1. 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

2. 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

3. Introduction
Introduction
Physics Motivation
E391a Experiment

4. 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

5. E391a Experiment E391a Run2 Result Introduction
KL→π0νν 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 , )

6. 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

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

8. Strategy for E391a Run3 Analysis
Strategy for Run3 Analysis
Strategy for E391a Run3 Analysis
Strategy for Run3 Analysis

9. 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

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

11. 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 !

12. 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)

13. 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%)

14. Optimization Optimization Run2 Opened Box Analysis
Improved halo-neutron MC

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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)

21. Summary E391a experiment Status of Run3 analysis
KL  p0nn 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

22. backup

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

24. 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

25. 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

26. 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. center crystal)
beam
Run3 BA

27. 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)