Dispersive Bounds on the Shape of B -> D^(*) l nu Form Factors

Dispersive constraints on the shape of the form factors which describe the exclusive decays B -> D^() l nu are derived by fully exploiting spin symmetry in the ground-state doublet of heavy-light mesons. The analysis includes all twenty B^() -> D^(*) semileptonic form factors. Heavy-quark symmetry, with both short-distance and 1/m_Q corrections included, is used to provide relations between the form factors near zero recoil. Simple one-parameter functions are derived, which describe the form factors in the semileptonic region with an accuracy of better than 2%. The implications of our results for the determination of |V_cb| are discussed.