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

Contents Introduction Strategy for Run3 analysis Overview Contents Introduction Physics of KL  p0nn 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

Introduction Introduction Physics Motivation E391a Experiment

Physics Motivation for Kpi0nunu Introduction Physics Motivation for Kpi0nunu KLp0nn “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

E391a Experiment E391a Run2 Result Introduction 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. )

Principle of E391a Experiment Introduction Principle of E391a Experiment E391a principle require (p0)2g + nothing else CsI calorimeter (2g detection) hermetic veto system reconstruct decay vertex assuming p0 mass Mgg = Mp0 3. requre missming pT & vertex inside the fiducial region

E391a Detector Introduction Back Anti Added Upgraded Aerogel Photon Counter (APC) Added

Strategy for E391a Run3 Analysis Strategy for Run3 Analysis Strategy for E391a Run3 Analysis Strategy for Run3 Analysis

Strategy for Run3 Analysis Step0 Preparation Calibration [MC] Develop Run3 MC Step1 Confirmation [Run3 Data] Data quality check [MC] MC mass production Step2 Optimization kdecay, halo-n, eta [Run2 Data / MC] Cut optimization Step3 Physics Output Results

Current Status of Run3 Analysis Calibration & Run Stability KL flux estimation with 3 decay modes

Calibration & Run Stability Check Current Status of Run3 Analysis Calibration & Run Stability Check 6g invariant mass Calibration & run stability completed / confirmed peak 497.6 MeV RMS~1% M6g (GeV/c2) Run Stability of CsI gain value Run Stability of reconstructed KL mass (6g sample : KL3p0) ±1% (typical crystal)  quite stable !

Reconstructed Mass Distribution Current Status of Run3 Analysis Reconstructed Mass Distribution Reconstructed mass for 6g(KL3p0), 4g (KL2p0) KL mass spectrum matches well to MC Rec. mass of 6g sample Rec. mass of 4g sample dots : Run3 data blue :K3p0MC red : K2p0MC # of events # of events reconstructed mass (GeV) reconstructed mass (GeV) ratio (data/MC) ratio (data/MC)

KL Flux Estimation Flux estimation is done by K3p0, K2p0, Kgg Current Status of Run3 Analysis KL Flux Estimation Flux estimation is done by K3p0, K2p0, Kgg mode # of events in data in MC(addbg) acceptance flux 3p0 56350 3464 (stat. 2.0e9) 8.09 x 10-5 3.56 x 109 (-2.7%) 2p0 1254 12646 (stat. 1.5e9) 3.94 x 10-4 3.66 x 109 (---) gg 11111 21896 (stat. 2.0e8) 5.12 x 10-3 3.96 x 109 (+8.2%) Run3 Statistics : ~71% of Run2 cf. Run2 3p0 : 5.02 x 109 (-2.1%) 2p0 : 5.13 x 109 (---) gg : 5.45 x 109 (+6.2%)

Optimization Optimization Run2 Opened Box Analysis Improved halo-neutron MC

Acceptance Study with Run2 Data Optimization : Get Further from Run2 Analysis 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) g-RMS : 57.9% p0 kinematics : 77.7% g-RMS cut has the largest acceptance loss  try to loosen this cut and see other cuts

Functionality of g-RMS cut Optimization : Get Further from Run2 Analysis Functionality of g-RMS cut fusion like normal g-RMS cut cut for g cluster shape on CsI to reject fusion cluster Fusion cluster gamma + gamma : KL 2p0 BG which effectively results in photon inefficiency gamma + neutron : “CV halo neutron BG” which results in p0 misreconstruction  see next page

Mechanism of CV Background Optimization : Get Further from Run2 Analysis Mechanism of CV Background Mechanism of CV-bg halo neutron hits on CV create p0 + something misreconstruct 2g to signal box due to extra activity extra particle beam line reconstructed vertex

Alternative to RMS cut : Fusion-NN Cut Optimization : Get Further from Run2 Analysis Alternative to RMS cut : Fusion-NN Cut pT vs z plot w/ and w/o g-RMS cut g-RMS cut requires cluster RMS smaller than 4cm Fusion Neural Network cut study with K3p0 MC 0 : fusion like, 1 : normal requires >0.7 for NN val. black : w/ g-RMS cut red : w/o g-RMS cut pT (GeV/c) i : run over all crystals di : distance from g center z (cm) veto

Alternative to RMS cut : Fusion-NN Cut Optimization : Get Further from Run2 Analysis Alternative to RMS cut : Fusion-NN Cut replacing g-RMS cut by fusion-NN cut no events in signal box acceptance : 58%  77% (33% recovery) pt vs z plot w/ & w/o g-RMS cut pt vs z plot w/ & w/o fusion NN cut pT (GeV/c) pT (GeV/c) z (cm) z (cm) black : w/ g-RMS cut red : w/o g-RMS cut black : w/ fusionNN cut red : w/o fusionNN cut

Improved Neutron MC For getting better understanding to neutron BG… Optimization : Other Analysis Effort in Run3 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)

Summary E391a experiment Status of Run3 analysis KL  p0nn 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  Seems good quality  To get more acceptance  To understand more about neutron BG

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Mechanism of CV Background even+extra odd fusion 1g from p0 + extra and 2g extra

Mechanism of CV background removing Veto : odd & p01g+extra removing g-selection : even+extra with bifurcation for each mechanism, even+extra is dominant setup + box + g selection + veto veto - g selection all cuts all 3277 44 6 0 (.081) even+extra 306 4 2 0 (.026) odd 210 26 0 (0.0?) fusion 226 3 1 0 (.013) p0 1g + extra 2525 11 extra 2clustr 10 tighten loosen

Features of E391a apparatus CsI calorimeter Front Barrel (FB) Decay region High vacuum: 10-5 Pa to suppress the background from interactions w/ residual gas Detector components Set in the vacuum: 0.1 Pa separating the decay region from the detector region with “membrane”: 0.2mmt film Charged Veto (CV) Main Barrel (MB) detector region is also evacuated

Back Anti Upgrade Upgrade Back Anti Benefits Step0 : Preparation Run2 BA Upgrade Back Anti lead plate + plastic scinti. + quartz -> PWO crystal + quartz segmentation : longitudinal -> transverse Benefits better n/g separation (shower shape analysis) lower rate (typ. 1/2 @ center crystal) beam Run3 BA

Aerogel Photon Counter Step0 : Preparation Aerogel Photon Counter Aerogel Photon Counter (APC) Aerogel Cherenkov counter : only sensitive to fast particle insensitive to neutrons / sensitive to g shower Can be used as photon tag counter (for BA study) prototype of E14 BA γ e+ e- Cerenkov light Pb convertor : 2mm thick (~0.3 X0) Aerogel : 30cm(x) x 30cm(y) x 5cm(z)