1. October 3, 2005 @ ICFP 2005 Takao INAGAKI （ KEK) for the E391a collaboration Status of E391a experiment for the rare decay

2. Theoretically it is very pure and clean, but it is a very rare decay

3. Recently confirmed that the theoretical uncertainty is also very small in most of calculations of new physics. hep-ph/050517 Possible Scenario

4. Experimentally very challenging Small branching fraction  Any tiny effects are possible sources of background Weak kinematical constraint (all neutrals, three-body decay with invisible neutrinos, etc)  Not easy to separate the signal from background This is the reason why the present limit stays at 5.9X10 -7 (KTeV, 2000) and why E391a is the first dedicated experiment in spite of a theoretically dream channel.

5. Strategy: step-by-step approach E391a at KEK 12 GeV PS O(10 -10 ) An experiment at JPARC 50 GeV PS O(10 -14 )

6. Collaboration and status of E391a ● The third data taking (Run-III) is scheduled in Nov/2005 E391a collaboration 11 institutes, ~50 members from 5 countries Univ. of Chicago, JINR, KEK. Kyoto Univ., NDA, National Taiwan Univ., Osaka Univ., Pusan National Univ., RCNP, Saga Univ., Yamagata Univ. Status of experiment First data taking was carried out from Feb/2004 to July/2004 Second data taking (Run-II) was carried out from Feb/2005 to April/2005

7. 0.25 GeV/c 0.13 GeV/c

8. Three important tools

9. Additional discussion about detection inefficiency, which causes photon veto miss ・ The inefficiency strongly depends on gamma energy. It is large at low energy ↓ Hi ｇｈ P T selection ・ The inefficiency strongly depends on the detection threshold ↓ Low detection threshold Concern ： how we can lower it under high rate environment. Understand the detector response at low energy deposit.

13. Final plot Using 10% sample of Run-1

16. What we have learned

17. Background By understanding the sources of side-band events

23. Core neutron Halo neutron Kpi2: negligibly small

26. Back splash It is clearly observed for the main barrel having both-end readout. Mean time Time difference(position)

29. Introduction of J-PARC our next playground

30. Configuration of the accelerators Neutron Muon

33. Layout K0 line is included in the construction plan

34. Construction Schedule

35. Summary 1. is one of the pure and clean channels in the field of quark flavor physics. It provides a critical and unambiguous check of the Standard Model. 2. E391a, which is the first dedicated experiment, has started at KEK 12-GeV PS successfully. 3. Using 10 % sample of Run I, we obtained a new limit of 2.9×10 -7 (90%cl). 4. We are learning many things, background sources, the rate of extremely low energy deposit etc. They must be quite important for a future extension. 5. J-PARC, which is expected to be our next play-ground, is now being constructed steadily. We are preparing a proposal.