Professor Chris Hays

Abstract:

We present a measurement of the top quark pair production cross section in pp̄ collisions at s=1.96TeV using 318pb-1 of data collected with the Collider Detector at Fermilab. We select tt̄ decays into the final states eν+jets and μν+jets, in which at least one b quark from the t-quark decays is identified using a secondary vertex-finding algorithm. Assuming a top quark mass of 178GeV/c2, we measure a cross section of 8.7±0.9(stat)-0. 9+1.1(syst)pb. We also report the first observation of tt̄ with significance greater than 5σ in the subsample in which both b quarks are identified, corresponding to a cross section of 10.1-1.4+1.6(stat)-1.3+2.0(syst) pb. © 2006 The American Physical Society.

Abstract:

We present the results of a search for standard model Higgs boson production with decay to WW*, identified through the leptonic final states e+e-ν̄ν,e±μ.

Abstract:

A measurement of the inclusive bottom jet cross section is presented for events containing a Z boson in pp̄ collisions at s=1.96TeV using the Collider Detector at Fermilab. Z bosons are identified in their electron and muon decay modes, and b jets with ET>20GeV and |η|<1.5 are identified by reconstructing a secondary decay vertex. The measurement is based on an integrated luminosity of about 330pb-1. A cross section times branching ratio of σ(Z+bjets)×B(Z→ℓ+ℓ-)=0.93±0.36pb is found, where B(Z→ℓ+ℓ-) is the branching ratio of the Z boson or γ* into a single flavor dilepton pair (e or μ) in the mass range between 66 and 116GeV/c2. The ratio of b jets to the total number of jets of any flavor in the Z sample, within the same kinematic range as the b jets, is 2.36±0.92%. Here, the uncertainties are the quadratic sum of statistical and systematic uncertainties. Predictions made with next-to-leading order QCD agree, within experimental and theoretical uncertainties, with these measurements. © 2006 The American Physical Society.

Abstract:

We present the first precise measurement of the Bs0-B̄s0 oscillation frequency Δms. We use 1fb-1 of data from pp̄ collisions at s=1.96TeV collected with the CDF II detector at the Fermilab Tevatron. The sample contains signals of 3600 fully reconstructed hadronic Bs decays and 37000 partially reconstructed semileptonic Bs decays. We measure the probability as a function of proper decay time that the Bs decays with the same, or opposite, flavor as the flavor at production, and we find a signal consistent with Bs0-B̄s0 oscillations. The probability that random fluctuations could produce a comparable signal is 0.2%. Under the hypothesis that the signal is due to Bs0-B̄s0 oscillations, we measure Δms=17.31-0.18+0.33(stat)±0. 07(syst)ps-1 and determine |Vtd/Vts|=0.208-0.002+0.001(expt)-0.006+0.008(theor). © 2006 The American Physical Society.

Abstract:

We present a search for t over(t, ̄) events with a tau lepton in the final state. The data sample corresponds to an integrated luminosity of 194 pb-1 collected with the CDF II detector from p over(p, ̄) collisions at a center of mass energy of 1.96 TeV. We observe two events with an expected signal of 1.0 ± 0.2 events and a background of 1.3 ± 0.3 events. We determine a 95% confidence level upper limit on rτ, the ratio of the measured rate of t → τ ν q to the expectation, of 5.2. © 2006 Elsevier B.V. All rights reserved.