The Last experiment at the 12 GeV proton Synchrotron
 
One of the most challenging experiments in particle physics was completed last December at East Counter Hall in KEK. That was the E391a experiment to measure the decay branching ratio of . E391a was the last particle-physics experiment using the 12-GeV proton-synchrotron, which terminated its operation in December 2005..
 
The decay is expected to be extremely rare as the branching ratio is predicted to be 10-11 by the Standard Model. All the particles produced by the decay are neutral in electric charge, which are hard to be detected. Hence, no dedicated experiment had been carried out before, although its uniqueness was pointed out in 1989..
 

The decay is a pure and simple process without the complication from the ambiguity of strong interaction corrections. Its measurement provides us the decisive answer to several problems of particle physics, such as the imaginary part of the transition amplitude from down to top quarks, the effect of super-symmetry and the additional source for CP violation..
 
E391a detector, which was installed in a large volume of vacuum chamber (Figure below), was constructed in February 2004. Figure on page 10 shows the members celebrated a successful installation of the main barrel calorimeter. The data collected so far are under analysis.
 
E391a collaboration is composed of 60 members from 11 institutes from five countries (Korea, Russia, Taiwan, USA and Japan).