SU(3) BREAKING IN (B)OVER-BAR/D-]KL(V)OVER-BAR DECAYS AND THE VALUE OF V-CD/V-CS

We determine the SU(3)-breaking effects in the ($) over bar B/D --> Kl $$($) over bar nu decays from the ratio of the ($) over bar B/D --> Kl $$($) over bar nu over the ($) over bar B/D --> pi l $$($) over bar nu semileptonic form factors using a semi-analytic expression based on QCD spectral sum rules. We obtain f(+)(($) over bar B-->K) (0)/f(+)(($) over bar B-->pi) (0) = 1.007 +/- 0.020 and f(+)(D-->K)(0)/f(+)(D-->pi)(0) = 1.102 +/- 0.007, which combined with the present CLEO data, leads to V-cd/V-cs = 0.323 +/- 0.058 and then, to: \V-cs\ = 0.72 +/- 0.10. The prediction on the ratio of mixing angle is (surprisingly) much higher than the value 0.226 +/- 0.004 derived from the unitarity constraint on the CKM matrix, while the corresponding value of V-cs is lower. These results can open a window for new physics, though a stringent conclusion needs data with much higher statistics (from, e.g., tau-charm factory) or/and alternative QCD-based estimates of the ratio of the semileptonic form factors.