Leptonic decay modes of the τ lepton

Like any other elementary particle, the τ is thought to be described by the Standard Model. The total number of electrons/positrons from τ decay coupled to the known number of τ's in the samples gives the branching ratio (BR) for the process τ→eνantiν, which provides another consistency test for the Standard Model. Although the Standard Model is very successful phenomenologically, it is not expected to be the ultimate theory of elementary particles and forces. It is possible that effects which are strictly forbidden by the Standard Model are allowed to a certain extent by some of its extensions. The above analyses goals require a detector which can identify electrons, positrons and photons with high efficiency and which can measure the energy and direction of these particles with good precision. These requirements are pre-eminently met by the Crystal Ball detector. For the decay τ→eνantiν 2753 candidates were found. Using Monte Carlo simulations, 289 of these decays could be attributed to various sources of background; their energy spectrum was subtracted from the observed spectrum. After application of the other relevant corrections, the electron/positron energy spectrum was tested for the V-A hypothesis. The data were found to be consistent with this conjecture with aχ2 per degree of freedom (D.o.F.) of 78.8/66. Fitting the energy spectrum with a freely varying Michel parameter yielded ρ=0.64±0.06±0.07 (the first error being statistical, the second systematic) with aχ2/D.o.F. of 76.5/65. From the total number of observed events the BR for τ→eνantiν: BR(τ →eνantiν)=(16.3±0.3±3.2)% was obtained. These results are consistent with the Standard Model predictions. In a manner similar to that of the previous analysis, selection criteria were developed for the decays τ→eγ, τ→eπo and τ→eη. No significant signal consistent with the resolution was observed in any of these modes. This is again in agreement with the Standard Model. (author). 88 refs.; 58 figs.; 7 tabs