Professor Chris Hays

Abstract:

We report a measurement of the forward-backward charge asymmetry of electrons from W boson decays in pp̄ collisions at √s=1.96 TeV using a data sample of 170 pb^-1 collected by the Collider Detector at Fermilab. The asymmetry is measured as a function of electron rapidity and transverse energy and provides new input on the momentum fraction dependence of the u and d quark parton distribution functions within the proton. © 2005 The American Physical Society.

Abstract:

Using 180 pb ^-1 of data collected with the CDF II detector at the Tevatron, we measure the first two moments of the hadronic invariant mass-squared distribution in charmed semileptonic B decays. From these we determine the nonperturbative Heavy Quark Effective Theory parameters Λ and λ 1 used to relate the B meson semileptonic branching ratio to the CKM matrix element |V cb|. For a minimum lepton momentum of 0.7 GeV/c in the B rest frame we measure the first two moments of the D** → D ^(*)π component to be 〈m D** ²〉 = (5.83 ± 0.16 stat ± 0.08 syst) GeV ²/c ⁴ and 〈(m D** ² - 〈m D** ²〉) ²〉 = (1.30 ± 0.69 stat ± 0.22 syst) GeV ⁴/c ⁸. Combining these with the discrete mass terms from the D and D* mesons, we find the total moments to be 〈M Ξc ²〉 - m̄ D ² = (0.467 ± 0.038 stat ± 0.068 syst GeV ²/c ⁴ and 〈(M Ξc ² - 〈M Ξc ²〉) ²〉 = (1.05 ± 0.26 stat ± 0.13 syst) GeV ⁴/c ⁸, where m̄ D is the spin-averaged D mass. The systematic error is dominated by the uncertainties in the world-average branching ratios used to combine the D, D*, and D** contributions. The analysis makes no assumptions about the shape or resonant structure of the D** → D (*)π invariant mass distribution. © 2005 The American Physical Society.

Abstract:

We present a measurement of the tt̄ production cross section using events with one charged lepton and jets from pp̄ collisions at a center-of-mass energy of 1.96 TeV. In these events, heavy flavor quarks from top quark decay are identified with a secondary vertex tagging algorithm. From 162 pb^-1 of data collected by the Collider Detector at Fermilab, a total of 48 candidate events are selected, where 13.5 ± 1.8 events are expected from background contributions. We measure a tt̄ production cross section of 5.6-1.1^+1.2(stat.)-0.6^+0.9(syst.) pb. © 2005 The American Physical Society.

Abstract:

Using the exclusive decay Bs⁰ → J/ψ (μ⁺μ^-)φ (K⁺K^-), we report the most precise single measurement of the Bs⁰ lifetime. The data sample corresponds to an integrated luminosity of approximately 220 pb^-1 collected with the DO detector at the Fermilab Tevatron Collider in 2002-2004. We reconstruct 337 signal candidates, from which we extract the Bs⁰ lifetime, τ(Bs⁰) = 1.444-0.090^+0.098(stat) ± 0.020(sys) ps. We also report a measurement for the lifetime of the B⁰ meson using the exclusive decay B⁰ → J/ψ(μ⁺μ^-) K*⁰(&92)(K⁺π^-). We reconstruct 1370 signal candidates, obtaining τ(B⁰) = 1.473 -0.050^+0.052(stat)±0.023(sys) ps, and the ratio of lifetimes, τ(Bs⁰)/τ(B⁰) = 0.980 -0.0.71^0.076(stat) ± 0.003(sys). © 2005 The American Physical Society.

Abstract:

We report the results of a search for supersymmetry (SUSY) with gauge-mediated breaking in the missing transverse energy distribution of inclusive diphoton events using 263 pb-1 of data collected by the DO experiment at the Fermilab Tevatron Collider in 2002-2004. No excess is observed above the background expected from standard model processes, and lower limits on the masses of the lightest neutralino and chargino of about 108 and 195 GeV, respectively, are set at the 95% confidence level. These are the most stringent limits to date for models with gauge-mediated SUSY breaking with a short-lived neutralino as the next-to-lightest SUSY particle. © 2005 The American Physical.