KL → π0νν-bar experiment. KEK-PS E391a

The KL → π0νν-bar decay is one of the most attractive phenomena to access the CP violation. From the theoretical point of view, its branching ratio can be calculated with very small ambiguity, while it needs several orders of improvement of sensitivity to measure it experimentally. In stead of leaping over the large gap at once, it would be more reasonable to carry out a kind of pilot experiment having its sensitivity close to the SM prediction - KEK-PS E391a. The E391a scheduled to start data taking within 2003 will give us many important informations about the tiny branching ratio region, which will guide us to the next stage of the experiment using the high intensity proton machine (JHF) to be built. (J.P.N.)