Professor Chris Hays

Abstract:

We present the results of a search for a very light CP-odd Higgs boson a10 originating from top quark decays t→H⊃±b→W⊃± (⊃*)a10b, and subsequently decaying into τ⊃+τ⊃-. Using a data sample corresponding to an integrated luminosity of 2.7fb⊃-1 collected by the CDF II detector in pp̄ collisions at 1.96TeV, we perform a search for events containing a lepton, three or more jets, and an additional isolated track with transverse momentum in the range 3 to 20GeV/c. Observed events are consistent with background sources, and 95% C.L. limits are set on the branching ratio of t→H⊃±b for various masses of H⊃± and a10. © 2011 American Physical Society.

Abstract:

We report on a search for anomalous production of Z boson pairs through a massive resonance decay in data corresponding to 2.5-2.9fb-1 of integrated luminosity in pp̄ collisions at √s=1.96TeV using the CDF II detector at the Fermilab Tevatron. This analysis, with more data and channels where the Z bosons decay to muons or jets, supersedes the 1.1fb-1 four-electron channel result previously published by CDF. In order to maintain high efficiency for muons, we use a new forward tracking algorithm and muon identification requirements optimized for these high signal-to-background channels. Predicting the dominant backgrounds in each channel entirely from sideband data samples, we observe four-body invariant mass spectra above 300GeV/c2 that are consistent with background. We set limits using the acceptance for a massive graviton resonance that are 7-20 times stronger than the previously published direct limits on resonant ZZ diboson production. © 2011 American Physical Society.

Abstract:

A study of event-shape observables in proton-antiproton collisions at √s=1.96TeV is presented. The data for this analysis were recorded by the CDF II Detector at the Tevatron Collider. The variables studied are the transverse thrust and thrust minor, both defined in the plane perpendicular to the beam direction. The observables are measured using energies from unclustered calorimeter cells. In addition to studies of the differential distributions, we present the dependence of event-shape mean values on the leading-jet transverse energy. Data are compared with pythia Tune A and to resummed parton-level predictions that were matched to fixed-order results at next-to-leading-order (NLO) accuracy (NLO+NLL). Predictions from pythia Tune A agree fairly well with the data. However, the underlying event contributes significantly to these observables, making it difficult to make direct comparisons to the NLO+NLL predictions, which do not account for the underlying event. To overcome this difficulty, we introduce a new observable, a weighted difference of the mean values of the thrust and thrust minor, which is less sensitive to the underlying event, allowing for a comparison with NLO+NLL. Both pythia Tune A and the NLO+NLL calculations agree well within the 20% theoretical uncertainty with the data for this observable, indicating that perturbative QCD successfully describes shapes of the hadronic final states. © 2011 American Physical Society.

Abstract:

We report on the first measurement of the angular distributions of final state electrons in pp̄→γ*/Z→e +e-+X events produced in the Z boson mass region at √s=1.96TeV. The data sample collected by the CDF II detector for this result corresponds to 2.1fb-1 of integrated luminosity. The angular distributions are studied as a function of the transverse momentum of the electron-positron pair and show good agreement with the Lam-Tung relation, consistent with a spin-1 description of the gluon, and demonstrate that, at high values of the transverse momentum, Z bosons are produced via quark-antiquark annihilation and quark-gluon Compton processes. © 2011 American Physical Society.