Search for the neutral current top quark decay tZc using the ratio of Z-boson+4jets to W-boson+4jets production

T. Aaltonen et al. (CDF Collaboration)
Phys. Rev. D 80, 052001 – Published 8 September 2009

Abstract

We have used the Collider Detector at Fermilab (CDF-II) to search for the flavor-changing neutral-current (FCNC) top-quark decay tZc using a technique employing ratios of W and Z production, measured in pp¯ data corresponding to an integrated luminosity of 1.52fb1. The analysis uses a comparison of two decay chains, pp¯tt¯WbWbνbjjb and pp¯tt¯ZcWbcjjb, to cancel systematic uncertainties in acceptance, efficiency, and luminosity. We validate the modeling of acceptance and efficiency for lepton identification over the multiyear data set 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¯ 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(tZc) using a likelihood constructed from the cjjb top-quark mass distribution and the number of νbjjb 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 assumptions regarding the theoretical top-quark pair production cross section.

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  • Received 4 May 2009

DOI:https://doi.org/10.1103/PhysRevD.80.052001

©2009 American Physical Society

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Vol. 80, Iss. 5 — 1 September 2009

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