Light-Meson Spectroscopy with COMPASS

Abstract: COMPASS is a multi-purpose fixed-target experiment at the CERN Super Proton Synchrotron investigating the structure and spectrum of hadrons. One primary goal is the search for new hadronic states, in particular spin-exotic mesons and glueballs. After a short pilot run in 2004 with a 190 GeV/c $\pi^-$ beam on a Pb target, which showed a significant spin-exotic $J^{PC} = 1^{-+}$ resonance consistent with the controversial $\pi_1(1600)$, COMPASS collected large data samples with negative and positive hadron beams on H$_2$, Ni, W, and Pb targets in 2008 and 2009. We present results from a partial-wave analysis of diffractive dissociation of 190 GeV/c $\pi^-$ into $\pi^-\pi^+\pi^-$ final states on Pb and H$_2$ targets with squared four-momentum transfer in the range 0.1 < t' < 1 (GeV/c)^2. This reaction provides clean access to the light-quark meson spectrum up to masses of 2.5 GeV/c^2. A first comparison of the data from Pb and H$_2$ target shows a strong target dependence of the production strength of states with spin projections $M = 0$ and 1 relative to the $a_2(1320)$. The 2004 Pb data were also analyzed in the region of small squared four-momentum transfer t' < 10^{-2} (GeV/c)^2, where we observe interference of diffractive production and photoproduction in the Coulomb-field of the Pb nucleus.