FIRST LIMIT ON INCLUSIVE B-]X(S)NU(NU)OVER-BAR DECAY AND CONSTRAINTS ON NEW PHYSICS

The inclusive B --> <X(s) nu(nu)over bar>> decay rate, on which no exp erimental bound exists to date, can be constrained by searching for la rge missing energy events in B decays. Carefully examining the experim ental and theoretical aspects of such an analysis, we argue that the p ublished ALEPH limit on BR(B --> tau<(nu)over bar>) implies, conservat ively, the bound BR(B --> <X(s) nu(nu)over bar>) < 3.9 x 10(-4), which is less than one order of magnitude above the standard model predicti on. The LEP collaborations could significantly improve this bound by a dedicated experimental analysis. We study the constraints this new Li mit imposes on various extensions of the standard model. We derive new bounds on the couplings of third generation fermions in models with l eptoquarks, and in supersymmetric models without R-parity. We also con strain models where new gauge bosons are coupled dominantly to the thi rd generation, such as TopColor models and models based on horizontal gauge symmetries, For models which predict an enhanced effective bsZ v ertex, the constraint from B --> <X(s) nu(nu)over bar> is competitive with the limits from inclusive and exclusive B --> X(s)l(+)l(-) decays .