1. Experimental Search for the Decay K. Mizouchi (Kyoto University) (1) Physics Motivation (2) Detector (3) Selection Criteria (4) Branching Ratio (5) Background Subtraction (6) Conclusions

2. [1] Helicity suppressed decay : Physics Motivation : left-handed (in SM): spin 0 (A) Neutrino mass : implies. [3] Cosmological Interests Neutron star cooling model through pion pole mechanism : (B) Neutrino type : Majorana neutrino  x2 larger branching ratio. [2] Decay Form of (B) Decay into different neutrino flavors : (A) Sensitive to any hypothetical weakly-interacting neutrals.

3. Event Detection Strategy Hermetic photon detection system Charged particles from K + decay at rest (1) Clean K  2 selection (2)  0 to invisible final states K2K2 K2K2 Prior best limit : (E787)

4. E949 Detector E949 detector end view (upper half) (1) Barrel Veto (BV): Pb-scintillator sandwich (2) Barrel Veto Liner (BVL) : Pb-scintillator sandwich (3) Endcap Calorimeter: CsI crystals E949 detector side view (upper half)

5. Analysis Strategy (1) K  2 selection 1/3 sample 2/3 sample  0 sample (2) Find the best photon veto parameters  0 sample ( ) Signal candidate (N) Offline Data (K  2 rich) tuning Acceptance C acc

6. K  2 selection and none-K  2 bkgnd Impurity : ~10 -9 Real data (2/3 sample)

7. Overlapping ,e +/- (from  0 ) may cause disruption in the  + track reconstruction. Disruption Correction Factor C dis Disruption correction :

8. (2) Hermetic Photon Veto (1) K  2 Tag … Done.

9. Acceptance Measurement C acc Measure acceptance loss of K  2 decays (real data) by the photon veto, after all  + activities are removed. acceptance loss due to coincident accidentals ++ accidental

10. Maximization of the Sensitivity  0   rejection : (2)  0  acceptance : [ Hermetic photon veto ] Find the best parameters; the largest rejection with the given acceptance. Real data “1/3 sample” Photon veto rejects events with : E sum in [T 1,T 2 ] > E threshold Acceptance Effective  0 rejection (= rej×acc) Final photon veto

11. Opening the Box A total of 99 candidates were observed in the signal box Kaon decay time (ns)  + momentum (MeV/c) Real data “2/3 sample”

12. New upper limit : A factor of 3 improvement from the previous best result. Branching Ratio # signal < 113 (90%CL) subtracting the non-K  2 bkgnds; Conservative upper limit 2/3 sample Saturation at 3.5x10 6 1/3 sample

13.  0   Background subtraction K  2 w/ one photon missing event Measurement of the detector single photon inefficiency Relaxed photon veto (acc = 0.80) (1)Establish a background subtraction method (2)Understand the detector performance

14. Single Photon Inefficiency

15.  0   detection inefficiency from MC simulation (N events) (2) Photon kinematics(1) Single photon inefficiency P SPI =

16. Number of candidates with relaxed photon veto  0   background subtraction 4131 events A factor of 1.8 improvement Singal (90% C.L) : 2259 Arbitrary

17. Subtraction at various levels of photon veto Improvement (Before/After) A factor of ~ two improvement at various photon veto

18. Num of  0   backgrounds as a function of cos(   + ) Single photon inefficiency Signal discrimination capability from backgrounds Signal candidates

19. Background Subtraction with dip angle distribution Candidates : s raw = 4131 Best fit value : s = 1977 90 % C.L. : s 90 = 2449 A factor of 1.7 improvement Ref. w/o subtraction :

20. Conclusions (1) search was performed with 3.02x10 9 K  2 events, where impurity of 10 -9 was achieved. (2) New upper limit of was obtained with a total number of 99 candidates in the signal region; x3 improvement from the previous best limit. (1)Single photon inefficiency was measured with special data  0   background subtraction was performed with the inefficiency; (A) x1.8 improvement with simple subtraction (B) x1.7 improvement from cos(   + ) shape discrimination

21. Thank you !

22. Early accidental hits

23. Two peaks in BVL

24. Background distribution

25. K  2 photon kinematics

26. Unvetoed hits in Candidates Unvetoed hits in BV Outside the veto time window. Lower energy than threshold.

27. Unvetoed hits PV for single photon study PV for search (1) (2) (3) (4) (5) (6) (1) (2) (3) (4) (5) (6)

28. K  2 photon kinematics

29. Background understanding and detector inefficiency ? (2) Event reconstruction Missing photon kinematics (1)Different type of critical backgrounds. (2)Geometrical dependence : Detector hole, dead material (3)Energy dependence : Photonuclear interaction … Can we understand the remaining events from a view of photon inefficiency ? ( if possible, subtract them as backgrounds.) (1)Special trigger K  2 but one photon is missed. (3) Photon inefficiency as a function of its energy and direction An Idea : NOTE :

30. Barrel Veto Liner

31. K +   + + “nothing” in E949 (1) K +   + (above K  2 ) Published in PRL, 93 031801 (2004)  0  (on K  2 peak) This report. Need tighter photon rejection. (3) K +    (below K  2 ) Analysis ongoing. Require more sophisticated treatment in  + multiple scatterings. Charged track momentum from various K decay modes

32. Published in PRD as rapid communication Phys. Rev. D72, 091102 (2005)

33. Optimized Photon veto parameters

34. Performance of the clustering Method MC sample theta

35.  0  backgrounds 20~40MeV 40~60MeV 80~100MeV 60~80MeV 100~120MeV 120~140MeV 140~160MeV Detector photon inefficiency (measured with real data) Photon inefficiency 20<E  [MeV]<225 Low energy  : sampling fluctuation High energy  : photonuclear interaction ( hard to simulate reliably.)

36. Phase space correction factors Polar angle distribution Correction factors Monte Carlo simulation Real data

37. Self-vetoing effect due to split photon MC simulation Missing photon kinematics Missing-side

38. (3) : Trigger prescale compensasion, (2) : mis- detected photon distribution (1) : raw photon distribution Single Photon Inefficiency K  2 w/ one photon missing event Measure single photon inefficiency with real data.

39. Analysis Strategy K  2 w/ one photon missing event (1)Reconstruct (tagging) photon (2)Extract kinematics of the mis- detected photon. (3)Correction factors

40. (1) Photon Clustering Method Reconstruct photons and extract their (A) positions (B) energies and (C) timings.

41. (2) Kinematical Fitting [Lagrange Multiplier]   minimization with constraints. (A) Four Constraints (B) Five inputs

42. Correction factors (1)C L1.1after : unwanted trigger rejection embedded in online photon veto (2)C acc : over-rejection by photon veto with accidentals (3)C split : self-vetoing effect by splitting tagging photon C L1.1after = 1.14 C acc = 0.80

43. High Purity K  2 Identification (1) background rejection (2) Single beam background rejection (3) Two-beam background rejection Dominant non-K  2 backgrounds

44. Top half of side view K  2 backgrounds ++

45. High Purity K  2 Identification (1) background rejection (2) Single beam background rejection (3) Two-beam background rejection Dominant non-K  2 backgrounds

46. Top half of side view  Single beam backgrounds Cerenkov B4

47. High Purity K  2 Identification (1) background rejection (2) Single beam background rejection (3) Two-beam background rejection Dominant non-K  2 backgrounds

48. Top half of side view Beam 1 Beam 2 Beam wire chamber  K+K+ veto Two-beam backgrounds

49. E949 Detector (1) Target: Kaon decay at rest (2) Drift chamber: Momentum (3) Range Stack (scintillator): Energy / Range E949 detector side view (upper half) E949 detector end view (upper half) Target Drift Chamber Range Stack

50. Error distribution w/ Daughter Table Method w/ Binominal error 300 daughter tables Daughter tables produced by random number generator Convoluted inefficiency

51. DAQ Summary Before data taking After data taking Platinum target used in 2002 Accumulated K + : # of accumulated Kaons

52. Data Acquisition Before data taking After data taking Platinum target used in 2002 Accumulated K + :

53. Single Photon Inefficiency

54. Subtraction at various levels of photon veto search Estimation from photon inefficiency Improvement (Before/After)     rejection at various photon veto A factor of ~ two improvement at various photon veto

55. Overlapping ,e +/- (from  0 ) may cause disruption in the  + track reconstruction. Disruption Correction Factor C dis Disruption correction : Estimation (Pure MC Study) : (1) Normal K  2 decays (2) K  2 decays but was forced. Difference in the  + recon. efficiency  correction