Is the lepton flavour changing observable in $Z \to \mu^\pm + \tau^\mp$   decay?
  Neutrino oscillations as suggested by the recent Super-Kamiokande result imply that lepton numbers could be violated, and Z$\to \mu^{\mp} +\tau^{\pm}$ is a typical manifestation. We point out that for this mode, the GIM cancelation is much milder with only a logarithmic behavior $\log (m_3 /m_2)$ where $m_j$ is the neutrino mass. The ratio $\Gamma$(Z $\to \mu^{\mp} + \tau^{\pm})/\Gamma($Z $ \to \mu^{-} + \mu^{+})$ could be about $10^{-5}-10^{-6}$, in sharp contrast with the vanishingly small rate $\tau^{\pm}\to \mu^{\pm} + \gamma$ strongly suppressed by a quadratic power $(m_3^2-m_2^2)/ M_{\rm W}^2$. Being complementary to neutrino oscillation experiments, measurements of Z$\to \mu^{\mp} +\tau^{\pm}$ -- which are at hand with the present colliders -- would give one more constraint to the lepton mixing angle $\sin 2\theta_{jk}$ and the neutrino mass ratio $m_j/m_k$. 
X.Y. Pham