The tau weak-magnetic dipole moment

We calculate the prediction for the anomalous weak-magnetic form factor of the tau lepton at $q^2=M_Z^2$ within the Standard Model. With all particles on-shell, this is a electroweak gauge invariant quantity. Its value is $a_\tau^w (M_Z^2)= - \;(2.10 + 0.61\, i) \times 10^{-6}$. We show that the transverse and normal components of the single-tau polarization of tau pairs produced at $e^+e^-$ unpolarized collisions are sensitive to the real and absorptive parts of the anomalous weak-magnetic dipole moment of the tau. The sensitivity one can achieve at LEP in the measurement of this dipole moment is discussed.Comment: 16 pages, Latex, 2 figure

First limit on inclusive B→XsννˉB \to X_s \nu \bar\nu decay and constraints on new physics

The inclusive $B \to X_s \nu \bar\nu$ decay rate, on which no experimental bound exists to date, can be constrained by searching for large missing energy events in $B$ decays. Carefully examining the experimental and theoretical aspects of such an analysis, we argue that the published ALEPH limit on \BR(B \to \tau \bar\nu) implies, conservatively, the bound \BR(B \to X_s \nu \bar\nu)<3.9\times10^{-4}, which is less than one order of magnitude above the standard model prediction. The LEP collaborations could significantly improve this bound by a dedicated experimental analysis. We study the constraints this new limit imposes on various extensions of the standard model. We derive new bounds on the couplings of third generation fermions in models with leptoquarks, and in supersymmetric models without R-parity. We also constrain models where new gauge bosons are coupled dominantly to the third generation, such as TopColor models and models based on horizontal gauge symmetries. For models which predict an enhanced effective $bsZ$ vertex, the constraint from $B \to X_s \nu \bar\nu$ is competitive with the limits from inclusive and exclusive $B \to X_s \ell^+ \ell^-$ decays.Comment: 43 pages (paper) + 3 pages (erratum), REVTeX, no figures. Erratum added (Paper unchanged). Two effects neglected in the original analysis are discussed in the Erratum. The numerical estimate of the limit on the branching ratio is weaker by about a factor of thre