Search for a dark vector gauge boson decaying to $\pi^+ \pi^-$ using   $\eta \rightarrow \pi^+\pi^- \gamma$ decays
  We report a search for a dark vector gauge boson $U^\prime$ that couples to quarks in the decay chain $D^{*+} \to D^0 \pi^+, D^0 \to K^0_S \eta, \eta \to U^\prime \gamma$, $U^\prime \to \pi^+ \pi^-$. No signal is found and we set a mass-dependent limit on the baryonic fine structure constant of $10^{-3} - 10^{-2}$ in the $U^\prime$ mass range of 290 to 520 MeV/$c^2$. This analysis is based on a data sample of 976 fb$^{-1}$ collected by the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider. 
E. Won, I. Adachi, H. Aihara, S. Al Said, D. M. Asner, T. Aushev, R.   Ayad, I. Badhrees, A. M. Bakich, V. Bansal, E. Barber...
hep-ex