WEAK ANNIHILATION AND THE END-POINT SPECTRUM IN SEMILEPTONIC B-DECAYS

A general relationship is formulated between the contributions of 'Wea k Annihilation' (WA) to inclusive semileptonic decays of heavy flavour hadrons and the matrix elements of four-fermion operators. We argue t hat non-perturbative contributions from low-energy hadronic final stat es provide the dominant impact of WA on the semileptonic b --> u width of B- mesons. In that case WA affects the lepton spectrum mainly in i ts end-point region -i.e. beyond the kinematical boundary for b --> c transitions - and we expect the latter to look quite different in B- t han in B(d) decays. At present we are unable to make a truly quantitat ive prediction; yet a detailed experimental comparison of the B- and t he B(d) lepton spectra will enable us to determine the size of the rel evant matrix elements and to check directly to what extent the factori zation approximation works here. Using universal analyticity propertie s of the decay amplitude we analyse WA in the perturbative regime and rederive our earlier results about the absence of a power enhancement due to gluon emission in a more general way. We also address the probl em of the invariant mass of hadrons in the final state in semileptonic B decays and identify non-trivial cancellations in [M(hadr)2]