Precision Studies of Light Mesons at COMPASS

The COMPASS experiment at CERN's SPS investigates the structure and excitations of strongly interacting systems. Using reactions of 190 GeV/c pions with protons and nuclear targets, mediated by the strong and electromagnetic interaction, an unprecedented statistical precision has been reached allowing new insight into the properties of light mesons. For the first time the diffractively produced 3pi 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 f0(980)pi has been found in previously unchartered territory. 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 radiative couplings and polarizabilities of mesons, and thus constitute a test of model predictions such as chiral perturbation theory.