Preshower detector for DIRAC experiment (CERN PS-212)

The DIRAC (DImeson Relativistic Atom Complex) experiment with a magnetic double arm spectrometer, aims to measure the lifetime of π+π- atomic bound state A2π or dimesoatom (pionium) in the ground state, using the high energy and high intensity proton beam of the CERN Proton Synchrotron. There exists a precise model - independent relationship between the lifetime τ and the difference of the isoscalar a0 and isotensor a2 S-wave ππ scattering lengths: 1/τ=C·|a0 - a2|2. The direct experimental study of the ππ scattering is a difficult task, first of all due to impossibility to prepare a pion target. At the same time, the theoretical description of the ππ scattering at low momentum transfer is still an unsolved problem in the frame of QCD, due to the fact that the strong coupling constant αs becomes large, and a perturbative approach cannot be justified. Several other methods of approaching QCD have been developed. An approach based on Effective Lagrangian and Chiral Perturbation Theory (ChPT) is strongly motivated to look for a simpler Lagrangian that displays essential features of QCD. This approach allows to predict the S-wave ππ scattering lengths within 5%. These predictions have not yet been tested at such a level of accuracy. To determine |a0-a2| down to 5%, the A2π lifetime must be measured, at least with 10% accuracy. Such a measurement would submit the understanding of chiral symmetry breaking of QCD to a crucial test. The main experimental task is to separate the real events (the two charged pions of the broken-up dimesoatom) from the large electron and pion background. For an efficient electron/pion separation, the DIRAC setup includes a preshower detector (PSD). Design, preparation and test of this detector along with data control and acquisition program, are the main tasks of the Romanian DIRAC team. The PSD performs the sampling of the early part of the particle shower produced by incident relativistic particles interacting with the converter material. Along some radiation lengths of converter thickness, the electron shower have begun to develop, but the pions, interacting as minimum ionizing particle (mip), produce a shower so shallow that only a small fraction of the charged pions have interacted. Therefore, the corresponding amplitude distributions are well separated in the experimental spectra. We tried to find the experimental conditions accomplishing the best possible separation of the pion end electron distributions. The electron rejection and pion loss have been measured experimentally. The results show an electron rejection better than 85% with a pion loss less than 5%. (authors)