Status of the KEK PS-E391a experiment

~ Measurement of the decay, KL π0νν ∼

Hiroaki WatanabeKEK, IPNS

( Univ. of Chicago)

JPS Mar 25th, 2005

E391a collaboration• KEK

T.Inagaki, T.Komatsubara, G.Y.Lim, T.Sato, M.Sekimoto, H.Okuno, Y.Yoshimura, M.Doroshenko• Saga University

S.Kobayashi, A.Sugiyama, Y.Akune, S.Ishibashi• Yamagata University

T.Iwata, Y.Tajima, H.Yoshida, M.Itaya, H.Sato• RCNP

T.Nakano• Osaka University

T.Yamanaka, M.Yamaga, K.Sakashita, N.Komatsu, N.Nishi, Y.Sugaya, T.Mizuhashi, H.Ishii• National Defense Academy in Japan

T.Shinkawa, T.Matsumura• Joint Institute for Nuclear Research

A.Kurilin, Z.Tsamalaidze, V.Baranov, S.Podolsky, S.Porokhovoy, E.Kuzmin, S.Perov, A.Moiseenko• University of Chicago

Y.Wah, H.Watanabe, J.Nix, G.N.Perdue, A.Lednev• Taiwan National University

Y.B.Hsiung• Pusan National University

J.K.Ahn, S.Y.Lee, H.S.Lee• Kyoto University

T.Sumida, N.Sasao, T.Nomura, Morii

11 Institutes & ~50 Members

Outline

• Physics Motivation• Detection Principle• E391a Detector• Data taking• Analysis• Summary & Prospect

Physics Motivations• KL→π0νν

- Direct CPV process (Δs=1)- BR(KL → π0 ν ν)

=1.94x10-10 .η2. A4.X2 ～ 3x10-11

- Sensitive to a CKM-matrix elementIm(Vtd) = η CPV in Quark sector

- Theoretical Uncertainty

~1.5%

New Physics• Unitarity Triangle vertices

K+ π+νν

KL π0νν

ACP(B J/ΨKS)ΔMBd/ΔMBs

η

G. Buchalla hep-ph/0110313

New prediction by Buras et.al.

A systematic approach to the recent B→ππ, πK data by Belle and BaBar.Suggesting a New Physics in the EW penguin sector :

↓

Br(KL→π0νν) :(2.6±0.5)×10-11 (SM)

↓New physics

Br : (3.1±1.0)×10-10 ρ

Very Interesting Probe for New Physics !

KL→π0νν• Theoretically Attractive

Experimentally Challenging

• Step-by-step approachCurrent upper limit (10-7 )

↓

KEK-E391a ( ~10-10)↓

J-PARC (~ 10-13) Δη/η~5%

• KEK-E391a Experiment– First Dedicated Search– Establishment of Method– Unexpected Background

E391 Detection PrincipleKL→π0 νν ( 2γ + νν ) Measuring 2γ + requesting no other visible particle- Hermetic Veto system

Pencil BeamDetermining decay vertex assumed by π0 mass.

- Selecting in higher transverse momentum (PT) events - Other neutral KL decays : PT=0 ( c.f. KL 2γ ,2π0, 3π0 )

~10m~3m

Detection Principle ~Rejection of other Kaon-decay backgrounds~

Pb/Scinti. Sandwich Calorimeter(14X0)

☆ Main background : KL πoπo 4 γBr(KL π0νν)/Br(KL π0π0)~10-8

↓

1. High detection efficiency for γLow energy threshold for vetoing.

2. 4π veto system without any dead space

Detection Principle ~ Rejection of Neutron backgrounds~

• π0 production by core neutron with residual gasHigh vacuum :

KEK-E391a Detector

KEK-E391a Detector

576 pure-CsI

EndcapMain BarrelFront Barrel

Detector Integration was finished on Jan 22, 2004

Data taking• Run time (physics run)

– Run-I : Feb. 16th – Jul. 1st 2004: (~60days) – Run-II : Feb. 1st – cont. ( ~ Mar 27th ?)

• KEK 12 GeV PS : Incident protons– 2.2 X 1012/spill at target– 2-sec spill length & 4-sec repetition

• KL flux in front of detector – 5x105 /spill– Peak momentum : ~2 GeV/c

• Trigger : No. of cluster >=2• Energy threshold for a cluster : 60 MeV.• DAQ live-time ratio : 75 %• Vacuum : ~10-5 Pa

Online event display

Analysis ~Run-I~– 1-day data (1/60) :

• Calibration• Normalization Channels : KL 3π0 KL 2π0 KL γγ• 2γ analysis• Result

– 1-week data (1/10)

– Full data

Calibration• Energy & timing

– Cosmic-ray muon. CsI, barrels– Punch-through muon. Collar counters– π0 production at Al target.

Precise calibration of CsI

Muon Data

(KEK-preprint-2004-85, accepted for publication in NIM.)Ishii@JPS-TUS, Sato@JPS-TUS

π0 production at target

Kaon Reconstruction (KL 3π0 ,2π0 ,γγ)• No. of reconstructed γ-cluster : equal to 2,4,6

(E >= 50MeV)• No other cluster ( E < 20 MeV) in CsI• π0 reconstruction (assuming π0-mass vertex Z)• Best chi2• Kinematical cuts

- Chi2 < 2- 2nd Chi2 >20- Pt < 10(MeV/c)- Beam size< 4cm

γ1

γ2

γ3γ4

z1

z2

beam directionPt

Example : KL 2π0

• KL momentum• KL decay vertex

Normalization channels KL 3π0

M.C. reproduces data. KL Momentum (GeV/c)

Decay Vertex (cm)

Normalization channels (without tight vetoing)

Invariant Mass of 4γ (GeV/c2)

MCdata

−•

Reconstructed vertex of KL (cm)Invariant Mass of 6γ (GeV/c2)

03πK02πKγγKKL ποποKL π0ποπο KL γγ

6γ events 4γ events 2γ events

Vertex-finding cut Vertex-finding cut + fusion-cut PT+decay-position cut

Pure kaon sample.Monte-Carlo simulation well reproduces data.

Comparison of normalization channelsKL 3π0 KL 2π0 KL γγ

BR(PDG) 21.13% 9.27x10-4 5.86x10-4

No. of event 29669 991.4 12951Acceptance 1.41x10-5 1.09x10-4 2.16x10-3

Backgrounds contribution

2γ analysis

–Background study– Cut optimization – Background estimation– Acceptance estimation– Result

Exp. Data without tight veto

Reconstructed vertex (cm)

PT(

GeV

/c)

Hard to explain using KL decay only

M.C. for Kaon decays2-γ Analysis

PT(

GeV

/c)

Reconstructed vertex (cm)

MC ~material in front of CV?~

CH2, ~1g/cm310 g/cm3& 2 mm-thick

P(G

eV/c

)T

Reconstructed vertex (cm)Well reproduces data distributions.

2γ analysis– Background study

–Cut optimization – Background estimation– Acceptance estimation– Results

1. CsI- Shower shape- Kinematical cuts

2. Veto detectors- Timing- Energy

2-γ Analysis ~Cut optimization~

B C

A

D

“A” sample – KL π0νν MC

“B,C,D” samples – exp. data

Reconstructed vertex (cm)

PT(

GeV

/c)

Exp. Data without tight veto

BC

D

A

Simple Shower-shape analysis• “D” sample

– Fusion dominate– Neutron events exist

• “C” sample – Mixing gamma and neutron clusters

• “B” sample– Contains only gamma clusters

clusterEEEE 321 ++

clusterEEmax

B C

A

D

B C

A

D

Kinematical cut• Clusters produced by neutrons

small distance between two γ’s• Fusion

unbalanced energy of two γ’s

=Eγ1-Eγ2 / (Eγ1+Eγ2)

2γ analysis– Background study

–Cut optimization – Background estimation– Acceptance estimation– Results

CsI- shower shape- Kinematical cuts

2. Veto detectors- Timing- Energy

Veto Optimization ~Main-barrel timing (low E sample)~KL γγ pure sample γγ B.G.sampleupstream

downstream

Backsplash should NOT veto! ① Real photon hit should veto.② Backsplash should NOT veto.③ Leakage of neutron should veto.

③

① ②early late

Veto optimization ~Energy Threshold~• Using Pure Kaon sample and background sample.

π3Kπ2K

γγK

Main barrel (inner layer) 3 MeV-threshold.

1 ~ 3 MeV threshold were applied.

D region.

Energy Threshold(MeV) Energy Threshold(MeV)

Acce

ptan

ce

Bac

kgro

und

Rej

ectio

n

Energy Threshold(MeV)

B C

A

D

Acceptance loss due to veto

• After being applied ALL veto cuts :– KL 3π0 acceptance : (37.4 ±0.8)%– KL 2π0 acceptance : (38.5±3.8)%– KL γγ acceptance : (37.9±0.7)%

• Well consistent with each other!

• Mean acceptance is (37.7± 0.5)%KL π0νν acceptance after being applied veto.

2γ analysis– Background study

– Cut optimization

– Background estimation– Acceptance estimation– Result

Signal box• PT vs. Z (decay vertex)

– 120MeV/c < PT < 250MeV/c– 300cm < Z < 500 cm

Raw data All cuts & all vetoskinematical & shower-shape

cuts

Reconstructed vertex (cm) Reconstructed vertex (cm) Reconstructed vertex (cm)

PT(

GeV

/c)

PT(

GeV

/c)

PT(

GeV

/c)

Background estimation in signal box− π0 production at CC02 by halo neutrons

• ~0.03 events

– KL decays : M.C.•

Signal boxNo events insideEvents around signal box:

① KL γγ decays② π0 production at CC02 by neutron interaction③ Core neutron events were suppressed by kinematical cuts④ ( KL 2π0 ? )

Reconstructed vertex (cm)

①

②

③

④

M.C. without tight vetoing.

Reconstructed vertex (cm)

PT(

GeV

/c)

PT(

GeV

/c)

Exp. Data

Acceptance for KL π0νν• Acceptance loss due to kinematical M.C.• Acceptance loss due to veto Pure Kaon samples

– Decay probability is included

Cuts Acceptance

Decay + geometry + skimming 5.49x10-3

E γ > 200MeV 3.55x10-3

Distance between 2-γ‘s >50 cm 2.77x10-3

0.92 < E1+E2+E3/Ecluster

KL π0νν branching ratio upper limit

• Acc = (1.21 ± 0.03) x10-4 (decay probability included)• Normalized by KL 3π0:

)()3(

)3()()3().(.. 0

0

0

000

ννππ

πννππννπ

→→

→→

→=→L

L

L

LLL KAcc

KAccKN

KNKBRKSES

74

5

10)2.03.8(1021.11041.1

2966912113.0 −−

−

×±=××

⋅⋅=

• c.f. ) KTeV result : BR < 5.9x10-7 (90%CL)

Upper limit (1-day ): 1.9 x10-6 (90% C.L.)

Summary ~1-day data analysis~

• Analysis : 1 day data (1/60) : – Understanding Kaon : ○ pure Kaon sample.

– Acceptance loss due to shower leakage & accidental loss. • Pure Kaon sample.

– Background sources : KL 2π0 ( Fusion) , Halo-neutron π0

Shower shape and kinematical Cut : important

– Membrane problem: solved by Kinematical cut (Run-I) Fixed (Run-II).

Upper limit : 1.9 x10-6 (90% C.L.)

Analysis– 1day (1/60) : M. Doroshenko, Ph.D. thesis

• Upper limit : 1.9 x10-6 (90% C.L.) • Fruitful Experiences and many implications

– 1-week (1/10) : in progress.

– Full data

Run-I

Run-II : in data taking.

Prospect ~Run-I analysis~• 1 week analysis (1/10) is in progress.

K. Sakashita will present its status on 27th, Mar in this JPS.• M.C. well reproduces detector responses.• Detail shower shape and kinematics studies & better energy calibration.• More statistics to study backgrounds.

Veto-counter Energy distribution Eγ at CsI

Prospect ~Run-II~

• Membrane was fixed.

• Additional collar counter was installed. to reduce halo-neutron effects.

• Be absorber was installed in the beam line.better n/Κ ratio, less halo-neutron. cc00

Membrane at Endcap

W/Scintillator Sandwich(2.5λI)in front of detector.

We are working hard on (and enjoying)this challenging experiment.

Status of the KEK PS-E391a experiment��~ Measurement of the decay, KLp0nn ~E391a collaborationOutlinePhysics MotivationsNew PhysicsE391 Detection PrincipleDetection Principle ~Rejection of other Kaon-decay backgrounds~Detection Principle ~ Rejection of Neutron backgrounds~ KEK-E391a DetectorKEK-E391a DetectorData takingAnalysis ~Run-I~CalibrationKaon Reconstruction (KL3p0 ,2p0 ,gg)Normalization channels (without tight vetoing)Comparison of normalization channels2g analysis�MC ~material in front of CV?~2-g Analysis ~Cut optimization~Simple Shower-shape analysisKinematical cutVeto Optimization ~Main-barrel timing (low E sample)~Veto optimization ~Energy Threshold~Acceptance loss due to vetoSignal boxBackground estimation in signal boxSignal boxAcceptance for KLp0nnKLp0nn branching ratio upper limitSummary ~1-day data analysis~AnalysisProspect ~Run-I analysis~Prospect ~Run-II~