$\eta\gamma Z$ anomaly from the $\eta-->\gamma\mu+\mu-$ decay
  We show that the $\eta\gamma Z$ anomaly can be measured by analysing parity-violating effects in the $\eta-->\gamma\mu+\mu-$ decay. In this sense, we find that the longitudinal polarization of the outgoing $\mu^+$ is an appropriate observable to be considered in future high-statistics $\eta$ factories. The effect is expected to lie in the range $10^{-5}-10^{-6}$ in the Standard Model. 
J. Bernabeu, D. Gomez Dumm, J. Vidal