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