BNL-E949実験における 崩壊の探索 (1) Physics Motivation (2) E949 Detector 京大理，KEK素核研A，阪大RCNPB，阪大理C，福井大工D，防衛大理工E，AlbertaF, UBCG, BNLH, FNALI, IHEPJ, INRK, UNML, Stony BrookM, TRIUMFN 溝内健太郎，大橋永治A，小林正明A，小松原健A，笹尾登，新川孝男E,　 杉本章二郎A，関口哲郎A，玉川洋一D，常見俊直B，中野貴志B，能町正治C，野村正，藤原庸博，村松憲仁B，吉村喜男A，他BNL-E949 Collaboration (1) Physics Motivation (2) E949 Detector (3) Kp2 Selection Criteria (4) Background Estimation (5) Opening the Box (6) Conclusion / Summary

K+  p+ + “nothing” (1) K+  p+nn (above Kp2) (2) p0 nn (on Kp2 peak) Published in PRL, 93 031801 (2004) Charged track momentum from various K decay modes (2) p0 nn (on Kp2 peak) This report. Need tighter photon rejection. (3) K+  p+nn (below Kp2) Analysis ongoing. Require more sophisticated treatment in p+ multiple scatterings.

Physics Motivation [Helicity suppressed decay] Theoretical BR : : left-handed (in SM) : spin 0 [Helicity suppressed decay] Theoretical BR : (for a single Dirac-neutrino type) nt mass limit ( ) implies . [ through beyond the standard model] (1) x2 larger BR in the case of Majorana neutrino : (2) Decay into different neutrino flavors : (3) Sensitive to any hypothetical weakly-interacting neutrals. [Cosmological Interests] Neutron star cooling model through pion pole mechanism :

E949 Detector Momentum : Drift chamber Energy/range: RS (scintillator) side view (upper half) E949 detector end view (upper half) Momentum : Drift chamber Energy/range: RS (scintillator) Photon veto : BV (Pb-scinti) EC (CsI) etc ...

DAQ Summary # collected Kaon Before data taking After data taking Platinum target used in 2002 Before data taking After data taking Accumulated K+:

Analysis Strategy decay decay 1/3 sample 2/3 sample Offline selection p0 sample p0 sample ( ) decay Find the best photon veto paramters Open the box (N) Correction ( )

High Purity Kp2 Identification (1) background rejection Decay sequence : Range deviation (2) Single beam background rejection Particle ID in beam line Requiring 2ns wait in the target (3) Double beam background rejection Beam line vetoes at Kaon decay time (4) Kp2 signal box (kinematics)

Bifurcation Analysis Bifurcation Analysis Ex.) Km2(g) rejection Cut1 : Range deviation Uncorrelated Cut-1 and Cut-2 Rcut-1 = (C+D)/C Signal Region A/B = C/D A = BC/D = B / (Rcut-1 -1) Cut2 : Neural Net output

# tagged Kp2 and Backgrounds in the Kp2 Tag Number of tagged p0 : Impurity : ~10-9

Disruption Correction Factor g,e+/- from p0 Overlapping g,e+/- (from p0) may cause a mis-reconstruction in p+ tracking. Pure MC Study : (1) Normal Kp2 decays (2) Kp2 decays but was forced. Difference in the p+ recon efficiency  correction Disruption correction :

Acceptance Measurement Signal loss due to accidentals in coincidence with the .  Correction factor : Hermetic photon detection cuts to Km2 decays (real data) identified with monochromatic kinematics (E/P/R) after all m+ activity removed.

Maximization of Sensitivity [ Hermetic photon detection cut ] Rejection : Acceptance : Find the best parameters providing the largest rejection and the smallest acceptance loss. (for ~20 photon detection subsystems)

Opening the Box A total of 99 events observed in the signal box Kaon decay time (ns) p+ momentum (MeV/c)

Result New upper limit : Conservative upper limit # signal < 113 at 90%CL (Poisson) and subtracting the non-Kp2 backgrounds; Saturation at 4x106 New upper limit : A factor of 3 improvement from the previous best result.

Conclusions / Summary (1) search was done at Kp2 kinematics peak. (2) A total number of 3.02x109 Kp2 were obtained. with 3.12 non-Kp2 backgrounds (estimation) (3) A total number of 99 candidates were observed in the signal box. (4) Obtained a conservative upper limit of x3 improvement from the previous best limit.