$B \to\tau^+ \tau^- (X)$ decays: first constraints and phenomenological implications

The study of $B \to \tau^+ \tau^- (X)$ decays can provide us with a better understanding of the third generation, and can be a useful probe of physics beyond the standard model. We present a model independent analysis of these decays. We classify new physics that can largely enhance the decay rates and we discuss the constraints implied by other processes. Experimentally, flavor changing neutral current $B$ decays into final state $\tau$'s are still unconstrained. Searches for $B$ decays with large missing energy at LEP provide the first limits. We estimate that existing data already imply bounds on the $B_d \to \tau^+ \tau^-$, $B_s \to \tau^+ \tau^-$, and $B \to X \tau^+ \tau^-$ decay rates at the few percent level. Although these bounds are over four orders of magnitude above the standard model predictions, they provide the first constraints on some leptoquarks, and on some R-parity violating couplings.