RARE EXCLUSIVE SEMILEPTONIC B-]S TRANSITIONS IN THE STANDARD MODEL

We study long-distance effects in rare exclusive semileptonic decays B -->(K,K)(l*l(-), v (v) over bar) and analyze dilepton spectra and asy mmetries within the framework of the standard model. The form factors, describing the meson transition amplitudes of the effective Hamiltoni an, are calculated within the lattice-constrained dispersion quark mod el: the form factors are given by dispersion representations through t he wave functions of the initial and final mesons, and these wave func tions are chosen such that the B-->K transition form factors agree wi th the lattice results at large q(2). We calculate branching ratios of semileptonic B-->K,K transition modes and study the sensitivity of o bservables to the long-distance contributions. The shape of the forwar d-backward asymmetry and the longitudinal lepton polarization asymmetr y are found to be independent of the long-distance effects and mainly determined by the values of the Wilson coefficients in the standard mo del.