Calculating phases between B→K^*π amplitudes

A phase ΔΦ between amplitudes for B0→K ^*0π0 and B0→K^*+π ^- plays a crucial role in a method for constraining Cabibbo-Kobayashi-Maskawa parameters. We present a general argument for destructive interference between amplitudes for B0→K^{* +}π^- and B0→K^*0π0 forming together a smaller I(K^*π)=3/2 amplitude. Applying flavor SU(3) and allowing for conservative theoretical uncertainties, we obtain lower limits on ΔΦ and its charge conjugate. Values of these two phases favored by the BABAR collaboration are in good agreement with our bounds.