Tau decays involving the eta meson.

The decays of the tau lepton which involve the eta meson are examined within the standard model. Such modes are prime candidates to comprise the approximately 7% of tau decays (with a one-prong topology) that are not assignable to known exclusive channels. It is shown that the combination of existing electronpositron annihilation data and measurements of five-prong tau decays prevents modes involving the eta from contributing more than 2% to one-prong tau decays. Submitted to Physical Review Rapid Communications * Work supported by the Department of Energy, contract DE AC03 76SF00515. As now known, the properties of the tau lepton and the tau neutrino are generally in excellent accord with those expected in the standard model for the third generation charged and neutral leptons, respectively. The decays r + v,eDe, r --+ v,~D~, r --) L+K, r --) v72r, r t u73r, and r -+ u747r, occur [‘I at the expected rates and account for almost 90% of tau decays. However, there is a nagging problem in accounting for all tau decays.[21 It shows up in doing the bookkeeping for the sum of the exclusive modes that result in one charged prong and comparing the result with the corresponding inclusive topological branching ratio “I of 86.3 f 0.3%. Taking all the modes listed above plus Cabibbo-suppressed modes into account, there remains about 7% of tau decays that are not assignable to known exclusive channels. The least disturbing explanation for this discrepancy, that the measurements of the major one-prong exclusive channels needed to be scaled up, became less likely as new and more accurate measurements became available.13’ By the time of the Kyoto Conference in 1985, the World-average branching ratios had significantly reduced errors and the discrepancy remained.[“’ However, data published in the past year seemed to point toward a resolution of the problem in a still relatively conventional manner through decays of the tau involving eta mesons. The MarkIIisl and TPC”’ collaborations each used slightly different tagging techniques and, constraining the sum of all the branching fractions to be unity, found that tau decays into a neutrino, charged pion, and two or more neutral hadrons were well in excess of the theoretical expectations”’ for the sum of just r+ ~~7r-271-O and r+ ~,~r-37rO. As other modes involving only pions and kaons had been shown to make very small contributions to the oneprong plus multi-neutrals topology,“’ by process of elimination decays involving the eta meson in the “multi-neutrals” became prime suspects.[” The finger was put more directly on the eta by the Crystal Ball[” and the HRS[” collaborations, who both claimed evidence for an eta signal in tau decays, with HRS reporting a branching fraction for r4 ~~7r-r]... of 5.0 f 1.0 f 1.5%.