KEK PS E391a実験における_KL→π0νν探索の最終結果
特定領域「フレーバー物理の新展開」研究会2010(2010 Feb 23)

Hideki MORII (Kyoto Univ.)

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

1

Contents

Contents• Introduction_• Physics of KL→π0νν decay• E391a実験の測定原理• Data analysis• “halo neutron background”• イベント選択の最適化• Results• sensitivity & results

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Introduction

Introduction
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_

Introduction

0
KL→π νν Decay with NP• もし新物理があれば...?
新粒子がloop diagramを回る→崩壊振幅を変化させる& 理論的不定性 : still 1-2%

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗+

2010/Feb/23

5_

Introduction

0
KL→π νν Decay with NP

http://www.lnf.infn.it/wg/vus/content/Krare.html
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2010/Feb/23

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_

Introduction

0
History of KL→π ννSearch• 上限値更新の歴史• KTeV0→e+e-γ
π
•
-7
Br
<
5.9
10
•
• KEK E391a (Run2)
0→γγ
π
•
-8
Br
<
6.7
10
•
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

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E391a ExperimentE391a ExperimentE391a Experiment_E391a Experiment• KL → π0νν 探索実験 @ KEK 12GeV PS• 世界初のこのモードに特化した実験• 次期実験 KOTO (J-PARC E14) のためのパイロット• Three Physics Runs

KEK 12GeV PS East Counter Hall

• Run1 (2004 Feb-Jul)

A-line“membrane” problem• Run2 (2005 Feb-Apr)• Run3 (2005 Nov-Dec)

C-line

E391a

最終解析にはRun2 + Run3 のsampleを使用
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

9Experimental Principles

E391a Experiment• シグナルモードの同定
ー stateKL→π0νν→cannot detect→2γ• “2γ + nothing”• 2γ → CsI calorimeter (energy, position)• nothing → hermetic veto detector

θ• 崩壊点をM(π0)を仮定する事で再構成
M(π0)2 = 2E1E2(1-cosθ)“pencil” beam で pT 分解能を確保• pTと崩壊点の情報から
signal regionを定義
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E391a ExperimentE391a Detector
Signal Mode

KL

ν

γ

ν

γ

11

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

E391a ExperimentE391a Detector
Signal Modepure CsI crystal
7x7x30cm (5x5x50cm)
576 channels
CsI Calorimeter

KL

ν

γ

ν

γ

11

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

E391a ExperimentE391a Detector
Background : KL →π0π0pure CsI crystal
7x7x30cm (5x5x50cm)
576 channels
γKL

CsI Calorimeter
γ
γ
γ

11

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

E391a ExperimentE391a Detector
Background : KL →π0π0pure CsI crystal
7x7x30cm (5x5x50cm)
576 channels
γKL

CsI Calorimeterγγ
γ

Photon Veto Detector

Back-Anti : veto γ
escaping into beamhole11
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

E391a ExperimentE391a Detector
Background : KL →π+π-π0 pure CsI crystal

7x7x30cm (5x5x50cm)
576 channels

KL

π+

CsI Calorimeterγπγ
Photon Veto Detector

Back-Anti : veto γ
escaping into beamhole11
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

E391a ExperimentE391a Detector
Charged Particle+π-π0 pure CsI crystal
Background:K→π
L
Veto Detector

7x7x30cm (5x5x50cm)
576 channels

KL

π+

CsI Calorimeterγπγ
Photon Veto Detector

Back-Anti : veto γ
escaping into beamhole11
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

E391a ExperimentE391a Detector
Charged Particle+π-π0 pure CsI crystal
Background:K→π
L
Veto Detector

7x7x30cm (5x5x50cm)
576 channels

KL

π+

CsI Calorimeterγπγ
Photon Veto Detector

Back-Anti : veto γ
escaping into beamhole11
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

Strategy to Run2+3 Data Analysis

Strategy to Run2+3 Data Analysis

Strategy to Run2+3 Data Analysis

Review of Run2 Analysis• 一つ前の解析 : Run2 Result• blind analysis• No event observed in
the signal box• Upper limit 6.7 x 10-8(90% C.L.)(Phys. Rev. Lett. 100 201802, 2008)• Run2解析から得られた事• 最大のバックグラウンド源→ halo neutron BG• Collar Counter (CC02)-π0 BG• CV-π0 BG• CV-η BG
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

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13Strategy to Run2+3 Data Analysis• Halo neutron• neutron ﬂux surrounding

rec. pT

Halo Neutron BackgroundCC02-π0CV-π0ﬁducialregionbeam core• Halo neutron BG

CV-ηrec. z vertex

halo-n hits detector
around beam core
→ creates π0,η→2γ

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

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14Strategy to Run2+3 Data Analysis• Collar Counter (CC02) π0 BG
Eγを実際より低く見積もる(shower leakage

rec. pT

Mechanism of Neutron BackgroundCC02-π0CV-π0ﬁducialregion& photo-nuclear eﬀect)••→θを大きく見積もる
CV-π0 BG
Eγを実際より大きく見積もる

CV-η

(due to fusion cluster)γ→ θを小さく見積もる
CV-η BG
M(π0) と M(η) の違い→ θを小さく見積もる

rec. z vertex

E1

θ

γ

E2

M(π0)2 = 2E1E2(1-cosθ)

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Strategy to Run2+3 Data Analysis

Motivation for the Current Analysis• halo neutron BG• CC02 π0 BG (→ extrapolation of the Al-target data)• CV π0 BG (→ bifurcation)(→ geant4 + geant3 MC)• CV η BG
以前のRun2解析では別々の方法で見積もり
バックグラウンドの統一的な扱いが困難
シグナル/バックグラウンドの効率的な最適化が難しい

新しい解析では
halo neutron BGの見積もりを統一的な方法で行う→ シンプルで効果的なS/Nの最適化→ バックグラウンドの統一的な理解
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Study on halo-n BG

Study on Halo Neutron BG

Study on halo-n BG

Halo Neutron BG Study• Halo neutron BG studyの手順
1. FLUKAのhadronic interaction modelの信頼性を確認→ 確認用に取られた測定データ(Al-plate run)を使用
2. イベント選択の最適化
3. バックグラウンドの見積もり

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The
Aluminumplaterun
Al Target Run
4.4. Calibration Runs

• Setting 5 mm thick Al plate
確認すべき事at 6.5 cm from the CC02’s surface• Statistics
π0, ηの生成率- 5.57x1016 POT (data: 1.40x1018)→ データとFLUKA simulation

confirmation of fluka model
MB

•

•

Study on halo-n BG

FB! 0, "!##

CV

neutronCC02
5mm AlCsI• BG estimation
using the Al plate runを比較する事で確認- CC02 events
Figure 4.1: Illustration of the Al plate run set-up. The Al plate was located at 6.5 cm dow$ contamination to downstream thebyedge of CC02.
10
invariant mass(GeV/c2):- shower leakage
assuming Z = Z_plate
4.4 Calibration Runs
Al
targetrun
10- photo nuclear effectIn addition to the physics run and the special runs for the physics analysis, we took data for th5•45mm厚のAl targetをビームライン中へ挿入•
calibration.

103% production
4.4.1 Cosmic Run→2γの質量を再構成可能(with
fixed z-vertex)
10
When the beam from the accelerator was stopped, the data with the cosmic trigger were tak$ evaluate the cross section
the pre-scaling. The gains of the CsI calorimeters were monitored with data taken in the cos
2• Amount of statisitics

4.4.2 Muon Run- 5.57 x 1016 POT

10Muons came from the upstreamof the neutral beam line through the collimators and the1
0 detector
0.1 uniformly
0.2
0.3and almost
0.4
0.5
0.6
0.7 When theshield. The muons entered theperpendicularly.
2
mass(
GeV/cand) only the h
the collimatorsystemwas closed, neutral particles in theinvariant
beam were
blocked0031
l. 1, 2008
“Search for the Decay”
T. SumidaL!!
muons Kcouldbe""#enhanced;this condition was called “Muon” run. CC00,
CC02, CC04–07,
19
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗
2010/Feb/23
BHCV, and BA were calibrated by the data with the muon trigger. The Muon run was carrie

Study on halo-n BG

confirmation of fluka model0
π , η Production Rate

• Ratio π0 / η → OK
# of events

Reconstructed mass of 2γ

measured by CsI

M(2γ)2 = 2E1E2(1-cosθ)

calculated fromfixed vtx-position

dot : Al target run data
blue : MC (fluka)

η peak↓↑π0 peakrec. z-vtx. (cm)reconstructed mass (GeV/c2)

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Cut Optimization

Study on halo-n BG

cut optimization• Cut condition最適化の方針• S/NをRun2の結果と同等に保ちながら
acceptanceを最大化する• 最適化の間は実データのシグナル領域を隠す→ human-biasingを防ぐため• 具体的には？
• 新しいカット“cluster-shape NN”の導入• いくつかのカットを置き換え• パラメータの自動最適化
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Study on halo-n BG

Cluster Shape NN Cut (for CV-η BG)• CsIのヒットパターンを用いたNeural Network
CV-η BG は広がりを持ったクラスタを生成(γが浅い角度でCsIに当る & γのエネルギーが高い)

Neural Network value
black : signal MC
red : CV-η MC

NN value for γ2• NNへの入力: energy, r, phi-position (each crystal)correlation colored : signal
dots : etaBG

veto
Neural-Net value

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NN
value for γ122Study on halo-n BG

γ-fusion NN Cut (for CV-π0 BG)•

CV-π0 BG

normalfusion like

CsIでの“fusion” clusterが原因• Run2からカットの変更で最適化
cluster size cut → fusion NN cut~40% accept. loss → ~20% accept. loss
rejection power is similar (~70% reduction)veto

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Study on halo-n BG

Result of Optimization

condition

Signalcut optimization

S/N (arb.)

Run2(prev.)
30328
5054
New
45945(+51%)5105
S/N : 以前のRun2解析と同等
acceptance : 以前のRun2解析から50%増加
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Background Estimation

Background Estimation
Halo neutron backgroundKL backgroundBackground Estimation

Background Estimation• Halo neutron background• CC02-π0 : from upstream• CV-π0 : from downstream• CV-η• KL originated background0, K →γγ
neutral
mode
:
K
→2π
L
L
•
• charged mode : KL→π+π-π0

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Background Estimation

halo-n BG
0CC02-π Background (upstream)• CC02-π0 BG (BG from upstream)• 0.66

0.39 events
CC02 π0 backgroundw/ all selections

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Background Estimation

halo-n BG
0CV-π Background (downstream)• CV-π0 BG (BG from downstream)• no events remained → < 0.36 events
CV π0 background
w/o fusion NN cut

with fusion NN cut
w/ all selections

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Background Estimation

halo-n BG

CV-η Background• CV-η BG
0.19

0.13 events
CV η background

w/o shape NN cut

with shape NN cut
w/ all selections

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Background Estimation• KL decay backgrounds• GEANT3 simulation• KL→2π0vetoで余分な2つのγを検出
統計量 : Run2+3の約65倍
全カット適用後 : 2events: 0.024

0.018

PT (GeV/c)

KL Decay Backgrounds

KL BG

KL→2π0 BG
0.35
w/ all selections
0.4

1

0.3
0.25

10-1

0.0 0.02

0.2

10-2

0.15
0.1
0.05

10-3

0.0

0.0

0.0

0 250 300 350 400 450 500 550 600
200
Zvtx (cm)10-4• 他のKL decay BGʼs :KL→γγ : PT, kinematic selction → O(10-5)
Charged modes : reduced by CV → O(10-4)
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Background Estimation

Summary of Background Estimation• Summing up all background sources→ estimated # of background : 0.87

sourceKL→2π0KL
othersCC02-π0
halo-n
CV-π0
CV-η
total

0.41

estimated BG
0.024 ± 0.018
small (~O(10-4))

0.66 ± 0.39
0.0 (<0.36)
0.19 ± 0.13
0.87 ± 0.41
for Run2 + Run3 data

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Background Estimation

Summary of BG Estimation• シミュレーションによる見積もりとデータを比較→ データをよく再現している

Run2 + Run3 data

Background MC

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

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Sensitivity & Results

Sensitivity & Results# of KL decays
Sensitivity
Results

Sensitivity & Results# of KL Decays• E391a full dataで得られたKL 崩壊数• KL→3π0, 2π0, γγの3 modesで見積もり

Run2 + Run3 data

mode
KL→3π0KL→2π0KL→γγ# of eventsin data

acceptanceflux

118334(7.21±0.06) x 10-5

(8.41±0.03stat.±0.53syst.) x 109(-3.3%)

2573.9(3.42 ± 0.03) x 10-4

(8.70±0.17stat.±0.59syst.) x 109(---)(7.18 ± 0.03) x 10-3

(9.02±0.05stat.±0.51syst.) x 109(+3.7%)

35367

cf.) Run2 only : flux = 5.13x109→ Run2+Run3 = 統計量は以前の解析の1.7倍
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Signal Acceptance• Signal acceptanceA=(イベント選択後に残るイベント数)

Sensitivity & Results
accidental eﬀect
17.4% loss (Run2)
20.6% loss (Run3)(崩壊領域で崩壊したKL数)(# accept MC)= (# generated→decayed in MC)
(1.06
=
(1.01

(accidental loss)
x (loss by time cuts)0.08)% for Run20.06)% for Run3

(cf. previous analysis with Run2 : 0.670%)

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Sensitivity• KL ﬂux• (8.70

Sensitivity & Results0.61) x 109 KL decays for Run2 + Run3

• Single event sensitivity (S.E.S.)“1eventの観測が期待できる分岐比”
S.E.S. = 1/ (Acceptance x # of KL)= (1.11 0.10) x 10-8 for Run2 + Run3

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Sensitivity & Results

Now, Ready to Open the BOX• Opening the box for Run2 + Run3 data
Run2 + Run3 data

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Sensitivity & Results

Now, Ready to Open the BOX• Opening the box for Run2 + Run3 data
Run2 + Run3 data

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特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Sensitivity & Results

Results• Acceptance = 1.06% (Run2) and 1.01% (Run3)(cf. Run2 previous : 0.670%)• S.E.S. = 1/(Acc. x #KL)
Run2 + Run3 : 1.11 x 10-8

(cf. Run2 previous : 2.91 x 10-8)• 分岐比上限no events observed → x 2.3 with Poisson stat.

_E391a ﬁnal : BR(KL→π0νν) < 2.6 x 10-8(@90% C.L.)(cf. Run2 previous : 6.7 x 10-8 @ 90% C.L.)→ Improvement from the previous : x 2.6 (=1.7 x 1.5)
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

統計 acceptance38

2010/Feb/23

Milestone

Sensitivity & Results• 一歩前進!
• Next step : KOTO
E391aでの知見を活かして...
→next talk

特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Summary (1)Summary_• KL→π0νν崩壊• 新物理を探索する良い実験場 : CPV, theoretically clean• E391a experiment @ KEK 12GeV PS_• ﬁrst dedicated experiment for KL→π0νν• Features of Full Analysis• halo-n BG studyに重点
simpleな手法で見積もり
効果的にイベント選択を最適化 (+50% in acceptance)• データの統計量
Run2+Run3で 以前の解析(Run2)の約1.7倍
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Summary

Summary (2)• Acceptance : 1.06% (Run2) & 1.01% (Run3)• Sensitivity
S.E.S. : 1.11 x 10-8 (Run2 + Run3)• Opening the box for Run2 + Run3 data
→ No events observed!
ﬁnal)• Upper Limit (E391a_

arXiv : 0911.4789

BR(KL→π0νν) < 2.6 x 10-8 (@ 90% C.L.)(以前の解析から2.6倍の更新)• E391a実験の手法の有効性を証明→その知見を活かしてJ-PARC KOTO実験へ
特定領域「フレーバー物理の新展開」研究会2010@オーシャンビュー大洗

2010/Feb/23

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Step-bThank You!