The non-factorizable terms in the operator product expansion have been recognized as one of the theoretical obstacles for precision determination of the mixing parameter Vub from semileptonic B decays. It is pointed out here that the recent CLEO data on the parameters of the heavy quark expansion λ1 and [Formula: see text], combined with a theoretical bound on λ1 strongly favor the existence of a sizeable contribution of non-factorizable terms in semileptonic decays of D mesons. Thus these terms are likely to solve the long-standing problem of the deficit of semileptonic decay rate of the D mesons, and with better data their magnitude can be determined and used in studies of the parameter Vub.
Semileptonic decays of D mesons in unquenched lattice QCD