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 π − beam on a Pb target, which showed a significant spin-exotic J PC = 1 −+ resonance consistent with the controversial π 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 π − into π − π + π − 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 fourmomentum transfer t < 10 −2 (GeV/c) 2 , where we observe interference of diffractive production and photoproduction in the Coulomb-field of the Pb nucleus.

Understanding the baryon and meson spectra

“…This studies multihadron production at small momentum transfers with a 192 GeV pion beam on nucleon and nuclear targets, in particular studying πη and 3π final states. The πη data show a significant broad enhancement in 1 −+ waves around 1600 MeV, but with little relative phase variation compared with the reference 2 ++ wave with its pronounced (conventional qq) a 2 (1320) signal [38]. In the 3π channel, the first runs in 2004 showed a very crude enhancement in 1 −+ waves, which was fitted to a Breit-Wigner form with doubtful significance [39].…”