A High Statistics Study of the Decay

1
A High Statistics Study of the Decay
Outline 1.Introduction 2.Experiment Belle
detector 3.Analysis Event selection Mass
spectrum Muon anomalous magnetic moment
4.Summary
M. Fujikawa for the Belle Collaboration
2
Introduction

• The hadronic vacuum polarization term plays an
  important role in the theoretical calculation of
  the muon anomalous magnetic moment.
• The dominant part of the hadronic vacuum
  polarization term can be calculated from the 2p
  Spectral function measured with ee- or t data.
• Recent data indicate that there is a systematic
  difference between the 2p system in ee- reaction
  and t-decays, which needs to be understood.

CVC

• In this talk, results from Belle experiment are
  presented.
• Results are based on 72.2/fb data taken in
  2000-2002 period.
• One order of magnitude more data than preceding
  experiments.

3
What should be measured

• Branching Fraction
• Mass Spectrum

4
Experiment Apparatus
Belle detector
SC solenoid 1.5T
Aerogel Cherenkov cnt. n1.0151.030
CsI(Tl)16X0
p0 mass resolution sp 0 5 -8MeV
3.5 GeV e
TOF counter
Central Drift Chamber Tracking dE/dx Small
cell He/C2H6
8 GeV e-
Si vtx. det. 4 lyr. DSSD

• / KL detection
• 14/15lyr. RPCFe

Good tracking and particle identification
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Event Selection
ee-?tt- Selection

• Low multiplicity
• Number of charged tracks2 or 4, net charge0
• Beam background rejection Event Vertex Position
• Physics background rejection
• Use Missing Mass and Missing Angle
  information.(Bhabha,2photon)
• Low track and gamma multiplicity. (qq continuum)

charged track
t-?p-p0nt Selection
p0

• one charged track in the event hemisphere.
• one p0 in the event hemisphere.
• veto any additional gammas with

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p0 Signal
Sideband region
Signal region
right
left
Sideband region is used to estimate the non-p0
background
t-?p-p0nt
5.55106 event
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m2pp distribution

• B.G. estimated by MC

• non-t B.G.

Checked above kinematical limit of tau decay

• feed across B.G.

• t-?p-p0nt MC(TAUOLA)
• Includes r(770),r(1400)
• Does not include r(1700)

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Acceptance correction
After background subtraction, data are Unfolded
with the SVD method.
Singular Value Decomposition method
Ref.Nucl.Instr.Math.,A372,469(1996)
Acceptance
(GeV)2
(GeV)2
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Mass spectrum after Unfolding
Pion Form FactorFp2
Mass spectrum ?phase space
? Form Factor
r
Interference between r and r
r
r
Dip at s2.5 GeV2
Agrees with ALEPH data. Our result is more
precise especially in the high mass region.
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Parameterization of resonances
r(770), r(1400), r(1700)
Gounaris-Sakurai(GS) parameterization
Normalization BW1 at s0 GeV
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Fit result

• 1st Errors Statistical
• 2nd Errors Systematics.
• Sources
• 1. Photon energy scale
• 2. Unfolding Procedure
• (DRank /- 3)
• 3. Background Subtraction.

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Stat. error only
Comparison with preceding experiments
strong correlation between Mr and fg
fit parameter Belle ALEPH(t) CMD-2( )
774.60.2 775.50.7 773.30.6
150.60.3 149.01.2 145.21.3
133612 132815 133735
47129 46841 56981
0.0900.009 0.2100.008 0.1230.011
123.75.0 153.07.0 139.46.5
160013 1713(fixed) 171315
25519 235(fixed) 235(fixed)
0.0620.015 0.0230.008 0.0480.008
-64.17.9 0 (fixed) 0 (fixed)
55/51 119/110 --------
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Hadronic vacuum polarization am

• Threshold region(0.25GeV2ltMpp2) is not included
  because the experimental uncertainty is large.

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Internal Systematic Error
source
Background estimation non- t(ee-gthadron) feed-down h2p0n feed-down K-p0n 0.11 0.09 0.15
p0/g selection efficiency/shape cuts 0.35
Energy scale 0.10
Gamma veto 0.93
g/track overlap 0.24
Tagging Dependence lt0.1
Smearing/Migration effect
Total 1.04
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External parameters
Item Value
1.02330.0006 0.32
0.97340.0008 0.42
(17.840.06) 1.82
(25.420.11) 2.30
Total 3.0
World average is calculated combining our new
result and the preceding measurements of other
experiments.
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am(2p) after SU(2) correction

• Isospin breaking correction in this mass region.
• (-1.8 2.3)x10-10

r-w interference effects
in the phase space in the
width
Ref.Phys.Lett.B513,361(2001)

• Consistent with other t data

c.f.
Ref. Eur.Phys.C27,497(2003)
Ref. Eur.Phys.C31,503(2003)
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recent summery of a? status
Direct measurement
BNL(2004)a?exp (11 659 208.0 ? 5.8)10 ?10
Theoretical calculation
(using ee-or t measurement)
Exp.
ee-?pp-
t-? p-p0nt
Ref. hep-ph/0507078(2005)
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Summary

• We measure p-p0mass spectrum with 72.2/fb data
  collected with the Belle detector at KEKB.
• In addition to the r(770) and r(1400) , the
  production of the r(1700) in t decays is
  observed and its parameters are determined.
• We evaluate the 2p contribution to the muon
  anomalous magnetic moment am using mass
  spectrum.
• Our result agrees well with the preceding t-based
  results but is higher than the ee- results.

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Backup slides
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2nd iteration

• r,r resonance parameter obtained in the fit are
  used input signal MC generation in the TAUOLA
  program.

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m2pp distribution
Original
2nd iteration
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Measurement of Branching Fraction

• ?Many systematics cancel in the ratio.

Method From the ratio
72.2/fb
Data
No. of tau-pairs 22.71x106 events No. pp0
5.55x105 events

Use
for p0 efficiency calibration
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Br Systematics
Source
1) Tracking efficiency 2) p0 efficiency 3)Background in t-pair 4)Feed-down background 0.12 0.25 0.09 0.04
5)Non-t background 6) Gamma veto 0.05 0.05
7) Trigger 0.08
8)MC Statistics 0.04
Total 0.31
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Br hpi0 result
Preceding Exp.

• Consistent with preceding results.
• The systematics is the same level as CLEO/OPAL
• World average is used for am evaluation.

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Br pipi0 result
Subtract kaon branching fraction Br(Kpi0)(0.450.
03)
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Mass spectrum after Unfolding
Mass spectra Phase space
Form Factor
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Pion Form FactorFp2
r
Interference between r and r
r
r
Dip at s2.5 GeV2
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Pion Form FactorFp2
r
Interference between r and r
r
r
Dip at s2.5 GeV2
Agrees with ALEPH data. But our results are more
precise in particular in high mass region.
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Pion Form FactorFp2
Comparison with CLEO.
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Comparison of t data

• (Data-Fitted curve)/Fit

• This fitted curve is the fit of the Belle 2p data
  to the GS model.

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Pion Form FactorFp2-r mass region-
ALEPH
CLEO
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Experiment apparatus
KEKB Collider
Belle detector
KEKB

• KEKB Collider
• High Luminosity
• Asymmetric energy collider
• 8GeV e- 3.5GeVe
• ?s 10.58GeV (U(4S))
• ee-?U(4S)?B?B
• Integrated Luminosity 550 fb-1
• 30fb-1 gt off-resonance

Lgt1.6x1034cm-2s-1 !!