We have used the Collider Detector at Fermilab (CDF II) to
search for the flavor-changing neutral-current (FCNC) top quark decay
t --> Zc using a technique employing ratios of W and Z production,
measured in p anti-p data corresponding to an integrated luminosity of
1.52 fb-1. The analysis uses a comparison of two decay chains, p anti-p
--> tt-bar --> WbWb --> el nu bjjb and p anti-p --> tt-bar
--> ZcWb --> ell ell c jjb, to cancel systematic uncertainties
in acceptance, efficiency, and luminosity. We validate the modeling of
acceptance and efficiency for lepton identification over the multi-year
dataset using another ratio of W and Z production, in this case the
observed ratio of inclusive production of W to Z bosons. To improve the
discrimination against standard model backgrounds to top quark decays, we
calculate the top quark mass for each event with two leptons and four jets
assuming it is a tt-bar event with one of the top quarks decaying to Zc.
For additional background discrimination we require at least one jet to
be identified as originating from a b-quark. No significant signal is
found, and we set an upper limit on the FCNC branching ratio Br(t -->
Zc) using a likelihood constructed from the ell ell cjjb top quark mass
distribution and the number of ell nu bjjb events. Limits are set as
a function of the helicity of the Z boson produced in the FCNC decay.
For 100% longitudinally polarized Z bosons we find limits of 8.3% and
9.3% (95% C.L.) depending on the assumption regarding the theoretical
top quark pair production cross-section.
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Updated: Wednesday, 2009 September 16 14:09:28 CDT automatically from input from carol picciolo