Experimental Search for the Decay νν K. Mizouchi (Kyoto University) (1) Physics Motivation (2) Detector (3) Selection Criteria (4) Branching Ratio (5) γγ Background Subtraction (6) Conclusions
νν : Physics Motivation ν ν : spin ν : left-handed (in SM) [1] Helicity suppressed decay Br( νν ) = 3 1 8 ( m ν / m ) 2 1 4 ( m / m ) 2 1 (A) eutrino mass : m <18.2 MeV / c implies Br < 5 1. ν τ (B) eutrino type : Majorana neutrino x2 larger branching ratio. [2] Decay Form of " nothing" (A) Sensitive to any hypothetical weakly-interacting neutrals. (B) Decay into different neutrino flavors : [3] Cosmological Interests ν ν1ν 2 eutron star cooling model through pion pole mechanism : γγ νν 2
Event Detection Strategy K + + ( K ) 2 νν ( nothing) Hermetic photon detection system γγ, e + e γ Charged particles from K + decay at rest K 2 K µ2 (1) Clean K 2 selection (2) to invisible final states Prior best limit : Br < 8.3 1 7 (E787)
E949 Detector 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
Analysis Strategy Br( νν ) = 1 1 C acc C dis Offline Data (K2 rich) 1/3 sample 2/3 sample (1) K 2 selection tuning sample (2) Find the best photon veto parameters Acceptance C acc sample ( ) Signal candidate ()
K2 selection and none-k2 bkgnd Real data (2/3 sample) Impurity : ~1-9 Br( νν ) = 1 1 C acc C dis
Disruption Correction Factor C dis Overlapping γ,e +/- (from ) may cause disruption in the + track reconstruction. Disruption correction : C dis =1.14 Br( νν ) = 1 1 C acc C dis
(1) K2 Tag Done. (2) Hermetic Photon Veto " nothing" (background : γγ )
Acceptance Measurement C acc νν acceptance loss due to coincident accidentals accidental ν + ν Br( νν ) = 1 1 C acc C dis Measure acceptance loss of K µ2 decays (real data) by the photon veto, after all µ + activities are removed.
Maximization of the Sensitivity Photon veto rejects events with : E sum in [T 1,T 2 ] > E threshold Real data 1/3 sample Br( νν ) = 1 1 C acc C dis [ Hermetic photon veto ] (1) γγrejection : (2) ννacceptance : C acc Effective rejection (= rej acc) C acc =.117 Final photon veto Acceptance Find the best parameters; the largest rejection with the given acceptance.
Opening the Box Real data 2/3 sample A total of 99 candidates were observed in the signal box Kaon decay time (ns) + momentum (MeV/c)
Branching Ratio Br( Conservative upper limit # signal < 113 (9%CL) subtracting the non-k 2 bkgnds; 11 νν ) < 3.2 1 7 = 2.7 1 9 1.117 1.14 1/3 sample 2/3 sample Saturation at 3.5x1 6 7 ew upper limit : Br( νν ) < 2.7 1 A factor of 3 improvement from the previous best result.
γγ Background subtraction Measurement of the detector single photon inefficiency K 2 w/ one photon missing event νν search K + + γ ( γ ) Relaxed photon veto (acc =.8) (1)Establish a background subtraction method (2)Understand the detector performance
Single Photon Inefficiency
γγ detection inefficiency (1) Single photon inefficiency P SPI = (2) Photon kinematics γ γ ( i i ) ( i i θ,, θ E ) 1 E1 2, 2 from MC simulation ( events)
γγ background subtraction umber of νν candidates with relaxed photon veto 4131 events Br( νν ) < 1.5 1-6 Arbitrary Singal (9% C.L) : 2259-7 Br( νν ) < 8.2 1 A factor of 1.8 improvement
Subtraction at various levels of photon veto Improvement (Before/After) A factor of ~ two improvement at various photon veto
um of γγ backgrounds as a function of cos(θ + ) Signal candidates Single photon inefficiency Signal discrimination capability from backgrounds
Background Subtraction with dip angle distribution Candidates : s raw = 4131 Best fit value : s = 1977 9 % C.L. : s 9 = 2449 Br ( νν ) < 8.9 1 A factor of 1.7 improvement -7 Ref. w/o subtraction : Br( νν ) < 1.5 1-6
Conclusions (1) νν search was performed with 3.2x1 9 K 2 events, where impurity of 1-9 was achieved. Br( νν ) < 2.7 1 7 (2) ew 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 (2) γγbackground subtraction was performed with the inefficiency; (A) x1.8 improvement with simple subtraction (B) x1.7 improvement from cos(θ + ) shape discrimination
Thank you!