Long-distance contribution to exclusive rare decays of heavy mesons in quark model

Nonperturbative effects in exclusive reactions $B\to K^{(\ast )}ll$ are discussed. Form factors which describe the main long-distance contribution to the meson transition amplitude are calculated within the dispersion formulation of the quark model: namely, the form factors in the decay region are expressed as relativistic double spectral representations through the wave functions of the initial and final mesons. The dilepton forward-backward asymmetry and the lepton polarization turn out to be largely independent of the particular choice of the quark model parameters and can be predicted with high accuracy. At the same time, these asymmetries are sensitive to the short-distance structure of the theory and might be used as a test of the Standard Model and probe of new physics.