7 events is consistent with the standard model expectation of 6.4 events. We constrain the mSUGRA model of supersymmetry and rule out chargino masses up to 145GeV/c2 for a specific choice of parameters. © 2008 The American Physical Society.

Abstract:

We present a search for the Higgs boson in the process qq̄→ ZH→ℓ+ℓ-bb̄. The analysis uses an integrated luminosity of 1fb-1 of pp̄ collisions produced at s=1.96TeV and accumulated by the upgraded Collider Detector at Fermilab (CDF II). We employ artificial neural networks both to correct jets mismeasured in the calorimeter and to distinguish the signal kinematic distributions from those of the background. We see no evidence for Higgs boson production, and set 95% C.L. upper limits on σZHB(H→bb̄), ranging from 1.5 to 1.2 pb for a Higgs boson mass (mH) of 110 to 150GeV/c2. © 2008 The American Physical Society.

Abstract:

We present the first measurement of σ(gg→tt̄)/ σ(pp̄→tt̄). We use 0.96fb-1 of s=1.96TeV pp̄ collision data recorded with the CDF II detector at Fermilab. Using charged particles with low transverse momentum in tt̄ events, we find σ(gg→ tt̄)/σ(pp̄→tt̄)=0.07±0.14(stat)±0. 07(syst), corresponding to a 95% confidence level upper limit of 0.33, in agreement with the standard model next-to-leading-order prediction of 0.15±0.05. © 2008 The American Physical Society.

Abstract:

The Standard Model of particle physics has a number of asymmetries suggestive of a more fundamental theory, where the symmetries are restored. If the symmetry breaking scale is the electroweak scale, new Higgs bosons beyond that of the Standard Model could be observed at the Tevatron. Recent Tevatron searches have set the most stringent limits on new Higgs bosons in a variety of models.