Professor Chris Hays

Abstract:

A search has been performed for Bs0→μ+μ- and B0→μ+μ- decays using 7fb-1 of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron collider. The observed number of B0 candidates is consistent with background-only expectations and yields an upper limit on the branching fraction of B(B0→μ+μ-)<6.0×10-9 at 95% confidence level. We observe an excess of Bs0 candidates. The probability that the background processes alone could produce such an excess or larger is 0.27%. The probability that the combination of background and the expected standard model rate of Bs0→μ+μ- could produce such an excess or larger is 1.9%. These data are used to determine B(Bs0→μ+μ-)=(1.8-0.9+1.1)×10 -8 and provide an upper limit of B(Bs0→μ+μ -)<4.0×10-8 at 95% confidence level. © 2011 American Physical Society.

Abstract:

We present a search for a new particle T′ decaying to a top quark via T′→t+X, where X goes undetected. We use a data sample corresponding to 5.7fb-1 of integrated luminosity of pp̄ collisions with √s=1.96TeV, collected at Fermilab Tevatron by the CDF II detector. Our search for pair production of T′ is focused on the hadronic decay channel, pp̄→T′T ̄′→tt̄+XX̄→bqq̄b̄ qq̄+XX̄. We interpret our results in terms of a model where T ′ is an exotic fourth generation quark and X is a dark matter particle. The data are consistent with standard model expectations. We set a limit on the generic production of T′T̄′→tt̄+XX̄, excluding the fourth generation exotic quarks T′ at 95% confidence level up to m T′=400GeV/c2 for mX≤70GeV/c2. © 2011 American Physical Society.

Abstract:

We search for resonant production of tt̄ pairs in 4.8fb-1 integrated luminosity of pp̄ collision data at √s=1.96TeV in the lepton+jets decay channel, where one top quark decays leptonically and the other hadronically. A matrix-element reconstruction technique is used; for each event a probability density function of the tt̄ candidate invariant mass is sampled. These probability density functions are used to construct a likelihood function, whereby the cross section for resonant tt̄ production is estimated, given a hypothetical resonance mass and width. The data indicate no evidence of resonant production of tt̄ pairs. A benchmark model of leptophobic Z′→tt̄ is excluded with m Z′<900GeV/c2 at 95% confidence level. © 2011 American Physical Society.

Abstract:

We present a search for new physics in events with two high pT leptons of the same electric charge, using data with an integrated luminosity of 6.1fb-1. The observed data are consistent with standard model predictions. We set 95% C.L. lower limits on the mass of doubly charged scalars decaying to like-sign dileptons, mH±±>190-245GeV/ c2, assuming 100% BR to ee, μμ or eμ. © 2011 American Physical Society.

Abstract:

This paper reports a search for nonstandard model topquark resonances, Z′, decaying to tt̄→W+bW -b̄, where both W decay to quarks. We examine the top-antitop quark invariant mass spectrum for the presence of narrow resonant states. The search uses a data sample of pp̄ collisions at a center of mass energy of 1.96 TeV collected by the CDF II detector at the Fermilab Tevatron, with an integrated luminosity of 2.8fb-1. No evidence for top-antitop quark resonant production is found. We place upper limits on the production cross section times branching ratio for a specific topcolor assisted technicolor model in which the Z′ has a width of Γ Z′=0.012MZ′. Within this model, we exclude a Z′ boson with masses below 805GeV/c2 at the 95% confidence level. © 2011 American Physical Society.