Search for the neutral current top quark decay t→Zc using the ratio of Z-boson+4 jets to W-boson+4 jets production
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AuthorAaltonen, Timo Antero; Akimoto, Takashi; Álvarez González, Bárbara; Amerio, Silvia; Amidei, Dante; Anastassov, Anton; Annovi, Alberto; Antos, Jaroslav; Apollinari, Giorgio; Apresyan, Artur; Casal Laraña, Bruno; Cuevas Maestro, Francisco Javier; Gómez Gramuglio, Gervasio; Rodrigo Anoro, Teresa; Ruiz Jimeno, Alberto; Scodellaro, Luca; Vila Álvarez, Iván; Vilar Cortabitarte, Rocío; Arisawa, Tetsuo; [et al.]
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 pp̅ data corresponding to an integrated luminosity of 1.52 fb-1. The analysis uses a comparison of two decay chains, pp̅ →tt̅ →WbWb→ℓνbjjb and pp̅ →tt̅ →ZcWb→ℓℓcjjb, 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 tt̅ 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 ℓℓcjjb top-quark mass distribution and the number of ℓνbjjb 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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