Precision Measurement of the η →γγ Decay Width via the Primakoff Effect*

The η →γγ decay offers a sensitive probe for understanding QCD symmetries and the origin and the dynamics of QCD symmetry breaking. A precision measurement of the Γ(η →γγ ) width via the Primakoff effect  (the PrimEx-eta experiment) is currently on-going in Hall D at Jefferson Lab. Two data sets were collected in 2019 and 2021, and the third run will be expected in 2022. This decay width will be extracted from measured differential cross sections at forward angles on light nuclear targets, using a tagged photon beam with energies up to 11.7 GeV. Results of this experiment will not only potentially resolve a long-standing discrepancy between the previous Primakoff and the collider measurements, but will also reduce the experimental uncertainty by a factor of two on the average value of existing results listed in the Particle Data Group. It will directly improve all other η partial decay widths which rely on the accuracy of the η radiative decay width. The projected 3.2% precision on Γ(η →γγ ) for PrimEx-eta will have a significant impact on the determination of the fundamental QCD parameters, such as the light-quark mass ratio and the η-η mixing angle; and will offer a critical input for theoretical calculation of hadronic light-by-light contribution in the muonic g-2.  The experimental status and a preliminary result of the η yield reconstructed from the two-photon final state will be presented.