Scalar Meson σ(500) Phase Motion at D+→π−π+π+ Decay

Applying the Amplitude Difference Method in the E791 experiment data of the D+→π−π+π+ decay, we make a direct measurement of 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.