Measurement of diffractive 3π production on a polarized target at 17 GeV/c

The reaction π^?p↑→π^?π⁺π^?p has been measured at 17 GeV/c using a polarized target. The data sample contains about 60 000 interactions on polarized protons. The nucleon polarization as a function of momentum transfer is very similar to elastic π^?p scattering and is nearly independent of the π mass, except for a possible structure around 1.2 GeV.Using the isobar model, we have performed a partial-wave analysis and extracted the 3π amplitudes. The generalized 3π density matrix elements agree with earlier determinations. Relative phases obtained from density matrix elements agree well with the ones obtained from transversity amplitudes. This proves the validity of the coherence assumption made in the interpretation of unpolarized target results. Our results confirm the existence of an A₁ resonance in the region 1.2–1.3 GeV.