Measurement of the scattering length of pions and the test of QCD

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The aim of this project is to measure directly the lifetime of π^-π^+ atoms bound by the Coulomb force, and deduce the scattering length of the s-wave interaction. We aim at measuring it with a precision of 5%, and test the QCD in the non-perturbative region by comparing it with a prediction based on the chiral perturbation theory.The proposal of the "DIRAC" experiment (PS212) was approved by CERN in 1996, and since, we started the construction of the spectrometer. In 1999, a serious data taking has started. Since then, we were successful in gradually increasing the extracted proton beam intensity by taking measures to reduce the background. Concerning the trigger that the Japanese group is responsible for, tuning of the Scifi hodoscope and the dE/dx counter, improvement of the read-out electronics and the integration circuit, improvement of the trigger circuit contributed greatly to the improvement of the experimental conditions. The data taking now continues smoothly, and the data taken in 1999 have been roughly analyzed. The analysis of the data taken in year 2000 is under way. In the preliminary result of the 1999 data, 266±79 atomic pairs with a loose cut, 160±45 with a tight cut, and 240 atomic pairs as a whole have been identified.Considering the proton beam intensity already ten times higher than in 1999, we should be able to deduce the scattering length with 5% precision within 2-3 years. Part of the result is already reported in conferences. We plan to publish the result of the 1999 data in May 2001, and that of 2000 in this autumn.