Development of detectors for CP violating rare decays

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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.