Development of detectors for CP violating rare decays

我们正在计划一个新的实验，通过测量K_L→π^0vv^^-的分支比来测量标准模型η中的CP违背参数。这个衰变的主要背景是K_L→π^0π^0，在这里我们丢失了4个光子中的2个。为了抑制这个背景，我们将用铅/闪烁体夹层制成的密封光子计数器覆盖衰变区域。这是对我们所做的工作的总结。我们发现，如果将铅/闪烁体夹层层与光束线平行（0度），而不是以60度角对齐，则检测效率会降低。此外，如果将闪烁体层扩展到暴露的铅片表面，则检测效率也会提高。我们计划用嵌入在沟槽里的带波长移位器的光纤来读出闪烁体平面发出的光。我们研究了在光纤中收集的闪烁光的位置依赖性。我们发现，光输出由光纤的立体角、沟槽处的反射/扩散以及表面反射处的光损失决定。测量结果可通过简单的模型计算得到再现。为了在抑制背景事件的同时保持较高的灵敏度，我们对实验进行了优化。我们测量了150GeV质子束产生的介子和中子的通量和能谱。在上述研究的基础上，我们正在准备一份新实验的提案，将于4月份提交。

We are planning a new experiment to measure the CP violating parameter in the standard model, η, by measuring the branching ratio of K_L→π^0vv^^-. The major background to this decay is K_L→π^0π^0 where we miss 2 out of 4 photons. In order to suppress this background, we will cover the decay region by a hermetic photon veto counters made of lead/scintillator sandwich.Here is a summary of what we have done.1. We found that the detection inefficiency of lead/scintillator sandwich would be smaller if the layers were aligned at angle (60 degrees) instead of aligning them in parallel (0 degrees) to the beam line. Also, the detection efficiency increases if the scintillator layers are extended to coverthe surface of exposed lead sheets.2. We are planning to read out the light from scintillator planes by using optical fibers with a wave length shifter, embeded in grooves. We studied the position dependence of the scintillator light collected in the fiber. We found that the light output is determined by the solid angle of the fiber, reflection/diffusion at the grooves, and the light loss at the surface reflections. The measured results were reproduced by simple model calculations.3. We have optimized cuts for the experiment to keep a high sensitivity while suppressing background events from K_L→π^0π^0.4. We have measured the flux and the energy spectrum of kaons and neutrons produced by 150GeV proton beam.5. Based on above studies, we are now preparing a proposal for the new experiment to be submitted in April.