Light-front quark model analysis of rare B→Kl⁺l^- decays

Using the light-front quark model, we calculate the transition form factors, decay rates, and longitudinal lepton polarization asymmetries for the exclusive rare B→Kl⁺l^-(l=e,μ,τ) decays within the standard model. Evaluating the timelike form factors, we use the analytic continuation method in the q⁺ = 0 frame to obtain the form factors F₊ and F_T, which are free from the zero mode. The form factor F_{_} which is not free from the zero mode in the q⁺ = 0 frame and contaminated by the higher (or nonvalence) Fock states in the q⁺ ≠ 0 frame is obtained from an effective treatment for handling the nonvalence contribution based on the Bethe-Salpeter formalism. The covariance (i.e., frame independence) of our model calculation is discussed. We obtain the branching ratios for BR(B→Kl⁺l^-) as 4.96×10^-7|V_ts/V_cb|² for l=e,μ and 1.27×10^-7|V_ts/V_cb|² for l=τ.