Measurement of the branching fractions of the τ lepton using a tagged sample of τ decays

The branching fractions for the major decay modes of the τ lepton are measured from a sample of tagged τ decays selected from ${\tau }^{+}$${\tau }^{\mathrm{-}}$ pairs produced in $e^{+}$$e^{\mathrm{-}}$ annihilation. The decay products of one member of the ${\tau }^{+}$${\tau }^{\mathrm{-}}$ pair are used to identify the event. The opposite member of the pair is then included in the sample if it satisfies an invariant-mass cut and a charged-multiplicity cut. In this way, we attempt to select an unbiased sample of τ decays. The sample is divided into categories based on charged- and neutral-particle multiplicities, and charged-particle identification. The branching fractions are measured simultaneously, with an unfold technique and a maximum-likelihood fit, with the sum of the branching fractions constrained to be unity.

The branching fractions for the decay modes ${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$ $K^{\mathrm{-}}$ and ${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$ $K^{\mathrm{*}\mathrm{-}}$ are fixed at the world averages in the fit. The following branching fractions are measured: B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$$e^{\mathrm{-}}$ν${¯}_e$) =(19.1±0.8±1.1)%, B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$ ${\mu }^{\mathrm{-}}$ ν${¯}_{\mu }$)=(18.3±0.9±0.8)%, B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$${\pi }^{\mathrm{-}}$) =(10.0±1.1 *1,4)%, B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$${\pi }^{\mathrm{-}}$${\pi }^0$) =(25.8±1.7±2.5)%, B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$${\pi }^{\mathrm{-}}$(n${\pi }^0$, n>1))=(12.0±1.4±2.5)%, B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$${\pi }^{\mathrm{-}}$${\pi }^{+}$${\pi }^{\mathrm{-}}$) =(6.7 ±0.8±0.9)%, and B(${\tau }^{\mathrm{-}}$→${\nu }_{\tau }$${\pi }^{\mathrm{-}}$${\pi }^{+}$${\pi }^{\mathrm{-}}$ (n${\pi }^0$,n>0) )=(6.1±0.8±0.9)%.

The total branching fraction to one charged pion plus at least one neutral hadron is measured to be (37.8±1.2±1.0)%. Limits on allowed decay modes involving the η meson are discussed.