Studies of light mesons at COMPASS

Abstract

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 a 1 (1420), has been found with unusual properties, pointing to an exotic nature. The findings are confirmed by the analysis of the π − π 0 π 0 final state.