CLEO Results on Tau Michel Parameters

Heavy lepton (μ, τ) decays are mediated (in the Standard Model) by the charged weak current, carried by W force bosons with a V − A Lorentz structure. As a consequence, such decays exhibit maximal parity violation, while conserving CP. The couplings of all the charged leptons to the W are consistent with being exactly equal (universality). Many extensions to the Standard Model predict small deviations from the V −A law, and/or fermion universality. Such deviations can be observed in sensitive measurements of the Lorentz structure of the weak interaction. In leptonic decays of the μ and τ leptons, in which the outgoing neutrinos are unobserved, the dynamics of the decay can be described in terms of the Michel parameters [1]. The precise measurement of these parameters can be used to search for evidence of, or provide limits on, processes which go beyond the pure V − A structure of the charged weak current. In particular, they are sensitive to the presence of small scalar currents (such as those mediated by the charged Higgs of the Minimal Supersymmetric extensions [2] to the Standard Model, MSSM), or small deviations from maximal parity violation (such as those mediated by the right-handed WR of left-right symmetric extensions [3] to the Standard Model). In the decays of the τ to lνν, information on the decay can be extracted from the shape of the momentum distribution of the lepton l, and from its angular distribution relative to the parent τ spin direction [1,4]. After integration over the unobserved neutrino momenta and the spin of l, and neglecting radiative effects we can write the charged lepton momentum spectrum as: