1. Precision Measurement of η Radiative Decay Width via Primakoff Effect
Liping Gan
University of North Carolina Wilmington, USA
Outline
Physics Motivation
Symmetry of QCD in the chiral limit
Properties of π0, η and η’
Primakoff experimental program at Jlab
An approved experiment on measurement

2. 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:
Large global symmetry group:

3. Fate of QCD symmetries

4. Lightest pseudoscalar mesons
Chiral SUL(3)XSUR(3) spontaneously broken Goldstone mesons π0, η8
Chiral anomalies Mass of η 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.

5. Primakoff Program at Jlab 6&12 GeV
Precision measurements of electromagnetic properties of 0, ,  via Primakoff effect.
Two-Photon Decay Widths:
6 GeV
Γ(→)
Γ(’→)
Transition Form Factors at low
Q2 ( GeV2/c2): F(*→ 0), F(* →), F(* →)
Input to Physics:
precision tests of Chiral
symmetry and anomalies
determination of light quark
mass ratio
-’ mixing angle
Input to Physics:
0, and ’ electromagnetic
interaction radii
is the ’ an approximate
Goldstone boson?

6. Physics Outcome from New  Experiment
Resolve long standing discrepancy between collider and Primakoff measurements, and improve all  partial decay widths in PDG.
Determine Light quark mass ratio:
Γ(→3π) ∝ |A|2 ∝ Q-4 Q
Extract -’mixing angle:
H. Leutwyler Phys. Lett., B378, 313 (1996)

7. → 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
Error for individual experiments:
7.6% to 25%
→
PDG average for collider
experiments:
Γ(η) = ± keV ( ± 5.1%)
Major limitations of method
unknown q2 for **
knowledge of luminosity

8. Primakoff Method η Requirement:
ρ,ω
Challenge: Extract the Primakoff amplitude
Features of Primakoff cross section:
Beam energy sensitive
Peaked at very small forward angle
Coherent process
Requirement:
Photon flux
Beam energy
η production Angular resolution
Coherency of reaction

9. Challenges in the → Primakoff experimentη
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;
larger momentum transfer (coherency, form factors, FSI,…)

10. Cornell Primakoff Experiment
Cornell (PRL, 1974)
untagged bremsstrahlung  beam,
E=5.8, 9.0, 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.

11. 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) 11

12. Advantages of the Proposed Light Targets
Precision measurements require low A targets to control:
coherency
contributions from nuclear processes
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: , (2010)
4He:
higher Primakoff cross section:
the most compact nucleus
form factor well known
new theoretical developments for FSI
S. Gevorkyan et al., Phys. Rev. C 80, (2009) 12

13. 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

14. 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
Beam energy
0.2%
Detection efficiency
Branching ratio (PDG)
0.66%
Total Systematic
3.02%
Total uncertainty (added quadratically):
Statistical
1.0%
Systematic
3.02%
Total
3.2%

15. 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 GeV.
Physics outcome of experiment:
Quark mass ratio
Mixing angle of η―η׳
Test chiral anomaly and QCD symmetries