A Test of QCD at Low Energy by measuring a Lifetime of Di-mesonic Atoms

点击翻译按钮获取中文摘要

The DIRAC experiment (PS2 12) aims to measure the lifetime of π+π-atoms using 20GeV proton beam provided by the CERN PS. The precise measurement of the lifetime enables to determine a s-wave pion-pion scattering length in a model independent way. The ππ scattering length have been calculated by means of Chiral perturbation theory with a precision of 5% at the starting stage of the DIRAC experiment. Now the precision of the calculation is better than 2.5%. The results are based on the assumption of a spontaneous chiral symmetry breaking due to a strong quark condensation. Therefore, a precise comparison of the theoretical value with the experimental one provides an opportunity to verify the current understanding of chiral symmetry breaking in QCD and to check the magnitude of the quark condensation.Japan group has developed a topological trigger device consisting of scifi-hodos, a peak sensing circuit and a trigger logic circuit. They can select out on line a real π+π-pair due to π+π-atom breaking-up from a huge amount of background events.The expected statistical accuracy of |aO-a2| scattering length obtained in the DIRAC experiment will be about 6% at the present stage. Although this value was a goal at the beginning, now it is still 3 times larger than the corresponding theoretical accuracy. An extension of this experiment applied for the LOI of the J-PARC is adopted