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An Experiment for the K

The rare K decay K L → π 0 νν̄ is considered as one of ideal processes in the flavor physics to make a critical test for the standard model and a search for new physics beyond it. The E391a experiment at the KEK 12-GeV proton synchrotron is the first dedicated experiment for the K L → π 0 νν̄ decay. The experiment started a data taking in February 2004 aiming at an improvement of sensitivity from the present experimental limit by three orders of magnitude and is planned as a pilot experiment that the study would be extended to a higher sensitivity experiment at the 50-GeV PS in J-PARC. Here, we would like to make a brief report on the physics interest, principles of experimental method, the current data analysis and future plans.