Results from the E391a experiment at KEK

The E391a is the first experiment dedicated to the decay. Its primary goal is to establish an experimental method which is expressed as ‘pencil beam + hermetic veto’, for a precise measurement of this rare process. There were three times of data taking during 2004–2005 at the 12-GeV proton synchrotron in KEK, Japan. In this article, a brief introduction and results of the E391a is reported.