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

该项目的目的是直接测量受库仑力束缚的π^-π+原子的寿命，并推导出S波相互作用的散射长度。我们的目标是以5%的精度测量它，并将其与基于手征微扰理论的预测进行比较，在非微扰区测试QCD。1996年，“狄拉克”实验(PS212)的提议得到了欧洲核子研究中心的批准，从此我们开始了光谱仪的建设。1999年，一场严肃的数据采集已经开始。从那时起，我们通过采取措施降低本底，成功地逐步提高了提取的质子束强度。关于日本小组负责的触发器，科学显微镜和De/DX计数器的调谐，读出电路和积分电路的改进，触发电路的改进对实验条件的改善做出了很大贡献。目前数据采集工作进展顺利，对1999年采集的数据进行了粗略分析。目前正在对2000年的数据进行分析。在1999年数据的初步结果中，共识别出266±79个松切原子对，160±45个紧切原子对，240个原子对，考虑到质子束强度已经比1999年提高了10倍，我们应该可以在2-3年内以5%的精度推算出散射长度。部分结果已经在会议上得到了报道。我们计划在2001年5月公布1999年的数据，并在今年秋季公布2000年的数据。

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.