A "BPS expansion" for B and D mesons

We analyze consequences of the approximation μ_π^2 -μ_G^2 << μ_π^2 (a `BPS' limit) for B and D mesons. It is shown that neglecting perturbative effects many power corrections would vanish to all orders in 1/m_Q, in particular those violating heavy flavor symmetry. Among them are corrections to B->D formfactors. A number of relations receive corrections only to the second order expanding around the BPS limit to any order in 1/m_Q, including both f_+ and f_- at zero recoil. This allows an accurate evaluation of {\cal F}_+ for B->D lν. Its perturbative renormalization is computed analytically in the required Wilsonian scheme, yielding the dominant 3% enhancement.

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