[en] The multilevel trigger system of the DIRAC experiment at CERN is presented. It includes a fast first level trigger as well as various trigger processors to select events with a pair of pions having a low relative momentum typical of the physical process under study. One of these processors employs the drift chamber data, another one is based on a neural network algorithm and the others use various hit-map detector correlations. Two versions of the trigger system used at different stages of the experiment are described. The complete system reduces the event rate by a factor of 1000 with respect to detectors' single rates, the detection efficiency of events in the relative momentum range of interest being ≥95%

[en] The production of π⁺π^- pairs in the reaction pTa→π⁺π^-X at 70 GeV is studied. For pions with laboratory momenta from 0.8 to 2.4 GeV/c, the two-particle correlation function is measured for c.m.s. relative momenta from the interval 0≤q≤50 MeV/c. For pions produced in the region that is much smaller than 1/m_πα, Coulomb final-state interaction leads to a sharp increase in the yield of π⁺π^- pairs with q<10 MeV/c. The effect of Coulomb interaction allows separation of π⁺π^- pairs according to the size of the region of their production

[en] The Preshower (PSh) detector [1] is a component of the DIRAC setup [2]. It is designed to identify and reject the huge e"−e"+ pairs background in the ππ and Kπ pairs measurement produced in a hadronic atom ionization process. In the high energy region used for kaon detection, the small Nitrogen Cherenkov detector has low electron rejection efficiency. To increase the overall efficiency, a new two-layer scintillator Preshower detector has been developed and built. The new Preshower–Cherenkov combination ensures an electron rejection efficiency better than 99% in the momentum range 1–7 GeV/c

[en] The main objective of DIRAC experiment is the measurement of the lifetime τ of the exotic hadronic atom consisting of π⁺ and π^- mesons. The lifetime of this atom is determined by the decay mode π⁺ π^- → π⁰ π⁰ due to the strong interaction. Through the precise relationship between the lifetime and the S-wave pion-pion scattering length difference |a₀ - a₂| for isospin 0 and 2 (respectively), a measurement of τ with an accuracy of 10% will allow a determination of |a₀ - a₂| at a 5% precision level. Pion-pion scattering lengths have been calculated in the framework of chiral perturbation theory with an accuracy below 5%. In this way DIRAC experiment will provide a crucial test of the chiral symmetry breaking scheme in QCD effective theories at low energies

[en] The DIRAC spectrometer has been commissioned at CERN with the aim of detecting π⁺π^- atoms produced by a 24 GeV/c high intensity proton beam in thin foil targets. A challenging apparatus is required to cope with the high interaction rates involved, the triggering of pion pairs with very detector efficiency.ation of the imaging sof the latter with resolution around 0.6 MeV/c. The general characteristics of the apparatus are explained and each part is described in some detail. The main features of the trigger system, data-acquisition, monitoring and set-up performances are also given