1. Measurement of K L    decay G.Y.Lim IPNS,KEK

2. Why K L →    ? Flavor Changing Neutral Current Br(K L   o  ) = 6  1 ・ Im(V td V ts ) 2 X 2 (x t ) = 1.94 ・ 10 -10  2 A 4 X 2  Strong Interaction : Factorized by K e3 Decay Very Small Theoretical Uncertainty Negligible Charm contribution

3. In JHF era Unitarity Triangle vertices Beyond Standard Models K +    K L    A CP (B  J/  K S ) MBd/MBsMBd/MBs   G. Buchalla hep-ph/0110313 L.Littenberg HEPAP(2001) E391a

4. Search for the decay K L             e + e -  Phys. Lett. B447(1999) Phys. Rev. D61 (2000) SM Prediction L. Littenberg E731 E799 KTeV BR(K L   o ) Year

5. Decay Mode Branching Ratio Additional Particles  o  o    +    o       e          + e    e                       e + e   e + e ｰ  21.13 % 12.55 % 27.18 % 38.78 % 5.86 ・ 10 -4 <2.4 ・ 10 -7 1.68 ・ 10 -6 5.18 ・ 10 -5 3.62 ・ 10 -3 5.7 ・ 10 -4 4.61 ・ 10 -5 <5.6 ・ 10 -6 3.25 ・ 10 -7 10.0 ・ 10 -7 6.9 ・ 10 -7 4  2Ch 1  2  2Ch 3  2Ch Decay ModeBranching Ratio Additional Particles    e + e            e  e      e  e      e  e  e  e          e  e    e    e  e       e  <7.1 ・ 10 -7 2.06 ・ 10 -3 9.27 ・ 10 -4 7.15 ・ 10 -9 9 ・ 10 - 12 3.5 ・ 10 -7 2.9 ・ 10 -9 4.1 ・ 10 -8 <5.1 ・ 10 -9 <4.3 ・ 10 -9 <5.9 ・ 10 -7 <4.7 ・ 10 -12 <6.1 ・ 10 -9 <6.2 ・ 10 -9 1  2Ch 2Ch 2  2Ch 4Ch 2Ch 4Ch 2Ch K L Decay Modes

6. Inefficiency of  detection Inefficiency for charged particles Detector Inefficiency NIM A359 p478 (1995) B.G. from K L   - e + Decay Separating charged veto

7. High Energy Accelerator Research Organization, KEK Faculty of Science and Engineering, Saga University Department of Physics, Yamagata University Department of Physics, Osaka University Research Center for Nuclear Physics, Osaka University National Defense Academy of Japan Department of Physics, Ibaraki University Joint Institute for Nuclear Research (Dubna) Russia Department of Physics, University of Chicago Fermi National Accelerator Laboratory Department of Physics, Pusan National University http://psux1.kek.jp/~e391/index.html E391a Collaboration

8. KEK-PS E391a First dedicated experiment Pilot experiment for next step (JHF) Figure out sources of background Developing method of rejection/estimation Realizing detector setup in high vacuum system Compact detector system High acceptance Pencil beam Double decay chamber Highly evacuated decay region High P T selection

9. How far we have to go ? K e3, K  3, K  3 KTeV E391a JHF KOPIO(BNL) S. M. Prediction BR(K L   o )  Current Exp. Limit New Physics (?) Limit from K +   +  PRL 84, 3768 (2000)) hep- ph/9908399 hep- ph/9804412 hep- ph/9808487

10. E391a Detector Setup K L     Nothing  pure CsI calorimeter 4  veto system

11. Pencil Beam

12. Beam Profile

13. Layout of Beam survey Moving Rail Hodoscopes (15x15) Profile detectors Cerberus Thermal Neutron Detectors (LiI, Li glass) CsI n/  detector (SLSD) Lead glass K L monitor Liquid Scin. BGO Copper K L monitoring using regeneration H. Watanabe (2002) : Revised B.G. estimation

14. Schedule 2001 2002 2003 2004 Data Taking Assembling Detector setup Vacuum & Cosmic Test Assembling Front barrel Construction Main barrel Construction Cosmic Run Beam surveyEngineering Run Beam Test Data Taking Jan. Apr. Jul. Oct. CsI

15. High flux of K L Large acceptance Suppression of the Background Standard model event B.G. linear to beam intensity  Same S/N ratio (~3) New physics Newly designed detectors system Physical motivation Following talk Search  Measurement

16. Large Number of K L Decay Three orders larger K L flux Beam will be given from 2007 Two stages of construction Limited beam lines Many experimental programs K L    Measurement Experiment at the first stage of JHF Flexible approach JHF

17. E391a at JHF Experience at KEK-PS Same veto system Low cost, earlier starting New calorimeter resolution, clustering, efficiency Fast DAQ High counting rates Optimization of beam line K L /n/  ratio,  production T.Inagaki CP Violation in K Dec. (1998)

18. In future ・・・ E391a SM Expectation KOPIO JHF Year BR(K L   o )

19. Summary Golden channel to access CP violation (K L   0 ) KEK-PS E391a Pilot experiment for JHF The first high sensitivity experiment Plan for data taking on 2003 JHF Real measurement of branching ratio Most clean measurement of CPV parameter Unitary triangle in K-system Potential to access the New Physics LOI within 2002 !