Detection of Pionium with DIRAC

The aim of the DIRAC experiment at CERN is to provide an accurate determination of S-wave pion-pion scattering lengths from the measurement of the lifetime of the pi+ pi- atom. The measurement will be done with precision comparable to the level of accuracy of theoretical predictions, formulated in the context of Chiral Perturbation Theory. Therefore, the understanding of chiral symmetry breaking of QCD will be submitted to a stringent test

The WLS fiber time properties study

The pulse shape properties of different wave-length shifting fibers have been measured. This study intents to help in the right choice of fibers in terms of their further application in the LHCb Hadron Calorimeter as zero-level trigger detector. The measurements have been clone both at the X7 test beamline of the CERN SPS and using ultra-violet NZ laser as a short light pulse source. Fibers under study have been attached to the standard polystyrene based scintillating tile used in HCAL Prototype. The fast photo-multiplier FEU-115M and digital recording oscilloscope were used. It was found that the fiber BCF-91A is rather slow (rd > 10ns) to be accepted for calorimetry in LHCb. The Fibers Y-11 and Pol.Hi.Tech. are found to be faster (Td N 7 ns), and could be used provided their signals are considerably clipped. The fiber BCF-92 shows best results (rd N 3 ns)

Dirac experiment

The main objective of DIRAC experiment is the measurement of the lifetime tau of the exotic hadronic atom consisting of pi /sup +/ and pi /sup -/ mesons. The lifetime of this atom is determined by the decay mode pi /sup +/ pi /sup -/ to pi /sup 0/ pi /sup 0/ due to the strong interaction. Through the precise relationship between the lifetime and the S-wave pion-pion scattering length difference a/sub 0/-a/sub 2/ for isospin 0 and 2 (respectively), a measurement of tau with an accuracy of 10% will allow a determination of a/sub 0/-a/sub 2/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. (19 refs)