Symmetry-breaking corrections to heavy meson form-factor relations

In the heavy quark limit, the form factors for semileptonic and rare radiative B decays into light mesons are related by heavy quark spin symmetry. Here we compute the leading corrections of order Λ/mQ to these symmetry relations, showing also how to include hard gluon effects systematically to any order in αs(mQ). The subleading correction to the form factor relation for B→π can be computed exactly in the soft pion limit. For decays into vector mesons, the matrix elements of two local dimension-4 operators are needed, one of which vanishes in the constituent quark model. A few applications are briefly discussed.