Search for CP and $P$ violating pseudoscalar decays into $\pi\pi$
  Using a sample of $(225.2\pm 2.8)\times 10^6$ $J/\psi$ events collected with the Beijing Spectrometer (BESIII) at the Beijing Electron-Positron Collider, CP and $P$ violating decays of $\eta$, $\eta'$ and $\etac$ into $\pi^+\pi^-$ and $\pi^0 \pi^0$ are searched for in $\jpsi$ radiative decays. No significant $\eta$, $\eta'$ or $\etac$ signal is observed, and 90% confidence level upper limits of $\BR(\eta \to \pi^+\pi^-)<3.9\times 10^{-4}$, $\BR(\eta' \to \pi^+\pi^-)<5.5\times 10^{-5}$, $\BR(\etac \to \pi^+\pi^-)<1.3\times 10^{-4}$, $\BR(\eta \to \pi^0 \pi^0)<6.9\times 10^{-4}$, $\BR(\eta' \to \pi^0 \pi^0)<4.5\times 10^{-4}$ and $\BR(\etac \to \pi^0 \pi^0)<4.2\times 10^{-5}$ are obtained. 
The BESIII Collaboration, M. Ablikim, et al
hep-ex