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 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 --> tau<(nu)over bar>) implies, conservatively, 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 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 --> <X(s) nu(nu)over bar> is competitive with the limits from inclusive and exclusive B --> X(s)l(+)l(-) decays.