Search For The Flavor Changing Neutral Current Decay T To Zq In P Barp Collisions At Sqrts

We report a search for the flavor changing neutral current (FCNC) decay of the top quark t → Zq (q = u, c) in p{bar p} collisions at √s = 1.96 TeV using a data sample corresponding to an integrated luminosity of 1.9 fb−1 collected by the CDF II detector. This decay is strongly suppressed in the standard model (SM) and a signal at the Tevatron would be an indication of physics beyond the SM. Using Z+ ≥ 4 jet final state candidate events, both with and without an identified bottom quark jet, we discriminate signal from background by exploring kinematic constraints present in FCNC events and obtain an upper limit of [Beta](t → Zq)

This dissertation presents a blind search for the flavor changing neutral current decay of the top quark t → qZ in pp collisions at [special characters omitted] = 1.96TeV using a data sample corresponding to an integrated luminosity of 1.12 fb-1 collected by the Collider Detector at Fermilab (CDF), This decay is extremely rare in the standard model, and a signal at the Tevatron would be an indication of new physics. Dividing candidate events with a Z boson and four or more jets into a sample of those which have a heavy flavor jet identified by a secondary vertex algorithm and those which do not, we observe data yields consistent with background expectations. We set a 95% C.L. upper limit on the branching fraction B (t → qZ) of 11.3%, consistent with an expected upper limit of 7.8% ± 3.3%.

This thesis reports the results of a search for the flavor changing neutral current decay of the top quark, t → Zq, in decays of t{bar t} pairs produced in p{bar p} collisions at a center-of-mass energy of 1.96 TeV. This search is performed on a data sample recorded by the Collider Detector at Fermilab (CDF), corresponding to an integrated luminosity of 1.9 fb−1. This search follows a previous CDF analysis that resulted in an upper limit for the branching fraction [Beta](t → Zq) of 10.4% at 95% C.L. using a dataset equivalent to 1.1 fb−1 of integrated luminosity. This thesis extends to 1.9 fb−1 of data, and has improved sensitivity to the small signal with the introduction of a template fit technique that includes systematic uncertainties by a linear interpolation between templates. Using a Feldman-Cousins construction, an upper limit at 95% C.L. is set on [Beta](t → Zq) of 3.7%, with the expected upper limit in absence of a signal is 5.0 ± 2.2% for a top mass of 175 GeV/c2.

We report on a search for the flavor-changing neutral-current decay D° →??− in p{bar p} collisions at √s = 1.96TeV using 360 pb−1 of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron collider. A displaced vertex trigger selects long-lived D° candidates in the?+?−,?+?−, and K−?+ decay modes. We use the Cabibbo-favored D° → K−?+ channel to optimize the selection criteria in an unbiased manner, and the kinematically similar D° →?+?− channel for normalization. We set an upper limit on the branching fraction?(D° →?+?−)

This thesis reports the results of a search for the flavor changing neutral current decay of the top quark, t → Zq, in decays of t$ar{t}$ pairs produced in p$ar{p}$ collisions at a center-of-mass energy of 1.96 TeV. This search is performed on a data sample recorded by the Collider Detector at Fermilab (CDF), corresponding to an integrated luminosity of 1.9 fb-1. This search follows a previous CDF analysis that resulted in an upper limit for the branching fraction ?(t → Zq) of 10.4% at 95% C.L. using a dataset equivalent to 1.1 fb-1 of integrated luminosity. This thesis extends to 1.9 fb-1 of data, and has improved sensitivity to the small signal with the introduction of a template fit technique that includes systematic uncertainties by a linear interpolation between templates. Using a Feldman-Cousins construction, an upper limit at 95% C.L. is set on ?(t → Zq) of 3.7%, with the expected upper limit in absence of a signal is 5.0 ± 2.2% for a top mass of 175 GeV/c2.

Presented here are the results of a direct search for flavor changing neutral currents via the rare process Z° → bs and a measurement of R{sub bs} = [Lambda](Z° → bs)/[Lambda](Z° → hadrons). Because the decays Z° → b{bar b} and Z° → c{bar c} contribute significant backgrounds to Z° → bs, simultaneous measurements of R{sub b} = [Lambda](Z° → b{bar b})/[Lambda](Z° → hadrons) and R{sub c} = [Lambda](Z° → c{bar c})/[Lambda](Z° → hadrons) were also made. The standard double tag technique was extended and self calibrating tags were used for s, c, and b quarks. These measurements were made possible by the unique capabilities of the SLAC Large Detector (SLD) at the Stanford Linear Accelerator Center (SLAC): The b and c tags relied upon the SLD's VXD3 307 megapixel CCD vertex detector for topological and kinematic reconstruction of the B and D decay vertices; the s tag identified K{sup {+-}} mesons using the particle identification capabilities of SLD's Cherenkov Ring Imaging Detector (CRID), and K{sub S}° mesons and [Lambda] hadrons by kinematic reconstruction of their decay vertices in SLD's 5120 channel central drift chamber (CDC) particle tracking system.

We report on a search for the flavor-changing neutral-current decays K/sub L/° .-->. .pi.°ee/sup /minus//, K/sub L/° .-->. ee/sup /minus// and K/sub L/° .-->. .mu./sup +-/e/sup /minus plus//. Limits obtained for these processes are BR(K/sub L/° .-->. .pi.°e+e/sup /minus//) 3.2 x 10/sup /minus/7/, BR(K/sub L/° .--. e+e/sup /minus//) 1.2 x 10/sup /minus/9/, BR(K/sub L/° .--. .mu./sup +-/e/sup /minus plus//)