稀崩壊 k l の実験・ e391a 内容 物理の背景・動機...
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稀崩壊 KL の実験・ E391a
内容• 物理の背景・動機• 測定の難しさ・我々のやりかた• 測定器の建設• 解析の現状
稲垣隆雄 ( KEK)
2004年7月23日 理研E391a: http://www-ps.kek.jp/e391/
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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
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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
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Comparison between K- and B- system for the determination of Unitary Triangle
Test of the Standard Model
( A. Buras hep-ph/9905437 )
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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
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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)
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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)
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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
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Step-by-step approach
E391a(O(10-10))
↓ J-PARC(O(10-
13))
~
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E391a apparatus
100-ton calorimeters are installed in vacuum chamber
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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
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Upstream: Fy02Downstream: Fy01
Middle:Fy03
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Detector Integration
Jan 22, 2004
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Calibration in situ
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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
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Clearly reconstructed KL decay modes
• KL • KL
6- invariant mass (GeV/c2) 4- invariant mass (GeV/c2)
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Z-vertex distribution for KL
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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
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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.
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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.
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A-line plan at JPARC
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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.