We present the results of a search for a very light $CP$-odd Higgs boson $a_1^0$ originating from top quark decays $t\to H^{\pm }b\to W^{{\pm }^{(*)}}{a}_1^{0}b$, and subsequently decaying into ${\tau }^{+}{\tau }^{-}$. Using a data sample corresponding to an integrated luminosity of $2.7{\mathrm{fb}}^{-1}$ collected by the CDF II detector in $p\overline{p}$ 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 $20\mathrm{GeV}/c$. Observed events are consistent with background sources, and 95% C.L. limits are set on the branching ratio of $t\to H^{\pm }b$ for various masses of $H^{\pm }$ and $a_1^0$.

• Received 2 May 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.031801