Measurement of the D→ππ branching fractions

Using data from CLEO II at the Cornell Electron Storage Ring we provide a new measurement of the branching fraction for D0→π+π-, and we present the first measurements of D0→π0π0 and of D+→π+π0, which is due to an isospin changing ΔI=3/2 transition. From these data we extract the ratio of isospin amplitudes A2/A0, and the cosine of the relative phase δ=δ2-δ0 between these amplitudes, for D→ππ decays. Unlike the situation in kaon decay where A2 is strongly suppressed, we find A2/A0=0.72±0.13±0.11 and cosδ=0.14±0.13±0.09. © 1993 The American Physical Society.