Measurement of the kaon content of three-prong tau decays

We present branching fraction measurements of three-prong decays of the [tau][sup [minus]] lepton based on data from the TPC/Two-Gamma detector at SLAC PEP. The decays are classified according to the identities of the charged tracks to give simultaneous measurements of [ital B][[tau][sup [minus]][r arrow][nu][sub [tau]] [ital K][sup [minus]][pi][sup +][pi][sup [minus]]]=0.58[sub [minus]0.13][sup +0.15]% and [ital B][[tau][sup [minus]][r arrow][nu][sub [tau]] [ital K][sup [minus]][ital K][sup +][pi][sup [minus]]]=0.15[sub [minus]0.07][sup +0.09]%, and upper limits of [ital B][[tau][sup [minus]][r arrow][nu][sub [tau]][pi][sup [minus]][ital K][sup +][pi][sup [minus]]][lt]0.25%, [ital B][[tau][sup [minus]][r arrow][nu][sub [tau]] [ital K][sup [minus]][pi][sup +][ital K][sup [minus]]][lt]0.09%, and [ital B][[tau][sup [minus]][r arrow][nu][sub [tau]] [ital K][sup [minus]][ital K][sup +][ital K][sup [minus]]][lt]0.21%, where additional neutrals may be present in each case. The branching fractions are normalized to the world average three-prong topological branching fraction and there is an estimated 20% systematic error in addition to the listed statistical errors. We find the mass distributions in the [ital K][sup [minus]][pi][sup +][pi][sup [minus]] decay are consistent with [ital K][sub 1] dominance, and obtain branching fractions of [ital B][[tau][sup [minus]][r arrow][nu][sub [tau]] [ital K][sub 1][sup [minus]](1270)]=0.41[sub [minus]0.35][sup +0.41]% and [ital B][[tau][sup [minus]][r arrow][nu][sub [tau]] [ital K][sub 1][sup [minus]](1400)]=0.76[sub [minus]0.33][sup +0.40]%.