Professor Chris Hays

Abstract:

This Letter reports a search for a narrow resonant state decaying into two W bosons and two b quarks where one W boson decays leptonically and the other decays into a quark-antiquark pair. The search is particularly sensitive to top-antitop resonant production. We use the full data sample of proton-antiproton collisions at a center-of-mass energy of 1.96 TeV collected by the CDF II detector at the Fermilab Tevatron, corresponding to an integrated luminosity of 9.45 fb-1. No evidence for resonant production is found, and upper limits on the production cross section times branching ratio for a narrow resonant state are extracted. Within a specific benchmark model, we exclude a Z′ boson with mass, MZ′, below 915 GeV/c2 decaying into a top-antitop pair at the 95% credibility level assuming a Z′ boson decay width of ΓZ′=0.012M Z′. This is the most sensitive search for a narrow qq̄-initiated tt̄ resonance in the mass region below 750 GeV/c2. © 2013 American Physical Society.

Abstract:

We present a measurement of the mass difference between top (t) and antitop (t̄) quarks using tt̄ candidate events reconstructed in the final state with one lepton and multiple jets. We use the full data set of Tevatron √s=1.96 TeV proton-antiproton collisions recorded by the CDF II detector, corresponding to an integrated luminosity of 8.7 fb-1. We estimate event by event the mass difference to construct templates for top pair signal events and background events. The resulting mass difference distribution in data compared to signal and background templates using a likelihood fit yields ΔMtop=Mt-Mt̄=-1.95±1.11(stat) ±0.59(syst) GeV/c2 and is in agreement with the standard model prediction of no mass difference. © 2013 American Physical Society.

Abstract:

We present a search for new particles in an extension to the standard model that includes a heavy Higgs boson (H0), a lighter charged Higgs boson (H ±), and an even lighter Higgs boson h0, with decays leading to a W-boson pair and a bottom-antibottom quark pair in the final state. We use events with exactly one lepton, missing transverse momentum, and at least four jets in data corresponding to an integrated luminosity of 8.7 fb-1 collected by the CDF II detector in proton-antiproton collisions at √s=1.96 TeV. We find the data to be consistent with standard model predictions and report the results in terms of a simplified Higgs-cascade-decay model, setting 95% confidence level upper limits on the product of cross section and branching fraction from 1.3 pb to 15 fb as a function of H0 and H ± masses for mh0=126 GeV/c2. © 2013 American Physical Society.

Abstract:

This Letter reports a measurement of the cross section for producing pairs of central prompt isolated photons in proton-antiproton collisions at a total energy √s=1.96 TeV using data corresponding to 9.5 fb-1 integrated luminosity collected with the CDF II detector at the Fermilab Tevatron. The measured differential cross section is compared to three calculations derived from the theory of strong interactions. These include a prediction based on a leading order matrix element calculation merged with a parton shower model, a next-to-leading order calculation, and a next-to-next-to-leading order calculation. The first and last calculations reproduce most aspects of the data, thus showing the importance of higher-order contributions for understanding the theory of strong interaction and improving measurements of the Higgs boson and searches for new phenomena in diphoton final states. © 2013 American Physical Society.

Abstract:

Using 1.8 fb-1 of pp collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Υ(1S,2S,3S) mesons. Υ mesons are reconstructed using the dimuon decay mode. Total production cross sections for pT<70 GeV and in the rapidity interval |yΥ|<2.25 are measured to be, 8.01±0.02±0.36±0.31 nb, 2.05±0.01±0.12±0.08 nb, and 0.92±0.01±0. 07±0.04 nb, respectively, with uncertainties separated into statistical, systematic, and luminosity measurement effects. In addition, differential cross section times dimuon branching fractions for Υ(1S), Υ(2S), and Υ(3S) as a function of Υ transverse momentum pT and rapidity are presented. These cross sections are obtained assuming unpolarized production. If the production polarization is fully transverse or longitudinal with no azimuthal dependence in the helicity frame, the cross section may vary by approximately ±20%. If a nontrivial azimuthal dependence is considered, integrated cross sections may be significantly enhanced by a factor of 2 or more. We compare our results to several theoretical models of Υ meson production, finding that none provide an accurate description of our data over the full range of Υ transverse momenta accessible with this data set.