Model-independent semi-leptonic form factors using dispersion relations

SummaryWe present a method for parametrizing heavy-meson semi-leptonic form factors using dispersion relations, and from it produce a two-parameter description of the B→B elastic form factor. We use heavy-quark symmetry to relate this function to $$\bar B \to D*\ell \bar v$$ form factors, and extract |Vcb|=0.0355−0.0025+0.0029 from experimental data with a least-squares fit. Our method eliminates model-dependent uncertainties inherent in choosing a parametrization for the extrapolation of the differential decay rate to threshold.