1. Future prospects on K-decays at KEK Hyosang Lee Pusan National University CKM2006 @ Nagoya

2. Content Physics motivation KEK-PS E391a Experiment J-PARC Experiment Summary

3.  L    decay Theoretical uncertainty : 1~2%  Direct measurement of  (0,0) (1,0)   Comparison with K and B decays VtdVtd Powerful tool to check SM Signature of new physics

4.  L   experiment Extremely challenging small branching fraction many background sources : K L      … 3 body decay with neutrinos weak kinematical constraint all particles : neutral Standard model E391a Pilot experiment for the K L   

5. Experimental method KLKL    Period# of K L decay Run-IFeb.’04~Jul.’041.6 X 10 10 Run-IIMar.’05~Apr.’053 X 10 9 Run-IIINov.’05~Dec.’053 X 10 9 Total2.2 X 10 10

6. Analysis Z vtx PTPT   KLKL   KLKL    L   L       KLKL  Halo neutron Core neutron Halo neutron   KLKL  signal region

7. 1-week data of the Run I Run I 10% of RunI full data New upper limit x2.8 improvement from the current world limit published in Phys. Rev. D 74, 051105(R) (2006) 1/3 data of the Run I

8. Run II-III P T (GeV/c) Z vtx (cm) Run II-1/3 data Cleaner Data sample Main BG Halo neutron N KL =5.28x10 9 Sensitivity of RunII full 1.15x10 -8

9. E391a New upper limit BR<2.1x10 -7 (90%C.L.) Good Development of detection method for K L   0 Figure out subjects to improve for J-PARC experiment

10. Measure Br with over 100 K L   events at J-PARC Two-step approach – Step1: First observation E391 detector with necessary modifications To share production target with other experiment New readout system – Step2: Precise measurement Optimized KL beam line Larger detector J-PARC E14

11. Beam line at step 1 K L 8.1x10 6 /spill step1 PS-K0 softer neutron n/K=9(42 for E391) : reduce n-induced BG Momentum of KL Momentum of neutron

12. Detector at step 1 7cm x 7cm x 30cm 2.5cm x2. 5cm x 50cm

13. BR( ) in time

14. Summary Measurement of K L ->   BR  A golden mode for SM test, new physics New upper limit - RunI 10% data samples E391a full data analysis is under going. Final Sensitivity will be order of 10 -9 J-PARC E14 experiment to measure Br(K L   )  Step 1 first observation  Step 2 Measure Br with over 100 K L ->   events with < 10%

15. High Energy Accelerator Research Organization, KEK, Japan J oint I nstitute for N uclear R esearch (Dubna), Russia I nstitute of H igh E nergy P hysics (Protvino), Russia Department of Physics, Kyoto University, Japan N ational D efense A cademy of J apan, Japan Department of Physics, National Taiwan University, Taiwan Department of Physics, Osaka University, Japan Department of Physics, Pusan National University, Korea R esearch C enter for N uclear P hysics, Osaka University, Japan Faculty of Science and Engineering, Saga University, Japan Enrico Fermi Institute, University of Chicago, USA Department of Physics, Yamagata University, Japan Collaboration

24. J-PARC Japan Proton Accelerator Research Comp lex World’s highest intensity beam Construction at Tokai  First beam in 200 8 MR(50GeV PS) perimeter~1.6km 30 GeV for slow ext. 2  10 14 ppp 0.3MW 0.7s spill/3.3s repe. T1 Ni Target E391 det. at 16 deg line proton Exp Hall 20m neutral beamline