Precision studies of light mesons at COMPASS
Prof. Bernhard Ketzer (Hemholtz-Institut f. Strahlen, Bonn)

The COMPASS experiment at the CERN Super Proton Synchrotron investigates the structure and excitations of strongly interacting particles. Using reactions of 190 GeV/c hadron beams with proton and nuclear targets mediated by the strong and electromagnetic interactions, an unprecedented statistical precision has been reached allowing new insight into the properties of light mesons. For the first time the diffractively produced \(3\pi\) final state has been analyzed simultaneously in bins of invariant mass and four-momentum transfer using a large set of 88 waves up to a total angular momentum of 6. In addition to a precise determination of the properties of known resonances and including a model-indepedent analysis of the \(\pi\pi\) \(S\)-wave isobar, a new narrow axial-vector state coupling strongly to \(f_0(980)\) has been found in previously unchartered territory. A possible interpretation of this new state in terms of a dynamic singularity will be discussed. The status of spin-exotic resonances will be presented. By selecting reactions with very small four-momentum transfer COMPASS is able to study processes involving the exchange of quasi-real photons. These provide clean access to low-energy quantities such as polarizabilities of pions and processes governed by chiral dynamics, and thus constitute a test of predictions of chiral perturbation theory.