A limit on the branching ratio of the flavor-changing top quark decay t → Zc

We have used the Collider Detector at Fermilab (CDF-II) to set upper limits on the branching ratio of the flavor-changing neutral-current (FCNC) top quark decay t → Zc using a technique employing ratios of W and Z production, measured in 1.52 fb--1 of pp data. The analysis uses a comparison of two decay chains, pp¯ → tt¯ → WbWb → ℓ vbjjb and pp¯ → tt¯ → ZcWb → ℓ+ℓ -- cjjb, to cancel systematic uncertainties in acceptance, efficiency, and luminosity. We validate the MC modeling of acceptance and efficiency for lepton identification over the multi-year dataset also using a ratio of W and Z production, in this case the observed ratio of inclusive production of W to Z-bosons, a technique that will be essential for precision comparisons with the standard model at the LHC. We introduce several methods of determining backgrounds to the W and Z samples. To improve the discrimination against SM backgrounds to top quark decays, we calculate the top mass for each event with two leptons and four jets assuming it is a tt¯ event with one of the top quarks decaying to Zc. The upper limit on the Br( t → Zc) is estimated from a likelihood constructed with the ℓ+ℓ-- cjjb top mass distribution and the number of ℓvbjjb 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 a limit of 8.3% (95% C.L.).