kek-ps e391a run3 データを用いた k l →π 0 νν 解析の現状

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KEK-PS E391a Run3データを用いたKL→π0νν解析の現状

JPS 2008 Autumn Meeting Hideki MORII (Kyoto Univ.)　　他 E391a Collaboration

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Contents

• Introduction – Physics of KL 0

– E391a Experiment

• Strategy for Run3 analysis• Status of Run3 analysis

– calibration & stability

– KL flux estimation

• To get further from Run2– Run2 opened box analysis– better halo neutron MC

• Summary

Overview

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Introduction

Introduction

1. Physics Motivation2. E391a Experiment

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Physics Motivation for Kpi0nunu

• KL0– “direct” CP violation– “gold-plated” mode :

small theoretical uncertainty– measures eta of CKM matrix– small BR : 2.5x10-11

• Sensitive to…– precise check of standard model– sensitive to new physics

Introduction

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E391a Experiment

• E391a– KL→π0νν measurement @ KEK 12GeV PS

– pilot experiment for J-PARC E14– Three data taking

• Run1 : Feb 2004 – Jul 2004• Run2 : Feb 2005 – Apr 2005• Run3 : Nov 2005 – Dec 2005

• Run2 Result– Blind analysis– No events found in the signal box– Upper limit : 6.7x10-8 (90% C.L.)

(Phys.Rev.Lett. 100 201802, 2008. )

Introduction

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Principle of E391a Experiment

• E391a principle

1. require (0)2 + nothing else CsI calorimeter (2 detection) hermetic veto system

2. reconstruct decay vertex

assuming 0 mass M = M0

3. requre missming pT & vertex

inside the fiducial region

Introduction

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E391a DetectorIntroduction

Aerogel Photon Counter(APC)

Upgraded

Added

Back Anti

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Strategy for E391a Run3 Analysis

Strategy for Run3 Analysis

1. Strategy for Run3 Analysis

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Strategy for Run3 AnalysisCalibration

kdecay,halo-n, eta

Strategy for Run3 Analysis

Results

Step1Confirmation

Step2 Optimization

Step0Preparation

Step3Physics Output

[MC] Develop Run3 MC

[Run3 Data] Data quality check

[Run2 Data / MC] Cut optimization

[MC] MC mass production

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Current Status of Run3 Analysis

Current Status of Run3 Analysis

1. Calibration & Run Stability2. KL flux estimation with 3 decay modes

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Calibration & Run Stability Check

• Calibration & run stability– completed / confirmed

Current Status of Run3 Analysis

M6(GeV/c2)

Run Stability of CsI gain value Run Stability of reconstructed KL mass

(6 sample : KL30)±1% ±1%

(typical crystal)

peak497.6 MeVRMS~1%

quite stable !

6 invariant mass

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Reconstructed Mass Distribution

• Reconstructed mass for 6(KL30), 4 (KL20)– KL mass spectrum matches well to MC

Current Status of Run3 Analysis

reconstructed mass (GeV) reconstructed mass (GeV)

Rec. mass of 6 sample Rec. mass of 4 sample

dots : Run3 datablue :K30MCred : K20MC

# of

eve

nts

# of

eve

nts

ratio

(da

ta/M

C)

ratio

(da

ta/M

C)

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KL Flux Estimation

• Flux estimation is done by K30, K20, K

mode # of events

in data

# of events

in MC(addbg)

acceptance flux

30 56350 3464

(stat. 2.0e9)

8.09 x 10-5 3.56 x 109

(-2.7%)

20 1254 12646

(stat. 1.5e9)

3.94 x 10-4 3.66 x 109

(---)

11111 21896

(stat. 2.0e8)

5.12 x 10-3 3.96 x 109

(+8.2%)

cf. Run2 30 : 5.02 x 109 (-2.1%) 20 : 5.13 x 109 (---)

: 5.45 x 109 (+6.2%)

Run3 Statistics : ~71% of Run2

Current Status of Run3 Analysis

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Optimization

Optimization

1. Run2 Opened Box Analysis2. Improved halo-neutron MC

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Acceptance Study with Run2 Data

• To get more acceptance…– try to see Run2 data with opened box

• Acceptance list (veto)– single hit CsI : 64.4%– MainBarrel : 79.0%– ChargedVeto : 82.9%

• Acceptance list (kinematic cuts)– -RMS : 57.9%– 0 kinematics : 77.7%

-RMS cut has the largest acceptance loss try to loosen this cut and see other cuts

Optimization : Get Further from Run2 Analysis

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• -RMS cut– cut for cluster shape on CsI– to reject fusion cluster

• Fusion cluster– gamma + gamma : KL 20 BG

• which effectively results in photon inefficiency– gamma + neutron : “CV halo neutron BG”

• which results in 0 misreconstruction

see next page

Functionality of -RMS cutOptimization : Get Further from Run2 Analysis

normalfusion like

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• Mechanism of CV-bg1. halo neutron hits on CV

2. create 0 + something

3. misreconstruct 2 to signal box

due to extra activity

Mechanism of CV BackgroundOptimization : Get Further from Run2 Analysis

reconstructed vertex

beam line

extra particle

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veto

Alternative to RMS cut : Fusion-NN Cut

• -RMS cut

– requires cluster RMS smaller than 4cm

• Fusion Neural Network cut– study with K30 MC– 0 : fusion like, 1 : normal– requires >0.7 for NN val.

Optimization : Get Further from Run2 Analysis

i i

ii

E

dERMS

2

i : run over all crystalsdi : distance from center

pT vs z plot w/ and w/o -RMS cutblack : w/ -RMS cutred : w/o -RMS cut

z (cm)

p T (

Ge

V/c

)

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Alternative to RMS cut : Fusion-NN Cut

• replacing -RMS cut by fusion-NN cut– no events in signal box– acceptance : 58% 77% (33% recovery)

Optimization : Get Further from Run2 Analysis

pt vs z plot w/ & w/o -RMS cut pt vs z plot w/ & w/ofusion NN cut

black : w/ -RMS cutred : w/o -RMS cut

black : w/ fusionNN cutred : w/o fusionNN cut

z (cm)z (cm)

p T (

Ge

V/c

)

p T (

Ge

V/c

)

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Improved Neutron MC

For getting better understanding to neutron BG…• trying FLUKA model for halo-n MC

– better estimation for hadronic interaction

• Recycling Method– recycle MC seed for “dangerous” events– speeding up MC (need 2weeks for Run3 data equivalent)

Optimization : Other Analysis Effort in Run3

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Summary

• E391a experiment – KL 0 measurement @ KEK 12GeV PS

– Now, Run3 analysis is ongoing

• Status of Run3 analysis– calibration & stability

– KL flux estimation

• To get further from Run2– Run2 opened box analysis– better halo neutron MC

Summary

Seems good quality

To get more acceptance

To understand more about neutron BG

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backup

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Mechanism of CV Background

even+extra odd

and 2 extra

fusion 1 from 0 + extra

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Mechanism of CV background• removing Veto : odd & 01+extra• removing g-selection : even+extra• with bifurcation for each mechanism, even+extra is dominant

setup + box + selection + veto

-veto - selection all cuts

all 3277 44 6 0 (.081)

even+extra 306 4 2 0 (.026)

odd 210 26 0 0 (0.0?)

fusion 226 3 1 0 (.013)

1+ extra

2525 11 3 0 (.013)

extra 2clustr 10 0 0 0 (0.0?)

tighten

loosen

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Features of E391a apparatus

• Decay region– High vacuum: 10-5 Pa

• to suppress the backgroundfrom interactions w/ residual gas

• Detector components– Set in the vacuum: 0.1 Pa

• separating the decay regionfrom the detector regionwith “membrane”: 0.2mmt film

CsI calorimeterCsI calorimeter

Charged Veto (CV)Charged Veto (CV)Main Barrel (MB)Main Barrel (MB)

Front Barrel (FB)Front Barrel (FB)

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Back Anti Upgrade• Upgrade Back Anti

– lead plate + plastic scinti. + quartz

PWO crystal + quartz– segmentation :

longitudinal

transverse

• Benefits– better n/ separation

(shower shape analysis)– lower rate

(typ. 1/2 @ center crystal)

Run2 BA

beam

Run3 BA

Step0 : Preparation

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Aerogel Photon Counter• Aerogel Photon Counter (APC)

– Aerogel Cherenkov counter : only sensitive to fast particle• insensitive to neutrons / sensitive to shower

– Can be used as photon tag counter (for BA study)– prototype of E14 BA

Pb convertor : 2mm thick (~0.3 X0)Aerogel : 30cm(x) x 30cm(y) x 5cm(z)

γ

e+

e-

Cerenkov light

Step0 : Preparation