Upgraded DIRAC spectrometer at CERN PS for the investigation of ππ and πK atoms

Abstract The DIRAC spectrometer installed at CERN PS was upgraded in order to study simultaneously A 2π and A πK atoms, namely the bound states of two π mesons, and of π and K mesons, respectively. The detector system can now accept a high intensity beam of 2–6×10 11 primary protons per second. The electronics and the data-acquisition system can handle a very large amount of data to identify π, K, p, e and µ, allowing the selection of ππ and πK pairs in the offline analysis. The resolution of the longitudinal and transverse components of the relative momentum Q of each mesonpair in their center-of-mass system with respect to the direction of the pair was substantially improved. The analysis of their distributions allowed an reliable separation between the mesonpairs originating from hadronic atoms and the backgrounds permitting the measurement of the lifetimes of hadronic atoms in the ground state and π–π, π–K s-wave scattering lengths. The upgraded setup also allowed the study of the long-lived excited states of ππ atoms.