Professor Chris Hays

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.

Abstract:

We present a new measurement of the inclusive forward-backward tt̄ production asymmetry and its rapidity and mass dependence. The measurements are performed with data corresponding to an integrated luminosity of 5.3fb -1 of pp̄ collisions at √s=1.96TeV, recorded with the CDF-II Detector at the Fermilab Tevatron. Significant inclusive asymmetries are observed in both the laboratory frame and the tt̄ rest frame, and in both cases are found to be consistent with CP conservation under interchange of t and t̄. In the tt̄ rest frame, the asymmetry is observed to increase with the tt̄ rapidity difference, Δy, and with the invariant mass M tt̄ of the tt̄ system. Fully corrected parton-level asymmetries are derived in two regions of each variable, and the asymmetry is found to be most significant at large Δy and Mtt̄. For Mtt̄?450GeV/c2, the parton-level asymmetry in the tt̄ rest frame is Att̄=0.475±0.114 compared to a next-to-leading order QCD prediction of 0.088±0.013. © 2011 American Physical Society.

Abstract:

We present a measurement of the top quark mass in the lepton+jets and dilepton channels of tt̄ decays using the data sample corresponding to an integrated luminosity of 5.6fb-1 of pp̄ collisions at Tevatron with √s=1.96TeV, collected with the CDF II detector. We construct templates of two reconstructed top quark masses from different jets-to-quarks combinations and the invariant mass of two jets from the W decays in the lepton+jets channel, and a reconstructed top quark mass and mT2, a variable related to the transverse mass in events with two missing particles, in the dilepton channel. The simultaneous fit of the templates from signal and background events in the lepton+jets and dilepton channels to the data yields a measured top quark mass of Mtop=172.1±1.1(stat)±0. 9(syst)GeV/c2. © 2011 American Physical Society.

Abstract:

We present the report of the hadronic working group of the BOOST2010 workshop held at the University of Oxford in June 2010. The first part contains a review of the potential of hadronic decays of highly boosted particles as an aid for discovery at the LHC and a discussion of the status of tools developed to meet the challenge of reconstructing and isolating these topologies. In the second part, we present new results comparing the performance of jet grooming techniques and top tagging algorithms on a common set of benchmark channels. We also study the sensitivity of jet substructure observables to the uncertainties in Monte Carlo predictions.

Abstract:

We present a search for a new particle T' decaying to top quark via T' → t + X, where X is an invisible particle. In a data sample with 4.8  fb(-1) of integrated luminosity collected by the CDF II detector at Fermilab in pp collisions with √s = 1.96  TeV, we search for pair production of T' in the lepton + jets channel, pp → tt + X + X → ℓνbqq'b + X + X. We interpret our results primarily in terms of a model where T' are exotic fourth generation quarks and X are dark matter particles. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and 600  GeV/c2, the dark matter particle mass being anywhere below m(T'). The data are consistent with standard model expectations, and we set 95% confidence level limits on the generic production of T'T' → tt + X + X. For the dark matter model we exclude T' at 95% confidence level up to m(T') = 360  GeV/c2 for m(X) ≤ 100  GeV/c2.