Professor Daniela Bortoletto

Abstract:

The tt̄ spin correlation at production is a fundamental prediction of QCD and a potentially incisive test of new physics coupled to top quarks. We measure the tt̄ spin state in pp̄ collisions at √s=1.96TeV using 1001 candidate events in the lepton plus jets decay channel reconstructed in the CDF II detector. In the helicity basis, for a top-quark mass of 172.5GeV/c2, we find a spin correlation coefficient κ=0.60±0.50(stat)±0. 16(syst), consistent with the QCD prediction, κ 0.40. © 2011 American Physical Society.

Abstract:

The collection of a large number of B-hadron decays to hadronic final states at the CDF II Detector is possible due to the presence of a trigger that selects events based on track impact parameters. However, the nature of the selection requirements of the trigger introduces a large bias in the observed proper-decay-time distribution. A lifetime measurement must correct for this bias, and the conventional approach has been to use a Monte Carlo simulation. The leading sources of systematic uncertainty in the conventional approach are due to differences between the data and the Monte Carlo simulation. In this paper, we present an analytic method for bias correction without using simulation, thereby removing any uncertainty due to the differences between data and simulation. This method is presented in the form of a measurement of the lifetime of the B^- using the mode B^-→D0π ^-. The B^- lifetime is measured as τB ^-=1.663±0.023±0.015ps, where the first uncertainty is statistical and the second systematic. This new method results in a smaller systematic uncertainty in comparison to methods that use simulation to correct for the trigger bias. © 2011 American Physical Society.