Studies of light mesons at COMPASS

To investigate the spectrum of light mesons, the COMPASS experiment has recorded data with a 190 GeV/c negative pion beam impinging on a liquid hydrogen target. Diffractive dissociation reactions at squared four-momentum transfers to the target between 0.1 and 1.0 (GeV/c)2 are studied. The flagship channel is the π− π+ π− final state, for which COMPASS has recorded the at present world’s largest data sample. A new axial-vector signal, the a1(1420), has been found with unusual properties, pointing to an exotic nature. The findings are confirmed by the analysis of theπ− π0 π0 final state.