Phase motion in the scalar low-mass ππ amplitude in D+→π−π+π+ decay

Applying the amplitude difference method to Fermilab experiment E791 D+→π−π+π+ data, we measure the low mass π+π− phase motion. Our results suggest a significant phase variation, compatible with the existence of an isoscalar σ(500) meson, as previously reported using an isobar model fit to the full Dalitz-plot density.