Precision Meson Spectroscopy at COMPASS

We present first results of a partial wave analysis of the diffractive reaction $\pi -Pb\to \pi -\pi +\pi -Pb$ based on data from the COMPASS experiment taken during a pilot run in 2004 using a 190 GeV/c $\pi -$ beam on a lead target. The analysis was performed in the region of squared four-momentum transfer $t^{\mathrm{\prime }}$ between 0.1 and 1.0 (GeV/c)^2. The $\pi -\pi +\pi -$ final state shows a rich spectrum of well-known resonances. In addition a spin-exotic $J^{PC}=1^{-+}$ state with significant intensity was observed at 1.66 GeV/c^2 in the $\rho (770)\pi$ decay channel in natural parity exchange. The resonant nature of this state is manifest in the mass dependence of its phase difference to $J^{PC}=1^{++}$ and $2^{-+}$ waves. The measured resonance parameters are consistent with the disputed ${\pi }_1(1600)$. An outlook on the analyses of the much larger data set taken during 2008 and 2009 is given.