Dr. Karolos Potamianos

Abstract:

We present a measurement of the top-quark mass using a sample of tt̄ events in 5.7fb-1 of integrated luminosity from pp̄ collisions at the Fermilab Tevatron with √s=1.96TeV and collected by the CDF II Detector. We select events having large missing transverse energy, and four, five, or six jets with at least one jet tagged as coming from a b quark, and reject events with identified charged leptons. This analysis considers events from the semileptonic tt̄ decay channel, including events that contain tau leptons. The measurement is based on a multidimensional template method. We fit the data to signal templates of varying top-quark masses and background templates, and measure a top-quark mass of Mtop=172.32±2. 4(stat)±1.0(syst)GeV/c2. © 2011 American Physical Society.

Abstract:

We report the first observation of the baryonic flavor-changing neutral current decay Λb0→Λμ+μ- with 24 signal events and a statistical significance of 5.8 Gaussian standard deviations. This measurement uses a pp̄ collisions data sample corresponding to 6.8fb-1 at √s=1.96TeV collected by the CDF II detector at the Tevatron collider. The total and differential branching ratios for Λb0→Λμ+μ- are measured. We find B(Λb0→Λμ+μ-)=[1.73±0. 42(stat)±(syst)]×10-6. We also report the first measurement of the differential branching ratio of Bs0→μ+μ -, using 49 signal events. In addition, we report branching ratios for B+→K+μ+μ-, B0→K0μ+μ-, and B→K*(892) μ+μ- decays. © 2011 American Physical Society.

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.