Small amplitude effects in B0→D+D- and related decays

Intrigued by a recent Belle result for a large direct CP asymmetry in B0→D+D-, we study the effects of a b→uud quark transition by combining the asymmetry information with rates and asymmetries in isospin-related decays. Arguing for a hierarchy among several contributions to these decays, including an exchange amplitude which we estimate, we present tests for factorization of the leading terms, and obtain an upper bound on the ratio of b→uud and b→ccd amplitudes. We prove an approximate ΔI=1/2 amplitude relation for B→DD, and an approximate equality between CP asymmetries in B0→D+D- and B+→D+D0. Violations of these relations by Belle measurements, at 1.8σ and 3.6σ respectively, if confirmed, would indicate a possible new physics contribution in b→uud. Applying flavor SU(3), we extend this study to a total of ten processes, including ΔS=0 decays involving final Ds and initial Bs mesons, and ΔS=1 decays of B and Bs mesons into pairs of charmed pseudoscalar mesons. The decays Bs→DD provide useful information about a small exchange amplitude, responsible for a decay rate difference between B+→D+D0 and B0→D+D-. A method for determining the weak phase γ, based on CP asymmetries in B0(t)→D+D- and the decay rate for Bs→Ds+Ds- or B+(0)→Ds+D0(D-), is shown to involve high sensitivity to SU(3) breaking.