Search for a light pseudoscalar particle in the decay
KL0  π0 π0 X at the KEK-PS E391a experiment
KAON09 @ Tsukuba, Jun. 11

Yu-Chen Tung ( National Taiwan Univ.)

OutlineIntroduction– Motivation– Theoretical model– E391a experiment

Analysis– Reconstruction– Background study– Results

Motivation: HyperCP observation
H. K. Park et al. PRL 94, 021801• HyperCP experiment observed 3 events for Σ+ pµ+µdecays with a narrow Mμ μ distribution around 214.3 MeV– Probability to have the Mμ μ distribution within 1 MeV less than 1%• Signal interpretation: Σ+ p X, X  μ+μ-

GMSB model for X
D. S. Gorbunov and V. A. Rubakov, PRD 73, 035002

The GMSB (Gauge Mediated SUSY Breaking) model said
• Spontaneous SUSY breaking generates Goldstone fermion, calledGoldstino, which gives the longitudinal component of gravitino.
• There should exist superpartners of Goldstino: sgoldstinos• Mass of sgoldstino is arbitrary and can be only a few MeV

And shows HyperCP observation is consistent in model with• Parity conservation in sgoldstino interaction• mX is in reasonable pseudoscalar sglodstino mass region• Low SUSY breaking scale √F=2.5-60 TeV

GMSB model for X
D. S. Gorbunov and V. A. Rubakov, PRD 73, 035002

For X with mass = 214.3 MeV in GMSB model:••••

Possible decay channels: γγ, e+e-, μ+μ- & gravitinos.
X’s BR is almost saturated by γγ, μ+μ- channels.
BR(Xγγ) / BR(X μ+μ-) ~ 104.
The lifetime of X is generally small: cτ = 0.02 - 0.0002 cm.

GMSB model for X
Possible neutral Kaon decays for probing XD. S. Gorbunov and V. A. Rubakov, PRD 73, 035002

The GMSB model shows:• BR(KL0  π0 π0 X) is proportional to Re[h(D)12]• h(D)12 is complex coupling constant only its absolute value known• if Re[h(D)12] = h(D)12,BR(KL0  π0 π0 X, X  γγ) = 1.2x10-4
BR(KL0  π0 π0 X, X  μ+μ-) = 1.2x10-8• No other mode made the prediction for this decay

The E391a experiment
The experiment was located at the KEK 12 GeV
proton-synchrotron in Tsukuba, Japan.
 ~50 members from 14 institutions
 Japan, the US, Taiwan, South Korea, and Russia are participants.

Three main Data Runs:•

Run I: February – July of 2004
 discarded due to a membrane problem•

Run ll: February – April of 2005
 published results of KL0  π0 π0 X, X  γγ analysis•

Run lll: October – December of 2005

The E391a Detector• Use CsI calorimeter for photon detection (7x7x30 cm3)

• Veto detectors cylindrically cover the CsI calorimeter• No momentum-measurement + we use charged veto before CsI Hard to measure KL0  π0 π0 X, X  μ+μ-

KL0  π0 π0 X, X  µ+µ- analysis
Challenge to measure the decay using
the upgraded BA in runrun-III.
( Lead plate + plastic scinti. + quartz
 PWO crystal + quartz )Back Anti(B.A.)
Back Anti

Small qq-values of KL0  π0 π0 X
 X  μ+μ-BHCV•

Signal is signatured by 4 photons on CsI
 2 muons in BA & BHCV

Upper view of B.A
30mm

30mm
30mm

35mm

Beam

120mm

UP ( 90% C.L. ) <1.7 x 10-6KL0  π0 π0 X, X  μ+μ- analysis is performed byRisa Ogata from Saga University.

0123# of Layer

4Quartz•

Side view of B.APWO

 Small open angle of μ+μ-PWO•

245mmPMTdivider

KL0  π0 π0 X, X  γγ analysis:Monte Carlo & Signal signature
Both K3pi0 & K2pi0X MC were simulated using GEANT3 and wereoverlaid with accidental events taken from the targettarget-monitor
accidental trigger.
KL0  π0 π0 X, X  γγ events are first selected by• Six photons on CsI calorimeter – the same as K3pi0 decays• No in-time hits on other detectors

Reconstruction
Event is reconstructed using Constrained Fit to require Six photons to have KL massx1
 Two photon pairs to have Pion massx2
 The KL momentum from target to
COE of photons on the CsI planex2••••

5 constraints
3 unknowns: KL decay vertex (Vx, Vy, Vz)
Two-degree-of-freedom fit
45 possible combinations  The combination with Minimum χ2 is chosen

In successful reconstructions, M56 (mass of 3rd gamma pair)
Pion mass for K3pi0 decays=X mass for K2pi0X decays

Selection Condition
Photon Veto
CC02

CC03

CC04

CC05

3 MeV

0.35 MeV

1 MeV

5 MeV

Inner MBOuter MB

Inner CVOuter CV

5 MeV

7 MeV

5 MeV

0.3 MeVFB

BHCV

BA scint.
scint.

BA quartz

5 MeV

0.1 MeV0.06 GeV

1 MIPsKinematical Cut
KL Z region

250 – 550 cm

KL pt< 25 MeV

Center E of clusters< 6 cmGamma hit pos.

12.5 cm square from beam – 88 cm circle

Gamma fusion

Gamma NN > 0.2

Constrained fit χ2<6Reconstruction Results

EventsSolution to the dip in the Pion peak: 3 correct combinations for K3pi0 decays.

Red Histogram: KL0  π0 π0 π0Dots: data

10-1

10-2

10-310-4

 Only the mass of 1st & 2nd gamma pairs
is constrained.

05101520
χ2253035

 The combination with minimum
χ2 leaves the worst rec. Pion massin 3rd gamma pair.

Events / ( 2.3 MeV/c2 )

106
105
104

Shaded Histogram: KL0  π0 π0 X
Red Histogram: KL0  π0 π0 π0Dots: data

103102
1010204060

80

100120140 160

M56 ( MeV/c2 )180 200 220

40

Background Study
Mis-reconstructed events in 1st combination
Miswere Successfully reconstructed in 2nd combination

 Main background source K3pi0 background are mainly due towrong photon pairing.

220

M56 in the 2nd combination

K3pi0 decay:• Large BR, BR = 19.56 %• Six photons in final state sameas the K2pi0X decays

200180160

Pionpeak

140
120
100806040200020406080

100 120 140 160 180 200 220

Other backgrounds:
M56 in the best combination
K2pi0 decays, background from other decays….
• MC study shows those backgrounds are negligible

Sideband + signal region

K3pi0 background suppression
Shaded Histogram: KL0  π0 π0 X
Red Histogram: KL0  π0 π0 π0Dots: data

10-1

EventsUse “full constrained fit” to reject all K3pi0 events
 Six photons to have KL massx1
 Three photon pairs to have Pion mass x3
 The KL momentum from target to
COE of photons on the CsI plane
x2

10-2

10-3

•

Event, which is consistent with K3pi0 decay,
has small χ2.

050

K3pi0: 0.15 %K2pi0X: 61.0 %χ2200250300

350

Red Histogram: KL0  π0 π0 π0Dots: data

10-2

Events••150

full (Signal region)

10-1

Acceptance (cut: χ2 < 50)

100

10-3

10-4

10-5010

2030

40

50

60

full(Full mass region)

χ270

80

90

100

Signal extraction
Modeling result of signal shape

After all cuts on data
• 2 events in signal region ( 211.3 – 217.3 MeV )• 250 events in sideband( 165 – 211.3 MeV & > 217.3 MeV )

103

102

Signal PDF:
Double Gaussian10

The number of signal events yielded:+1.7• NS = -1.4 -0.9

10.209 0.21 0.211 0.212 0.213 0.214 0.215 0.216 0.217 0.218 0.219

Background PDF:
35

Events / ( 2.2 MeV/c2 )Use unbinned likelihood method to extract signal• Signal PDF: double Gaussian function
for asymmetric peak
• BKGD PDF: ARGUS function for tail shape

30
25
20
15
10
50170

180

190

200

M56 ( MeV/c2 )

210

220

Error estimation• The signal shape modeling ( 4.07 % )– Consider the 1 sigma error in signal shape modeling• The signal shape difference between data & MC ( 1.79 % )• The signal efficiency ( 0.93 % )• KL flux ( 2.88 % )– Calculated from K3pi0: ( 1.32 ± 0.04 )x1011• Use this value for further calculation– Calculated from K2pi0: ( 1.36 ± 0.04 )x1011• Background shape modeling (△Ns = 0.80 )– Error from the background shape assumption

Upper Limit• 2 events observed in signal region• Signal acceptance: (1.08 ± 0.01)10-4• KL flux: (1.32±(1.32±0.04)x1011– Calculated from K3pi0 decays• SES: (7.0 ± 4.0)10-8• U.L. at 90% confidence level: 2.4x10-7– Calculated by integrating the likelihood
function up to 90% area.

Results of KL0  π0 π0 X, X  γγ
The results of the X mass region from 194.3 to 219.3 MeV andthe X lifetime region from 0 to 10-9s are all performed.

90% C.L. upper limit of B.R.

 U.L. doesn’t change if X lifetime < 10-10s
 U.L. weaken by a factor of 2-3 if X lifetime 10-9s-65 ×10-6
4 ×10-63×10

τx = 0sτx = 10-10s-62×10τx = 10-9s-6105 ×10-7
4 ×10-7
3×10-72×10-7195200205

210

2X mass ( MeV/c )

215

220

Summary• We performed the KL0  π0 π0 X, X  γγ analysis at E391a. No evidence ofX is found and the upper limit for mX = 214.3 MeV is placed at 2.4x10-7[PRL 102, 051802].
• The results for different mX & τ X are also performed. There is no evidenceof X found in other mass region, either.
• The KL0  π0 π0 X, X  μ+μ- analysis is also performed at E391a by astudent from Saga university. There is also no evidence of X andthe upper limit is placed at 1.7x10-6.