DIRAC Experiment at CERN: LIFETIME MEASUREMENT OF PIONIUM Collaboration DIRAC

The DIRAC experiment, a magnetic double arm spectrometer, aims to measure the π + π − atom lifetime with 10% precision, using the high intensity 24 GeV/c proton beam of the CERN Proton Synchrotron. Since the value of this lifetime of order 10 s is dictated by strong interaction at low energy, a precise measurement of this quantity enables to study characteristic pion parameters in detail and to submit predictions of QCD to a severe check. INTRODUCTION. Pion scattering at low energies involves only the lightest observed hadrons and, therefore, is a key issue of low energy QCD. This pure process has been analysed in terms of scattering amplitudes and, hence, in terms of scattering lengths. By means of CHiral Perturbation Theory (CHPT) precise values for pion scattering lengths presumably at the 5% level are and will be predicted 1, experimentally not confirmed up to now (the best precision: ∼ 20% by Rosselet et al. 2). The Collaboration DIRAC (DImeson Relativistic Atom Complex, PS212 at CERN) intends to measure the ground-state lifetime of the atom “Pionium”. Pionium or A2π is the Coulomb bound state, formed by π and π− mesons, with the following predicted properties: 1. Bohr radius: rB = 387 fm 2. Coulomb energy of the ground state: E(1S) = −1.86 keV 3. Ground-state quantum numbers: J = 0 4. Pionium will decay in more than 99% of the cases by strong interaction into two neutral pions with a ground-state lifetime τ of about 3 fs.