Diffractive Dissociation of 190 GeV/c $π^-$ into $π^-π^+π^-$ Final States at COMPASS

We present results from a Partial-Wave Analysis (PWA) of diffractive dissociation of 190 GeV/c $\pi^-$ into $\pi^-\pi^+\pi^-$ final states on nuclear targets. A PWA of the data sample taken during a COMPASS pilot run in 2004 on a Pb target showed a significant spin-exotic $J^{PC} = 1^{-+}$ resonance consistent with the controversial $\pi_1(1600)$, which is considered to be a candidate for a non-$q\bar{q}$ mesonic state. In 2008 COMPASS collected a large diffractive $\pi^-\pi^+\pi^-$ data sample using a hydrogen target. A first comparison with the 2004 data shows a strong target dependence of the production strength of states with spin projections $M = 0$ and 1.