Letter of Intent for Experiments at 50 GeV PS of the JPARC Measurement of the K 0 L → π 0 νν Branching Ratio

The K0 L → π0νν decay is a uniquely pure and clean process. The precise measurement of the K0 L → π0νν decay branching ratio, which we are proposing here, will provide a clean determination of a basic parameter of the present particle physics, and will play an important role for a new-physics search and for a profound understand of the CP-violation. The 50 GeV Proton Synchrotron (PS) is the best place for such a measurement. It can deliver a K0 L beam whose intensity is much higher than that of the present 12 GeV PS at KEK by about three hundreds times. Since we are going to start a kind of pilot experiment using the present 12-GeV PS in KEK, the plan, which we are proposing as a high sensitivity experiment at the 50-GeV PS, is a natural extension of our program. After modest upgrades of the detectors, we will be able to get ready at the Time(0) for the experiment at the 50 GeV PS. The goal of the proposing experiment is to achieve a sensitivity of less than 3 × 10−13, which corresponds to an observation of more than hundred K0 L → π0νν events for the Standard Model prediction and to a determination of η in less than 5% accuracy. 1 Physics Motivation The K L → πνν decay is a rare decay process of flavor-changing-neutral-current (FCNC) with CP violation. A direct term, which proceeds through a ∆S=1 transition as expressed by diagrams shown in Figures 1, dominates a mixing term which proceeds through a ∆S=2 transition where K L changes into K 0 S via K 0−K0 mixing[1]. Then, the K L → πνν decay will offer a pure information of the ∆S=1 process. The hadronic matrix element can be factorized as the well-known branching ratio of the K → πeν decay [2]. The remaining corrections which couple to the virtual top quark are calculable and small due to the large top-quark mass [3]. A long-distance interaction little contributes to the K L → πνν decay, because neutrino is a weekly interacting particle[4]. Consequently an uncertainty in the theoretical calculation is estimated to be only a few % [5], and the K L → πνν decay will offer a clean information of a basic parameter of the present elementary particle physics. In the Standard Model the decay amplitude of K L → πνν is proportional to the imaginary part of a product of CKM matrix elements, Im(VtdV ∗ ts), which corresponds to