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. : Physics Motivation [1] Helicity suppressed decay [2] Decay Form of
: left-handed (in SM)
: spin 0
[1] Helicity suppressed decay
(A) Neutrino mass : implies
(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] Cosmological Interests
Neutron star cooling model through pion pole mechanism :

3. Event Detection Strategy
Charged particles from K+ decay at rest
Km2
Hermetic photon detection system
Kp2
Clean Kp2 selection
p0 to invisible final states
Prior best limit :
(E787)

4. E949 Detector (1) Barrel Veto (BV) : Pb-scintillator sandwich
E949 detector side view (upper half)
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

5. Analysis Strategy Offline Data (Kp2 rich) 1/3 sample 2/3 sample
(1) Kp2 selection
p0 sample
p0 sample ( )
tuning
(2) Find the best photon veto parameters
Acceptance Cacc
Signal candidate (N)

6. Kp2 selection and none-Kp2 bkgnd
Real data (2/3 sample)
Impurity : ~10-9

7. Disruption Correction Factor Cdis
Overlapping g,e+/- (from p0) may cause disruption in the p+ track reconstruction.
Disruption correction :

8. (1) Kp2 Tag　… Done.
(2) Hermetic Photon Veto

9. Acceptance Measurement Cacc
acceptance loss due to coincident accidentals p+
accidental n n
Measure acceptance loss of Km2 decays (real data) by the photon veto, after all m+ activities are removed.

10. Maximization of the Sensitivity
Photon veto rejects events with :
Esum in [T1,T2] > Ethreshold
Acceptance
Effective p0 rejection (= rej×acc)
Final photon veto
Real data “1/3 sample”
[ Hermetic photon veto ]
(1) p0  gg rejection :
(2) p0  nn acceptance :
Find the best parameters;
the largest rejection with the given acceptance.

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

12. Branching Ratio New upper limit :
2/3 sample Saturation at 3.5x106
1/3 sample
Conservative upper limit
# signal < 113 (90%CL) subtracting the non-Kp2 bkgnds;
New upper limit :
A factor of 3 improvement from the previous best result.

13. p0  gg Background subtraction
Measurement of the detector single photon inefficiency
Kp2 w/ one photon missing event
Relaxed photon veto
(acc = 0.80)
Establish a background subtraction method
Understand the detector performance

14. Single Photon Inefficiency

15. p0  gg detection inefficiency
(1) Single photon inefficiency
(2) Photon kinematics
PSPI=
from MC simulation
(N events)

16. p0  gg background subtraction
Number of candidates with relaxed photon veto
Arbitrary
4131 events
Singal (90% C.L) : 2259
A factor of 1.8 improvement

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

18. Num of p0  gg backgrounds as a function of cos(qp+)
Signal candidates
Single photon inefficiency
Signal discrimination capability from backgrounds

19. Background Subtraction with dip angle distribution
Candidates : sraw = 4131
Best fit value : s = 1977
90 % C.L : s90 = 2449
A factor of 1.7 improvement
Ref. w/o subtraction :

20. Conclusions (1) search was performed with 3.02x109 Kp2 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.
Single photon inefficiency was measured with special data
p0  gg background subtraction was performed with the inefficiency;
(A) x1.8 improvement with simple subtraction
(B) x1.7 improvement from cos(qp+) shape discrimination

21. Thank you !