Transcript Precision Measurement of η Radiative Decay Width via Primakoff Effect Liping Gan University of North Carolina Wilmington, USA Outline 1.Physics Motivation • • Symmetry of QCD in the.

Precision Measurement of η Radiative Decay
Width via Primakoff Effect
Liping Gan
University of North Carolina Wilmington, USA
Outline
1.Physics Motivation
•
•
Symmetry of QCD in the chiral limit
Properties of π0, η and η’
2. Primakoff experimental program at Jlab
3.An approved experiment on
    measurement

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Symmetries of QCD in the chiral limit
chiral limit: is the limit of vanishing quark masses
mq→ 0.
QCD Lagrangian with quark masses set to zero:
(o)
QCD
L
1  
 qL  iD qL  qR  iD q R  G G
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D     g s  / 2G
u 
 
q  d 
s 
 
qR , L
1
 (1   5 )q
2
Large global symmetry group:
SUL (3)  SUR (3) U A (1) U B (1)
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Fate of QCD symmetries
mq =0
mq  0
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Lightest pseudoscalar mesons
• Chiral SUL(3)XSUR(3)
spontaneously broken
Goldstone mesons
0
π , η8
•
Chiral anomalies
Mass of η0
P→γγ
( P: π0, η, η‫)׳‬
• Quark flavor SU(3)
breaking
The mixing of π0,
η and η‫׳‬
The π0, η and η’ system provides a rich laboratory to
study the symmetry structure of QCD at low energy.
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Primakoff Program at Jlab 6&12 GeV
Precision measurements of
electromagnetic properties
of 0, ,  via Primakoff
effect.
a) Two-Photon Decay Widths:
1)
2)
3)
Γ(0→) @ 6 GeV
Γ(→)
Γ(’→)
Input to Physics:
 precision tests of Chiral
symmetry and anomalies
 determination of light quark
mass ratio
 -’ mixing angle
b)
Transition Form Factors at low
Q2 (0.001-0.5 GeV2/c2):
F(*→ 0), F(* →), F(* →)
Input to Physics:
 0, and ’ electromagnetic
interaction radii
 is the ’ an approximate
Goldstone boson?
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Physics Outcome from New  Experiment
1. Resolve long standing discrepancy
between collider and Primakoff
measurements, and improve all 
partial decay widths in PDG.
Determine Light quark mass ratio:
3.
2
2
m

m
Q 2  s2
,
md  mu2
1
where mˆ  (mu  md )
2
Q
Γ(→3π) ∝ |A|2 ∝ Q-4
2.
Extract -’mixing angle:
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H. Leutwyler Phys. Lett., B378, 313 (1996)
→ Decay Width Experiments
(e+e- Collider Results)
 e+e-  e+e-** e+e-   e+e- 
 e+, e- scattered at small angles
(not detected)
 Only  detected
 PDG average for collider
experiments:
Γ(η) = 0.510 ± 0.026 keV ( ± 5.1%)
→
 Error for individual experiments:
7.6% to 25%
Major limitations of method
 unknown q2 for **
 knowledge of luminosity
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Primakoff Method
η
Primakoff
ρ,ω
d Pr
8 Z 2  3 E 4
2
2
 
F
(
Q
)
sin

e.m.
3
4
d
m Q
Challenge: Extract the Primakoff
amplitude
Features of Primakoff cross
section:
•Beam energy sensitive
Requirement:
Photon flux
Beam energy
η production Angular resolution
Coherency of reaction
•Peaked at very small forward angle
•Coherent process
 Pr
peak
m2

2E 2
 d Pr 
 E4


 d   peak
 d
Pr
 Z 2 log( E )
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Challenges in the → Primakoff experiment
η
Compared to 0:
  mass is a factor of 4 larger than 0 and has a smaller cross section
 larger overlap between Primakoff and hadronic processes;
 Pr
peak
m2

2E 2
 NC 
2
E  A1/ 3
 larger momentum transfer (coherency, form factors, FSI,…)
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Cornell Primakoff Experiment
 Cornell (PRL, 1974)
untagged bremsstrahlung  beam,
E=5.8, 9.0, 11.45 GeV
targets: Be, Al, Cu, Ag, U
conventional Pb-glass calorimeter
 Result: (η)=(0.3240.046) keV (14.2%)
 As a result:
 insufficient resolutions in
experimental parameters;
 hard to resolve Primakoff
from hadronic contributions;
 relatively large and
uncontrolled accidental
background over the nuclear
incoherent background.
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Measurement of Γ(→) in Hall D at 12 GeV
CompCal
FCAL
Incoherent tagged photon beam
 Pair spectrometer and a TAC detector for the photon flux control
 30 cm liquid Hydrogen and 4He targets (~3.6% r.l.)
 Forward Calorimeter (FCAL) for → decay photons
 CompCal and FCAL to measure well-known Compton scattering for control
of overall systematic uncertainties.
Solenoid detectors and forward tracking detectors (for background rejection)
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Advantages of the Proposed Light Targets
 Precision measurements require low A targets to control:
 coherency
m2
 contributions from nuclear processes
 Pr
peak

2E 2
 NC 
2
E  A1/ 3
Hydrogen:
 no inelastic hadronic contribution
 no nuclear final state interactions
 proton form factor is well known
 better separation between Primakoff
and nuclear processes
 new theoretical developments of Regge
description of hadronic processes
J.M. Laget, Phys. Rev. C72, (2005)
A. Sibirtsev, et al. arXiv:1001.0646, (2010)
4He:
 higher Primakoff cross section:  Pr  Z2
 the most compact nucleus
 form factor well known
 new theoretical developments for FSI
S. Gevorkyan et al., Phys. Rev. C 80, (2009)
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Approved Beam Time
Days
Setup calibration, checkout
2
Tagger efficiency, TAC runs
1
4He
target run
30
LH2 target run
40
Empty target run
6
Total
79
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Estimated Error Budget
 Systematical uncertainties (added quadratically):
Contributions
Estimated Error
Photon flux
1.0%
Target thickness
0.5%
Background subtraction
2.0%
Event selection
1.7%
Acceptance, misalignment
0.5%
Beam energy
0.2%
Detection efficiency
0.5%
Branching ratio (PDG)
0.66%
Total Systematic
3.02%
 Total uncertainty (added quadratically):
Statistical
1.0%
Systematic
3.02%
Total
3.2%
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Summary
 A comprehensive Primakoff program has been developed
at Jlab to test fundamental QCD symmetries at low
energy.
 A new Primakoff experiment on      with a 3%
precision has been preparing to run in Hall D at Jlab
12 GeV.
 Physics outcome of      experiment:
 Quark mass ratio
 Mixing angle of η―η‫׳‬
 Test chiral anomaly and QCD symmetries
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