Phenomenology of nonstandard Z couplings in exclusive semileptonic b -> s transitions

The rare decays B --> K((*))l(+)l(-), B --> K-(*) <(nu )over bar> and B-s --> mu (+)mu (-) are analyzed in a generic scenario where new physics effects enter predominantly via Z penguin contributions. We show that this possibility is well motivated on theoretical grounds, as the (s) over bar bZ vertex is particularly susceptible to nonstandard dynamics. In addition, such a framework is also interesting phenomenologically since the (s) over bar bZ coupling is rather poorly constrained by present data. The characteristic features of this scenario for the relevant decay rates and distributions are investigated. We emphasize that both sign and magnitude of the forward-backward asymmetry of the decay leptons in (B) over bar --> (K) over barl(+)l(-), A(FB)(((B) over bar)), carry sensitive information on new physics. The observable A(FB)(((B) over bar)) + A(FB)((B)) is proposed as a useful probe of nonstandard CP violation in (s) over bar bZ couplings.