Precision Meson Spectroscopy at COMPASS

Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching, Germany.

^a e-mail: bgrube@ph.tum.de

Abstract

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 J^PC = 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 J^PC = 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.