Hadron spectroscopy with COMPASS

COMPASS is a multi-purpose fixed-target experiment at the CERN Super Proton Synchrotron aimed at studying the structure and spectrum of hadrons. The two-stage spectrometer has a good acceptance for charged as well as neutral particles over a wide kinematic range and is thus able to measure a wide range of reactions. Light mesons are studied with negative (mostly π^-) and positive (p, π⁺) hadron beams with a momentum of 190 GeV/c. Their spectrum is investigated in various final states produced in diffractive dissociation or in central-production reactions. The COMPASS data not only allow for measuring the properties of known resonances with high precision, but also for searching for new states. Among these is a new resonance-like signal, the a₁(1420), with unusual properties. In addition, the resonance content of the partial wave with spin-exotic J^PC = 1^-+ quantum numbers, which are forbidden for quark-antiquark states, is of particular interest.