Studies of ${\tau }^{-}\to \eta K^{-}{\nu }_{\tau }$ and ${\tau }^{-}\to \eta {\pi }^{-}{\nu }_{\tau }$ at BABAR and a search for a second-class current

We report on analyses of tau lepton decays ${\tau }^{-}\to \eta K^{-}{\nu }_{\tau }$ and ${\tau }^{-}\to \eta {\pi }^{-}{\nu }_{\tau }$, with $\eta \to {\pi }^{+}{\pi }^{-}{\pi }^0$, using $470{\mathrm{fb}}^{-1}$ of data from the BABAR experiment at PEP-II, collected at center-of-mass energies at and near the $\mathit{{\rm Y}}(4S)$ resonance. We measure the branching fraction for the ${\tau }^{-}\to \eta K^{-}{\nu }_{\tau }$ decay mode, $\mathcal{B}({\tau }^{-}\to \eta K^{-}{\nu }_{\tau })=(1.42\pm 0.11(\mathrm{stat})\pm 0.07(\mathrm{syst}))\times {10}^{-4}$, and report a 95% confidence level upper limit for the second-class current process ${\tau }^{-}\to \eta {\pi }^{-}{\nu }_{\tau }$, $\mathcal{B}({\tau }^{-}\to \eta {\pi }^{-}{\nu }_{\tau })<9.9\times {10}^{-5}$.

Figure 1

Mass spectra for ${\pi }^{+}{\pi }^{-}{\pi }^0$ in ${\tau }^{-}\to {\pi }^{+}{\pi }^{-}{\pi }^0{K}^{-}{\nu }_{\tau }$ candidates, for (a) $e$-tag data and (b) $\mu$-tag data. The curves show the results of the fits described in the text. Note the suppressed zero on the $y$ axes.

Figure 2

The $\eta K^{-}$ mass distributions for the data and MC samples, for $e$- and $\mu$-tag events, obtained from the sideband subtraction method as described in the text. The MC samples are normalized to the data luminosity; in particular, the ${\tau }^{-}\to \eta K^{-}{\nu }_{\tau }$ sample is normalized to luminosity with the branching fraction reported in this paper.

Figure 3

Invariant ${\pi }^{+}{\pi }^{-}{\pi }^0$ mass distributions for ${\tau }^{-}\to {\pi }^{+}{\pi }^{-}{\pi }^0{\pi }^{-}{\nu }_{\tau }$ candidates, for (a) $e$-tag data (b) $\mu$-tag data. The curves show the results of the fits described in the text. Note the suppressed zero on the $y$ axes.

Figure 4

The $\eta {\pi }^{-}$ mass distributions for the data and MC samples, for $e$- and $\mu$-tag events, obtained from the sideband subtraction method as described in the text. The MC samples are normalized to the data luminosity; in particular, there are no ${\tau }^{-}\to \eta {\pi }^{-}{\nu }_{\tau }$ MC events.