SEARCH FOR LONG-LIVED STATES OF π + π − ATOMS Addendum to the DIRAC Proposal

The proposed experiment is a further development of the DIRAC experiment already running at CERN PS. Up to now more than 21000 π+π− pairs originated from the π+π− atom (A2π) breakup were identified and the overall accuracy of the A2π lifetime is about 9% in accordance with the DIRAC proposal. This measurement allowed to extract the difference |a0 −a2| of s-wave ππ-scattering length with accuracy of 4.3%. The observation of long-lived (metastable) A2π states will be performed with the same setup. This observation opens a possibility to measure the energy difference between ns and np states and to determine the value of another combination 2a0 +a2 of ππ scattering length in a model-independent way. In combination with the first measurement it allows to get a0 and a2 separately. An additional measurement of the multiple scattering angles in different materials with accuracy better than 1% will be performed in parallel with the observation. This measurement is needed to improve the systematic accuracy in the A2π lifetime for the data already collected in 2008–2010.