We present the results of a search for a very light CP-odd Higgs boson $a_1^0$ originating from top quark decays t -> H^+ b -> W^+ a_1^0 b, and subsequently decaying into tau-tau. Using a data sample corresponding to an integrated luminosity of 2.7 inv fb collected by the CDF II detector in p-anti-p collisions at 1.96 TeV, we perform a search for events containing a lepton, three or more jets, and an additional isolated track with transverse momentum in the range 3 to 20 GeV/c. Observed events are consistent with background sources, and 95% C.L. limits are set on the branching ratio of t -> H^+ b for various masses of H^+ and a_1^0.

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