The $A_{1}$ meson produced at 63 and 94 GeV/c in the reaction $\pi^{-} p\to\pi^{-}\pi^{-}\pi^{+}p$

The results are presented of partial wave analyses of almost 600000 events from the reaction pi /sup -/p to pi /sup -/ pi /sup -/ pi /sup +/p at 63 and 94 GeV/c and with a higher momentum transfer trigger. The shape of the 1/sup +/S ( rho pi ) mass spectrum changes substantially as a function of t, and is not compatible with predictions of the Deck model. In addition, the phase of this partial wave rises by approximately 90 degrees over a mass range of 400 GeV/c /sup 2/ relative to 0/sup -/P, 2/sup -/P and the 2/sup +/D production phases. These features provide compelling evidence for a resonant A /sub 1/ of mass approximately 1280 MeV/c/sup 2/ and width approximately 300 MeV/c/sup 2/ and indeed a rescattering model with such a state fits the data well. (7 refs).