稀崩壊 KL の実験・ E391a

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稲垣隆雄　（ KEK)

２００４年７月２３日　理研E391a: http://www-ps.kek.jp/e391/

E391a:The first dedicated experiment for the KL decay

KEK, Saga, Osaka, RCNP, Kyoto, NDA,Yamagata, Taiwan, Pusan, Chicago, JINR

KL decayIm(Vtd) measurement: CP violation processVery small theoretical ambiguityOnly top loop in SM clean and pureLast frontier in K-decay challenging

Small theoretical ambiguity

Can be factorized, because

Rstrong ～ 1 GeV-1= 0.2 fm and

Rweak ～ MW -1

= 0.002 fm.

Cancelled the strong part by taking ratio with K

K

K

d

d

e

s

Rstong

Rweak

d

s

d

s

t

d

W

Correction for strong part at the internal loop is small due to high top mass. 3 %≦

l

Comparison between K- and B- system for the determination of Unitary Triangle

Test of the Standard Model

( A. Buras hep-ph/9905437 )

New Physics (1)

• Unitarity Triangle vertices

• Beyond Standard Models

K+

KL

ACP(BJ/KS)MBd/MBs

G. Buchalla hep-ph/0110313 L.Littenberg HEPAP(2001)

E391a

New Physics (2)

• Several models beyond the Grossman-Nir limit

　 Br(KL→π0νν) ＜ 4.4× Br(K+→π+νν)

1.1×10-8 (proposal) 　⇒　 1.7×10-9 (present) One of models is based on a CP conserving process due to lepton mixing

• 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) 　⇒　 (3.1±1.0)×10-10 (New physics)

Naturalty

Too large gap in the mass scale between SM and GUT

for elementary scalar field (Higgs) 　↓

δMH ～ O(MX), O(MP)

↓

For MH ～ O(MW)

1. Compositeness at O(1 TeV)

2. Cancellation: SUSY at O(1 TeV)

Decay through a Flavor Changing Neutral Current (FCNC)

• Zero contribution at tree-level in the SM:

good field for hunting

Y

Decay branching ratio (Br) ～ (g/g’)4(MW/MY)4

Br MY

1 100 GeV

10- １２ 10 TeV

CP violation is a longtime but still big problem

• Strong CP problem: hard to check through direct way; EDM, axion, etc.• Source of Baryogenisis

One possible way is to measure all FCNC processes with CP violation with high precision

・ △ b=1, 2 B-factories (Belle, Babar, CDF, BTeV, LHC-b)

・ △ s=2　　　 ε + Lattice calculation

・ △ s=1

KL→π0νν is an only process which can be measure with a high precision. 　　　

　

Why it remains as a frontier?:

rare decay (Br ～ 10-11) and no definite kinematical constraint ⇒ very hard experiment

Two methods have been proposed:

・ KOPIO: Kinematical constraint as much as possible to be

a line shape for the main background from KL

・ E391a: “Meditation”, simply observe 2 γ(high PT )+ nothing

Difference is only in the last one order for background reduction, and S/N is different by at most a factor of 2. Basic problem is almost same.

Hard for experimental study

(1) Requirement for tight vetoing

Veto is an only way to identify the KL decay from other KL decays.

A rejection of 8 orders of magnitudes (10-3 → 10-11) of photon vetoing against the background from KL and 10 orders of magnitude of charged-particles vetoing against Kl3 decays.

Hard for experimental study

(1) Requirement for tight vetoing (continued)

• Full coverage by thick calorimeters

• They have to be sensitive down to the deposit energy below 1 MeV for photons and 0.1 MeV for charged particles

To build a large but sensitive detector:

against many accidentals, reemitted photons from activated nuclei and back splash in such a low energy ( because of 8-MeV binding energy of nucleus) using both of time and amplitude information.

Hard for experimental study

(2) Hard to define the signal fiducial 　　 Defining the fiducial region in both geometry and kinem

atics is crucial to reduce backgrounds from other decays, beam-related backgrounds and the backgrounds from outside decays

• Many particles exist at low momentum in the KL rest frame due to multi-prong decays of KL.

• Pointing to the decay vertex is not so accurate for photons.• Weak in vetoing at outside.

↓

Pencil Beam: to select high PT events and to minimize the beam hole of the detector.

Double decay chamber and several collar counters: A special care for the beam entrance and exit.

Hard for experimental study

(3) Special requirements

• High vacuum requirement KL Decay probability(/m): 　 e-(1/βγ ｃ τ ） ~0.01 (1%)

Interaction rate (/m 1atm) ： (n/K) ・ σnn→π0nn ・ Ntarget ~10 ・ 10-27 ・ 290.3×6×1023÷224 ~0.001 (0.1%)　　　→ 10-10 atm (10-5 Pa, 10-7 Torr) is required for single event at the 10-

11 sensitivity

• Severe tolerance for dead material in front of detectors Deposit energy of 0.1MeV for charged particles corresponds to only 50mg/cm2, which is a tolerable thickness of material

These are incompatible with each other.↓

Our answer is differential pumping. Make high and low vacuum with a thin separator of special membrane.

Hard for experimental study

(4) Requirement for high acceptance　　　 S = 1 / (A ・ T ・ D)• S: single event sensitivity• A: acceptance for KL 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

High acceptance is crucial for high sensitivity

KL beamDetector

Step-by-step approach

E391a(O(10-10))

↓ 　J-PARC(O(10-

13))

~

E391a apparatus

100-ton calorimeters are installed in vacuum chamber

History

• Dec.1996: conditionally approved• Mar.1999: constructed the beam line• July 2001: approved• Oct. 2002: engineering run• Nov.2003: middle section (last vacuum cha

mber) arrived• 18 Feb. –June 30 2004: Start data taking• In fall 2004: first publication of physics result

Upstream: Fy02Downstream: Fy01

Middle:Fy03

Detector Integration

Jan 22, 2004

Calibration in situ

Develpment or first applicationof new techniques

• CeF3 crystal Published in NIM and patent• Plastic scintillator (injection molding) Published in NIM• Plastic scintillator (extrusion) Preparing a publication• High QE PMT at 500nm Published in NIM• Techniques for WLSF readout Preparing a publication• Ineff. measurement for γ(ES147) 　 　　　 Published in NIM• Ineff. measurement for γ(ES171) 　　 　　 Preparing a publication • Ineff. measurement for charged Published in NIM• n / γ separation for BA counter Published in NIM• Pencil beam line Preparing a publication 　• Vacuum system Preparing a publication• Calibration method for CsI Preparing a publication• Large sampling calorimeter Preparing a publication• Electronics and DAQ Preparing a publication

Clearly reconstructed KL decay modes

• KL • KL

6- invariant mass (GeV/c2) 4- invariant mass (GeV/c2)

Z-vertex distribution for KL 　

Invariant mass of 4 (GeV/c2)

P T o

f ea

ch

0 (G

eV/c

)

Invariant mass of 4 (GeV/c2)

Cou

nts

Two event clusters in sample

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)+tuning with full

beam(20) ・ 3×15 shifts: 3 special runs (air, short bunch, π0 calibration)

～ 2.4×107

Br3π = 　 0.21

Sπνν ～ 4.3×10-10 ⇔ 8.6×10-10

　　

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

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 start a

nd 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 measure

ment (>100 SM events)”. Mile stones: conceptual design (scenario) by NP04 (August 2004) and

full proposal will be submitted within official schedule for day-one. 　　　

A-line plan at JPARC

Summary• KL decay is pure and clean mode to measure a basic

parameter of the SM and to look for new physics.• The decay is the last frontier remaining in K-decay field.• E391a is the first dedicated experiment for KL decay.

Several techniques have been developed to challenge this hard experiment.

• We will achieve the goal of 10-13 sensitivity (>100 SM events) redundantly using the present KEK 12-GeV PS and then the JPARC 50-GeV PS.

• E391a data taking started on 18 Feb.2004 as scheduled. and the Run-1 successfully finished on 30 June. We are asking to double the statistics by Run-2 in 2005.

• The quality of the collected data looks fine.• We are expecting a continuous support and specially asking for

your join.