Search for a dark vector gauge boson decaying to ${\pi }^{+}{\pi }^{-}$ using $\eta \to {\pi }^{+}{\pi }^{-}\gamma$ decays

In this report, we search for a dark vector gauge boson $U^{\text{'}}$ that couples to quarks in the decay chain $D^{*+}\to D^0{\pi }^{+},D^0\to K_S^0\eta ,\eta \to U^{\text{'}}\gamma$, $U^{\text{'}}\to {\pi }^{+}{\pi }^{-}$. No signal is discovered and we set a mass-dependent limit on the baryonic fine structure constant of ${10}^{-3}-{10}^{-2}$ in the $U^{\text{'}}$ mass range of 290 to $520\mathrm{MeV}/c^2$. This analysis is based on a data sample of $976{\mathrm{fb}}^{-1}$ collected by the Belle experiment at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.