Analyticity, shapes of semileptonic form factors

We give a pedagogical discussion of the physics underlying dispersion relation-derived parameterizations of form factors describing B → πlν̄ and B → Dlν̄. Moments of the dispersion relations are shown to provide substantially tighter constraints on the f+(t) form factor describing B̄ → πlν than the unweighted dispersion relation alone. Heavy quark spin symmetry relations between the B → πlν̄ and B → πlν̄ form factors enables such constraints to be tightened even further.