experimental search for the decay k. mizouchi (kyoto university) (1) physics motivation (2) detector...
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Experimental Search for the Decay 0
K. Mizouchi (Kyoto University)
(1) Physics Motivation(2) Detector(3) Selection Criteria(4) Branching Ratio(5) Background Subtraction(6) Conclusions
0
[1] Helicity suppressed decay
: Physics Motivation
/41)/(103)( 228000 mmmmBr
: left-handed (in SM)
0: spin 00
(A) Neutrino mass : implies .2/MeV 2.18 cm
[3] Cosmological InterestsNeutron star cooling model through pion pole mechanism :
0
(B) Neutrino type : Majorana neutrino x2 larger branching ratio.
210
[2] Decay Form of ""0 nothing
(B) Decay into different neutrino flavors :
(A) Sensitive to any hypothetical weakly-interacting neutrals.
10105 Br
0
Event Detection Strategy
)( 20
KK
Hermetic photon detection system
)(0 nothing Charged particles from K+ decay at rest
ee0
(1) Clean K2 selection(2) 0 to invisible final states
K2
K2
7103.8 BrPrior best limit :
(E787)
E949 Detector
E949 detectorend 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)
Analysis Strategy
(1) K2 selection
1/3 sample 2/3 sample
0 sample
(2) Find the best photon veto parameters
0 sample ( )
Signal candidate (N)
Offline Data (K2 rich)
disacc CCN
NBr
11)(
0
0
0N
tuning
Acceptance Cacc
K2 selection and none-K2 bkgnd
Impurity : ~10-9
Real data (2/3 sample)
disacc CCN
NBr
11)(
0
0
Overlapping ,e+/- (from 0) may cause disruption in the + track reconstruction.
Disruption Correction Factor Cdis
Disruption correction : 14.1disCdisacc CCN
NBr
11)(
0
0
""0 nothing) :background( 0
(2) Hermetic Photon Veto
(1) K2 Tag … Done.
Acceptance Measurement Cacc
Measure acceptance loss of K2 decays (real data) by the photon veto, after all + activities are removed.
acceptance loss due to coincident accidentals 0
+
acc
idental
disacc CCN
NBr
11)(
0
0
Maximization of the Sensitivity
disacc CCN
NBr
11)(
0
0
0 rejection :
(2) 0 acceptance :
[ Hermetic photon veto ]
accCN
N 0
Find the best parameters; the largest rejection with the given acceptance.
Real data “1/3 sample”
Photon veto rejects events with :Esum in [T1,T2] > Ethreshold
Acceptance
Effe
ctiv
e 0
re
ject
ion
(= r
ej×
acc
)
Final photon veto
117.0accC
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”
New upper limit : A factor of 3 improvement from the previous best result.
Branching Ratio
7
90
107.2
14.1117.0
1
103.02
110 )(
Br
# signal < 113 (90%CL) subtracting the non-K2 bkgnds;
70 107.2 )( Br
Conservative upper limit
2/3 sample Saturation at 3.5x106
1/3 sample
0 Background subtraction
K2 w/ one photon missing event
0 K)( 0
Measurement of the detector single photon inefficiency
search 0
Relaxed photon veto(acc = 0.80)
(1)Establish a background subtraction method
(2)Understand the detector performance
Single Photon Inefficiency
0 detection inefficiency
from MC simulation(N events)
iiii EE 2211 ,,,
(2) Photon kinematics
0
(1) Single photon inefficiency
PSPI=
Number of candidates with relaxed photon veto
0 background subtraction
0
4131 events
-70 108.2 )( BrA factor of 1.8 improvement
-60 101.5 )( Br
Singal (90% C.L) : 2259A
rbitr
ary
Subtraction at various levels of photon veto
Improvement (Before/After)
A factor of ~ two improvement at various photon veto
Num of 0 backgrounds as a function of cos(+)
Single photon inefficiency
Signal discrimination capability from backgrounds
Signal candidates
Background Subtraction with dip angle distribution
Candidates : sraw = 4131Best fit value : s = 197790 % C.L. : s90 = 2449
-70 108.9 )( BrA factor of 1.7 improvement
-60 101.5 )( BrRef. w/o subtraction :
Conclusions
(1) search was performed with 3.02x109 K2 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.
0
70 107.2 )( Br
(1)Single photon inefficiency was measured with special data20 background subtraction was performed with the in
efficiency; (A) x1.8 improvement with simple subtraction (B) x1.7 improvement from cos(+) shape discrimination
Thank you !
Early accidental hits
Two peaks in BVL
Background distribution
K2 photon kinematics
Unvetoed hits in Candidates
Unvetoed hits in BV
Outside the veto time window.Lower energy than threshold.
Unvetoed hits
PV for single photon studyPV for search 0
(1)
(2)
(3)
(4)(5)(6)
(1)
(2)
(3)
(4)(5)
(6)
K2 photon kinematics
Background understanding and detector inefficiency
?
(2) Event reconstructionMissing 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 K2 but one photon is
missed.
(3) Photon inefficiency as a function of its energy and direction
An Idea :
NOTE :
Barrel Veto Liner
K+ + + “nothing” in E949
(1) K+ + (above K2)Published in PRL, 93 031801 (2004)
20 (on K2 peak)
This report.Need tighter photon rejection.
(3) K+ (below K2)Analysis ongoing. Require more sophisticated treatment in + multiple scatterings.
Charged track momentum from various K decay modes
)(K 20
K 0
)( 0
Published in PRD as rapid communication Phys. Rev. D72, 091102 (2005)
Optimized Photon veto parameters
Performance of the clustering Method
MC sample
0 backgrounds
20~40MeV
40~60MeV
80~100MeV
60~80MeV
100~120MeV
120~140MeV
140~160MeV
miss
0
Detector photon inefficiency(measured with real data)
Photon inefficiency 20<E[MeV]<225Low energy : sampling fluctuation High energy : photonuclear interaction ( hard to simulate reliably.)
Phase space correction factors
Polar angle distribution Correction factors
Mon
te C
arlo
sim
ulat
ion
Rea
l dat
a
Self-vetoing effect due to split photon
0 K)( 0
MC simulation
Missing photon kinematics
Missing-side
(3) : Trigger prescale compensasion,
(2) : mis-detected photon distribution
(1) : raw photon distribution
Single Photon Inefficiency
K2 w/ one photon missing event
0 K)( 0
),(denom EN
),(numer EN
normA
Measure single photon inefficiency with real data.
21057.1
Analysis Strategy
K2 w/ one photon missing event
0 K)( 0
(1)Reconstruct (tagging) photon(2)Extract kinematics of the mis-
detected photon.(3)Correction factors
(1) Photon Clustering Method
Reconstruct photons and extract their (A) positions (B) energies and (C) timings.
(2) Kinematical Fitting
[Lagrange Multiplier]2minimization with constraints.
2
2
i
Fiti
measuredi XX
(A) Four Constraints
(B) Five inputs
Correction factors
(1)CL1.1after : unwanted trigger rejection embedded in online photon veto(2)Cacc : over-rejection by photon veto with accidentals(3)Csplit : self-vetoing effect by splitting tagging photon
CL1.1after = 1.14
Cacc = 0.80
High Purity K2 Identification
(1) background rejection
(2) Single beam background rejection
(3) Two-beam background rejection
)( 2 KK
Dominant non-K2 backgrounds
Top half of side view
K2 backgrounds
+
e
High Purity K2 Identification
(1) background rejection
(2) Single beam background rejection
(3) Two-beam background rejection
)( 2 KK
Dominant non-K2 backgrounds
Top half of side view
Single beam backgrounds
Cerenkov B4
High Purity K2 Identification
(1) background rejection
(2) Single beam background rejection
(3) Two-beam background rejection
)( 2 KK
Dominant non-K2 backgrounds
Top half of side viewBeam 1
Beam 2
Beam wire chamber
K+veto veto veto
Two-beam backgrounds
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 detectorend view (upper half)
TargetDrift Chamber Range Stack
Error distribution w/ Daughter Table Method
w/ Binominal error
300 daughter tables
Daughter tables produced by random number generator
Convoluted inefficiency
DAQ Summary
Before data taking
After data taking
Platinum target used in 2002
12108.1 KNAccumulated K+:
# of accumulated Kaons
Data Acquisition
Before data taking After data taking
Platinum target used in 2002
12108.1 KNAccumulated K+:
Single Photon Inefficiency
Subtraction at various levels of photon veto
search Estimation from photon inefficiency
Improvement (Before/After) rejection at various photon veto
0A factor of ~ two improvement at various photon veto
Overlapping ,e+/- (from 0) may cause disruption in the + track reconstruction.
Disruption Correction Factor Cdis
Disruption correction : 14.1disC
Estimation (Pure MC Study) :
(1) Normal K2 decays
(2) K2 decays but was forced. 0
Difference in the + recon. efficiency correction disC
disacc CCN
NBr
11)(
0
0