Precision Meson Spectroscopy at COMPASS

We present first results of a partial wave analysis of the diffractive reaction π− Pb → π− π+ π− Pb based on data from the COMPASS experiment taken during a pilot run in 2004 using a 190 GeV/c π− beam on a lead target. The analysis was performed in the region of squared four-momentum transfer t' between 0.1 and 1.0 (GeV/c)2 . The π− π+ π− final state shows a rich spectrum of well-known resonances. In addition a spin-exotic JPC = 1−+ state with significant intensity was observed at 1.66 GeV/c2 in the ρ(770) π decay channel in natural parity exchange. The resonant nature of this state is manifest in the mass dependence of its phase difference to JPC = 1++ and 2−+ waves. The measured resonance parameters are consistent with the disputed π1 (1600). An outlook on the analyses of the much larger data set taken during 2008 and 2009 is given.