The Spectrum of Light Isovector Mesons with C = +1 from the COMPASS Experiment

Abstract

Based on the largest event sample of diffractively produced π − π − π + , obtained by a pion beam of 190 GeV/c momentum, the COMPASS collaboration has performed the so far most advanced partialwave analysis on multi-body final states, using the isobar model. The large number of 88waves included in the analysis reduces truncation effects. We have used fourteen waves, to extract resonance parameters for eleven light-meson candidates, most of them observed previously. The coherence of the analysis and the large variety of systematic studies has allowed us to determine mass and width of most a J and π J states with a total of six different values of J PC below a mass of 2.1 GeV/c 2 , with high confidence. We exploit that the production rates of resonant and non-resonant contributions in these fourteen waves vary differently with the four-momentum transfer squared in the reaction. In addition, we have performed the first isobar-freed analysis in diffraction, from which we have determined the shape of the ππ S -wave isobar for different J PC of the 3π system.