DIRAC: A high resolution spectrometer for pionium detection

Pionium atom is a metastable electromagnetic boundstate system of a π+ and a π-, produced by Coulomb interaction of the components and decaying into π0π0 (∼ 99.6%) via strong interaction. In the last time the width of pionium (ground state) Γ2π has been calculated by considering the isospin breaking effects using the effective Lagrangian framework, i.e. Chiral Perturbation Theory (ChPT). The expression of Γ2π is given as a function of the K and the scattering amplitude at threshold of the π+π- → π0π0, decay which has been evaluated from relevant Feynman diagrams. Expression of A in the isospin symmetry limit (α = 0 and mμ = md) is given in terms of strong ππ scattering lengths evaluated in QCD in the isospin symmetry limit e 0, mμ = md. The quark masses are tuned so that the pion mass in the isospin symmetry world coincides with the charged pion mass mπ mπ+. Evaluating K one obtains K= (1.15 ± 0.03) · 10-2. DIRAC - a double-arm spectrometer has been built and installed at CERN with the purpose of detecting the signal of pionium (π+π-) atom breakup produced by 24 GeV/c high intensity proton beam collisions on thin-foil targets. The spectrometer achieves a resolution of 0.6 MeV/c (0.4 MeV/c) in the longitudinal (transverse) components of the relative momentum of the π+π- pairs in their center-of-mass frame. It is capable to handle a large particle fluence, corresponding to a 1.2 x 1010 primary protons impinging on the target within a 400-500 ms burst. The main spectrometer components are the tracking detectors (drift chambers and upstream trackers, both with single-detector space resolution better than 900 μm), two threshold Cherenkov counters and pre-shower detectors which provide jointly a e/π ratio better than 10-4, and the triggering and time-of-flight hodoscope system, with 110 ps single-counter resolution. The spectrometer has the additional capability to identify very small opening angle pairs by measuring the double-ionisation produced in dedicated hodoscope layers upstream the dipole magnet. The trigger system achieves selection of pion pairs with relative center-of-mass momentum less than approximately 30 MeV/c, suitable to study the Coulomb region where pionium production takes place as a narrow state. Accidental pairs within a time gate of 40 ns are recorded by the data acquisition system, along with prompt pairs, in order to facilitate Coulomb interaction analysis. A schematic view of the DIRAC spectrometer is given. (author)