According to heavy-meson chiral perturbation theory, the vector form factor f+(q2) of exclusive semileptonic decay B→πℓν is closely related, at least in the soft-pion region (i.e., q2∼(mB−mπ)2), to the strong coupling gB*Bπ or the normalized coupling g^. Combining the precisely measured q2 spectrum of B→πℓν decay by the BABAR and Belle collaborations with several parametrizations of the form factor f+(q2), we can extract these couplings from the residue of the form factor at the B* pole, which relies on an extrapolation of the form ...
