BNL-E949実験における崩壊の探索

Experimental Search for the
0
Decay  
K. Mizouchi (Kyoto University)
(1) Physics Motivation
(2) Detector
(3) Selection Criteria
(4) Branching Ratio
0
(5)    Background Subtraction
(6) Conclusions
  : Physics Motivation
0

 0 : spin 0



: left-handed (in SM)
0
[1] Helicity suppressed decay
Br( 0  )  3 108 (m / m 0 ) 2 1  4m / m 0 
2
2
(A) Neutrino mass : m  18.2 MeV/ c implies Br  5 1010.
(B) Neutrino type : Majorana neutrino  x2 larger branching ratio.
[2] Decay Form of  0 " nothing"
(A) Sensitive to any hypothetical weakly-interacting neutrals.
0

 1 2
(B) Decay into different neutrino flavors :
[3] Cosmological Interests
Neutron star cooling model through pion pole mechanism :
   0 
Event Detection Strategy
K     0 ( K 2 )
 0  (nothing)
Hermetic photon
detection system
 0    ee
(1) Clean K2 selection
(2) 0 to invisible final states
Prior best limit :
Br  8.3 107 (E787)
Charged particles from K+ decay at rest
K2
Km2
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( 0  ) 
N
1
1


N 0 Cacc Cdis
Offline Data (K2 rich)
1/3 sample
2/3 sample
0 sample
0 sample ( N )
(1) K2 selection
0
tuning
(2) Find the best photon
veto parameters
Acceptance Cacc
Signal candidate (N)
K2 selection and none-K2 bkgnd
Real data (2/3 sample)
Impurity : ~10-9
Br( 0  ) 
N
1
1


N 0 Cacc Cdis
Disruption Correction Factor Cdis
Overlapping ,e+/- (from 0) may
cause disruption in the + track
reconstruction.
Disruption correction :
Cdis  1.14
Br( 0  ) 
N
1
1


N 0 Cacc Cdis
(1) K2 Tag … Done.
(2) Hermetic Photon Veto
 " nothing"
0
(background:    )
0
Acceptance Measurement Cacc
 0  acceptance loss due to coincident accidentals
+


Br( 0  ) 
N
1
1


N 0 Cacc Cdis
Measure acceptance loss of Km2 decays (real data) by the
photon veto, after all m+ activities are removed.
Photon veto rejects events
with :
Esum in [T1,T2] > Ethreshold
Real data “1/3 sample”
Br( 0  ) 
N
1
1


N 0 Cacc Cdis
[ Hermetic photon veto ]
1
0
  rejection
:
N 0
N
(2) 0   acceptance : Cacc
Effective 0 rejection (= rej×acc)
Maximization of the Sensitivity
Cacc  0.117
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
Conservative upper limit
# signal < 113 (90%CL)
subtracting the non-K2 bkgnds;
1/3 sample
2/3 sample
Saturation at 3.5x106
110
1

3.02109 0.1171.14
 2.7 107
Br( 0   ) 
New upper limit : Br ( 0  )  2.7 107
A factor of 3 improvement from the previous
best result.
0   Background subtraction
Measurement of the detector single photon inefficiency
K2 w/ one photon missing event
 0   search
K     0
 0   ( )
Relaxed
photon veto
(acc = 0.80)
(1)Establish a background subtraction method
(2)Understand the detector performance
Single Photon Inefficiency
0   detection inefficiency
(1) Single photon inefficiency
PSPI=
(2) Photon kinematics
 0  
 , E ,  , E 
i
1
i
1
i
2
i
2
from MC simulation
(N events)
Number of  0 
candidates with relaxed
photon veto
4131 events
Br( 0  )  1.510-6
Singal (90% C.L) : 2259
Br( 0  )  8.210-7
A factor of 1.8 improvement
Arbitrary
0   background subtraction
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(+)
Signal candidates
Single photon inefficiency
Signal discrimination capability from backgrounds
Background Subtraction with dip angle distribution
Candidates : sraw = 4131
Best fit value : s = 1977
90 % C.L.
: s90 = 2449
Br( 0  )  8.910-7
A factor of 1.7 improvement
Ref. w/o subtraction :
Br( 0  )  1.510-6
Conclusions
(1)  0  search was performed with 3.02x109 K2 events,
where impurity of 10-9 was achieved.
(2) New upper limit of Br ( 0  )  2.7 107was
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0   background subtraction was performed with the
inefficiency;
(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
K2 photon kinematics
Unvetoed hits in Candidates
Unvetoed hits in BV
Outside the veto time window.
Lower energy than threshold.
Unvetoed hits
(6)
(5)
(4)
(1)
(3)
(2)
(2)
(3)
(4)
(5)
(6)
(1)
PV for single photon study
PV for  0  search
K2 photon kinematics
Background understanding and detector inefficiency
Can we understand the remaining events from a view of photon
inefficiency ? ( if possible, subtract them as backgrounds.)
An Idea :

(1) Special trigger
K2 but one photon is missed.
(2) Event reconstruction
Missing photon kinematics
(3) Photon inefficiency as a
function of its energy and direction

?
NOTE :
(1) Different type of critical backgrounds.
(2) Geometrical dependence : Detector hole, dead material
(3) Energy dependence
: Photonuclear interaction …
Barrel Veto Liner
K+  + + “nothing” in E949
(1) K+  + (above K2)
Published in PRL, 93 031801
Charged track momentum
from various K decay modes
(2004)
2 0  (on K2 peak)
K     0 (K 2 )
 0 
This report.
( 0   )
Need tighter photon rejection.
(3) K+   (below K2)
Analysis ongoing.
Require more sophisticated
treatment in + multiple
scatterings.
Published in PRD as rapid communication
Phys. Rev. D72, 091102 (2005)
Optimized Photon veto parameters
Performance of the clustering Method
MC sample
0  backgrounds
Photon inefficiency 20<E[MeV]<225
Low energy  : sampling fluctuation
High energy : photonuclear interaction ( hard to simulate reliably.)
Detector photon
inefficiency
(measured with
real data)
 0   
miss
20~40MeV
40~60MeV
60~80MeV
80~100MeV
100~120MeV
120~140MeV
140~160MeV
Phase space correction factors
Monte Carlo simulation
Real data
Polar angle distribution
Correction factors
Self-vetoing effect due to split photon
Missing-side
MC simulation
K     0
 0   ( )
Missing
photon
kinematics
Single Photon Inefficiency
K  


0
 0   ( )
Measure single photon
inefficiency with real data.
K2 w/ one photon missing event
(1) N denom ( , E ) : raw
photon distribution
(2) N numer ( , E) : misdetected photon distribution
(3) Anorm : Trigger prescale
compensasion, 1.57 102
Analysis Strategy
K     0
 0   ( )
K2 w/ one photon missing event
(1)Reconstruct (tagging) photon
(2)Extract kinematics of the misdetected 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]
c2 minimization with constraints.
(A) Four Constraints
(B) Five inputs
X
c   

2
measured
i
i
X
Fit
i



2
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 K2 Identification
Dominant non-K2 backgrounds
(1) K  m  m ( K m 2 ) background rejection
(2) Single beam background rejection
(3) Two-beam background rejection
   m   e
Top half of side view
m+

Km2 backgrounds
High Purity K2 Identification
Dominant non-K2 backgrounds
(1) K  m  m (Km 2 ) background rejection
(2) Single beam background rejection
(3) Two-beam background rejection
Top half of side view
Cerenkov

B4
Single beam backgrounds
High Purity K2 Identification
Dominant non-K2 backgrounds
(1) K  m  m (Km 2 ) background rejection
(2) Single beam background rejection
(3) Two-beam background rejection
Beam wire chamber
Beam 1
Top half of side view
Beam 2

veto
veto
vetoK+
Two-beam backgrounds
E949 Detector
E949 detector side view (upper half)
E949 detector
end view (upper half)
Drift ChamberTarget
Range
Stack
(1) Target
: Kaon decay at rest
(2) Drift chamber
: Momentum
(3) Range Stack (scintillator) : Energy / Range
Error distribution w/ Daughter Table Method
w/ Binominal error
Daughter tables produced by
random number generator
Convoluted inefficiency
DAQ Summary
Platinum target used in 2002
# of accumulated Kaons
Before data taking
After data taking
Accumulated K+: N K  1.8 10
12
Data Acquisition
Platinum target used in 2002
After data taking
Before data taking
Accumulated
K+:
N K  1.8 1012
Single Photon Inefficiency
Subtraction at various levels of photon veto
0   rejection at
various photon veto
Improvement (Before/After)
 0  search
Estimation from photon inefficiency
A factor of ~ two
improvement at
various photon veto
Disruption Correction Factor Cdis
Overlapping ,e+/- (from 0) may
cause disruption in the + track
reconstruction.
Estimation (Pure MC Study) :
(1) Normal K2 decays
(2) K2 decays but  0  was forced.
Difference in the + recon. efficiency  correction C dis
Disruption correction :
Cdis  1.14
Br( 0  ) 
N
1
1


N 0 Cacc Cdis

Download Report