Exotic meson π1 (1600) with JPC=1 −+ and its decay into ρ(770)π

We study the spin-exotic J(PC)=1(-+) amplitude in single-diffractive dissociation of 190 GeV/c pions into pi(-)pi(-)pi(+) using a hydrogen target and confirm the pi(1)(1600) -> rho(770)pi amplitude, which interferes with a nonresonant 1(-+) amplitude. We demonstrate that conflicting conclusions from previous studies on these amplitudes can be attributed to different analysis models and different treatment of the dependence of the amplitudes on the squared four-momentum transfer and we thus reconcile these experimental findings. We study the nonresonant contributions to the pi(-)pi(-)pi(+) final state using pseudodata generated on the basis of a Deck model. Subjecting pseudodata and real data to the same partial-wave analysis, we find good agreement concerning the spectral shape and its dependence on the squared four-momentum transfer for the J(PC)=1(-+) amplitude and also for amplitudes with other J(PC) quantum numbers. We investigate for the first time the amplitude of the pi(-)pi(+) subsystem with J(PC)=1(--) in the 3 pi amplitude with J(PC)=1(-+) employing the novel freed-isobar analysis scheme. We reveal this pi(-)pi(+) amplitude to be dominated by the rho(770) for both the pi(1)(1600) and the nonresonant contribution. These findings largely confirm the underlying assumptions for the isobar model used in all previous partial-wave analyses addressing the J(PC)=1(-+) amplitude.