Future prospects on K-decays at KEK Hyosang Lee Pusan National University Nagoya Content Physics motivation KEK-PS E391a Experiment J-PARC Experiment Summary 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 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 Experimental method KLKL Period# of K L decay Run-IFeb.04~Jul.041.6 X Run-IIMar.05~Apr.053 X 10 9 Run-IIINov.05~Dec.053 X 10 9 Total2.2 X 10 10 Analysis Z vtx PTPT KLKL KLKL L L KLKL Halo neutron Core neutron Halo neutron KLKL signal region 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, (R) (2006) 1/3 data of the Run I 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 E391a New upper limit BR 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 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% 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 J-PARC Japan Proton Accelerator Research Comp lex Worlds highest intensity beam Construction at Tokai First beam in MR(50GeV PS) perimeter~1.6km 30 GeV for slow ext. 2 ppp 0.3MW 0.7s spill/3.3s repe. T1 Ni Target E391 det. at 16 deg line proton Exp Hall 20m neutral beamline