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

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:

Fate of QCD symmetries

Lightest pseudoscalar mesons Chiral SUL(3)XSUR(3) spontaneously broken Goldstone mesons π0, 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.

Primakoff Program at Jlab 6&12 GeV Precision measurements of electromagnetic properties of 0, ,  via Primakoff effect. Two-Photon Decay Widths: Γ(0→) @ 6 GeV Γ(→) Γ(’→) Transition Form Factors at low Q2 (0.001-0.5 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?

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)

→ 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) = 0.510 ± 0.026 keV ( ± 5.1%) Major limitations of method unknown q2 for ** knowledge of luminosity

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

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; larger momentum transfer (coherency, form factors, FSI,…)

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.

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

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:1001.0646, (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

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

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%

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