E391a:
The first dedicated experiment for the KL  π0νν decay
KEK, Saga, Osaka, RCNP, Kyoto, NDA,Yamagata, Taiwan, Pusan,Chicago, JINRKL  π0νν decay
Im(Vtd) measurement
Very small theoreticalambiguity
Only top loop in SM
clean and pure
Last frontier in K-decay
challenging

Why it remains ?: rare decay (Br~10-11) without a definite
kinematical constraint ⇒ very hard experiment
Two methods are proposed:
・KOPIO: Kinematical constraint as much as possibleline shape for the main background from KL  π0 π0・E391a: Meditation
simply observe 2 γ(high PT )+ nothing
Pencil beam and full coverage by veto counter with high efficiency+Double decay chamber to define the fiducial region redundantly,differential pumping to achieve ultra high vacuum and very thin deadmaterial in front of the detector, etc.

High AcceptanceHigh Acceptance
It is crucial for high sensitivity••••

S = 1 / (A・T・D)
S: single event sensitivity
A: acceptance for KL  π0νν decay
T: data collection time
D: decay rate in the fiducial region
C(counting rate)＞D
S<10-13 , T=107 sec↓
C＞D＞10MHz for A=0.1
C＞D＞100MHz for A=0.01

Detector
KL beamStep-by-step approach

E391a(O(10-10))↓J-PARC(O(1013))

˜

E391a apparatus

100-ton calorimeters are
installed in vacuum chamber

Mile stones• Dec.1996: conditionally approved• Mar.1999: constructed the beam line
• July 2001: approved• Oct. 2002: engineering run• Nov.2003: middle section (last vacuumchamber) arrived• 18 Feb. 2004: Started data taking• In fall 2004: first publication of physics result

Downstream: Fy01Upstream: Fy02

Middle:Fy03

Detector Integration

Jan 22, 2004

Calibrationin situ

Publications of new techniquesdevelopment or first application to high-energy physics••••••••••••••

CeF3 crystal
Plastic scintillator (injection molding)
Plastic scintillator (extrusion)
High QE PMT at 500nm
Techniques for WLSF readout
Ineff. measurement for γ(ES147)
Ineff. measurement for γ(ES171)
Ineff. measurement for chargedn / γ separation for BA counter
Pencil beam line
Vacuum system
Calibration method for CsILarge sampling calorimeter
Electronics and DAQ

Published in NIM and patent
Published in NIM
Preparing a publication
Published in NIM
Preparing a publication
Published in NIM
Preparing a publication
Published in NIM
Published in NIM
Preparing a publication
Preparing a publication
Preparing a publication
Preparing a publication
Preparing a publication

Clearly reconstructed KL decay modes• KL  π0π0π06-γ invariant mass (GeV/c2)• KL  π0π04-γ invariant mass (GeV/c2)

Z-vertex distribution for KL  π0π0π0PT of each π0 (GeV/c)CountsTwo event clusters in π0π0 sample

Invariant mass of 4γ(GeV/c2)

Invariant mass of 4γ(GeV/c2)

Future prospect• Run 2 in 2005 (requesting)To make more critical check in the frontier region
Run must be more efficient because detector is ready for prompt startand valuable because tuning parameters can be re-polished.
New methods for JPARC experiment can be tested.
・High sensitive experiment at JPARC (LOI-05)
Regular video meeting is started for detailed planning.
Key issues: “based on E391a” and “goal must be an ultimate
measurement (>100 SM events)”.
Mile stones: conceptual design (scenario) by NP04 (August 2004)
and full proposal will be submitted within official schedule for dayone.

A-line plan at JPARC

Summary• E391a data taking started on 18 Feb.2004 asscheduled, and it will finish on 30 June.
• The quality of the data so far collected looks fine.
• We are expecting a continuous support to ourRun-2 and JPARK plan and specially asking for astrong recommendation from the present review
committee.

Expected sensitivity by Run 1
Sπνν=(A3π/Aπνν/Y3π)・Br3π,A3π/Aπνν ~ 1/20
Y3π ~ 19(/spill)・7.2 103 ( spill/shift)・(300-80-3 15)(shifts)
・80 shifts: cooling water trouble(30)+tuning with shared beam(30)+tuningwith full beam(20)・3 15 shifts: 3 special runs (air, short bunch, π0 calibration)~ 2.4 107
Br3π = 0.21Sπνν~4.310-10 ⇔ 8.6

10-10

If we get another 200 shifts, we can double the statistics.

Physics background for higher
sensitivity• Grossman-Nir limit
Br(KL→π0νν)＜4.4 Br(K+→π+νν)
1.1 10-8 (proposal) ⇒ 1.7 10-9 (present)
Require higher sensitivity• New prediction by Buras et.al
A systematic approach to the recent B→ππ, πΚdata by Belle and BaBar
Suggest New Physics in the EW penguin sector
Br(KL→π0νν) :(2.6±0.5) 10-11 (SM)Reachable level⇒(3.1±1.0)

10-10 (New physics)