SEARCHING FOR NEW PHYSICS IN RARE B-]TAU DECAYS - ART. NO. 012001

The rare decays B- --> tau<(nu)over bar>, B --> tau(+)tau(-), b --> X nu<(nu)over bar> and b --> X tau(+)tau(-) all contain third generation leptons in the final state, and hence are sensitive to new physics th at couples more strongly to the third family. We present model indepen dent expressions for these decays that can be useful to study several types of new physics effects. We concentrate on supersymmetric models without R-parity and without lepton number. We also assume a horizonta l U(1) symmetry with fermion horizontal charges chosen to explain the magnitude of fermion masses and quark mixing angles. This allows us to estimate the order of magnitude of the new effects, and to derive num erical predictions for the various decay rates and for the forward-bac kward asymmetry and the tau polarization components measurable in b -- > X tau(+)tau(-) . In some cases the branching ratios are enhanced by more than one order of magnitude, rendering foreseeable their detectio n at upcoming B-factories. We also discuss how a measurement of asymme tries in b --> X tau(+)tau(-) can be crucial in distinguishing between different sources of new physics. [S0556-2821(98)01811-6].