A test of QCD by measuring a life-time of hadoronic atoms.

在低能区，由于夸克间的相互作用太强而不能作为微扰来处理，QCD的有效性还没有得到证明。本研究的目的是通过测量π + π-束缚态的π原子寿命来检验未微扰低能量子色动力学<2pi>。（DIRACCERN-PS212）<2pi>用手征微扰理论精确地预言了A_（1-x）的寿命与S态π-π散射的散射长度有关。为了在实验上<2pi>从大量的背景事件中提取出真实的稀有A_事件，需要研制一种能在真实的时间内提供一定拓扑信息的触发装置。本文采用的拓扑触发装置由偏振光纤振迹仪（SCIFI）、位置敏感光电倍增器（PSPM）、峰值读出电路（PSC）和触发逻辑电路（TLC）组成。1995-1996年期间，我们研制了一种偏振光纤振迹仪，它能有效地测量多个碰撞粒子之间的距离。 ...更多信息 设计了示波器和峰值检测电路，并制作了样机。用24 GeV质子产生的高能次级粒子测试了它们的性能。我们测量了拓扑触发器所需的基本性能，如光纤衰减长度、时间分辨率、脉冲高度分布均匀性、通道间串扰、探测效率、多粒子识别能力等。触发模式和触发率特性的衰变从介子原子和那些竞争的背景过程被拒绝研究，蒙特卡罗模拟根据仿真和实验结果，最终设计出最适合的触发逻辑电路，并于1997年建成了一套完整的真实的尺寸拓扑触发装置，并利用BEAM进行了整体性能测试。实验结果与精确的离线分析和MonteCarlo模拟结果进行了比较，表明该装置能产生拓扑触发信号，在事件识别、时间分辨率、位置分辨率、触发速度、探测效率和粒子多重性识别等方面都有令人满意的性能。该触发装置将作为第二级触发发生器用于DIRAC实验。少

A validity of QCD has not been proved yet at low energy region, where the interaction between quarks is too strong to be treated as a perturbation. The aim of this research is to test the QCD at an unperturbed low energy by measuring a life time of pionic atoms A_<2pi> (bound state of pi+pi-) with 10% accuracy. (DIRACCERN-PS212) The life time of A_<2pi> is related to the scattering length of pi-pi scattering in S-state and is preciesly predicted by a Chiral perturbation theory. To extract real rare A_<2pi> events from huge amount of background events experimentally, we have to develop a new type trigger device providing some topological information in the real time. The topological trigger device employed here consists of scintillating fiber hodoscopes (SCIFI) followed by position sensitive photo-multipliers (PSPM), peak sensing circuit (PSC) for a read-out device, and a trigger logic circuit (TLC) giving distances among multiple hit particles.During 1995-1996, a scintillating fiber ho … More doscope and a peak sensing circuit were designed and prototypes of them were constructed. Their performances were tested by using high energy secondary particles produced by 24GeV protons. We measured such fundamental properties needed for our toporogical trigger device as an attenuation length of a scintillating fiber, a time resolution, an uniformity of pulse height distribution, cross talk among channels, a detection efficiency, an ability of multi-particle identification etc. At the same time, triggering patterns and triggering rates characteristic of the decay from the pionic atoms and those of competing background processes to be rejected were studied by a Monte Carlo simulation. Based on the simulation and the experimental results, the final design of the best suited triggering logic circuit was drawn.In 1997, a complete set of a real size topological trigger device was constructed and tested the performance as a whole by using beam. The experimental results obtained from the trigger device were compared with the precise off-line analysis and/or the Monte Carlo simulation, and showed that the device can generate topological trigger signals with satisfactorily high performance in the event identification, time resolution, position resolution, triggering speed, detection efficiency, and partlicle multiplicity identification.After the autumn in 1998, this trigger device will be used for thr DIRAC experiment as a second level trigger generator. Less