Limit on the production of a new vector boson in ${\mathrm{e}}^{+}{\mathrm{e}}^{-}\to \mathrm{U}\gamma$, U$\to {\pi }^{+}{\pi }^{-}$ with the KLOE experiment

The recent interest in a light gauge boson in the framework of an extra U(1) symmetry motivates searches in the mass range below 1 GeV. We present a search for such a particle, the dark photon, in ${\rm e^+ e^-}\rightarrow {\rm U}\gamma$,U$\rightarrow \pi^+\pi^-$basedon28million$${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ \rightarrow \pi^+ \pi^-\gamma$eventscollectedatDA$\Phi$NE by the KLOE experiment. The ${\pi }^{+}{\pi }^{-}$ production by initial-state radiation compensates for a loss of sensitivity of previous KLOE ${\rm U} \rightarrow ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$$,$\mu^+\mu^-$ searches due to the small branching ratios in the $\rho -\omega$ resonance region. We found no evidence for a signal and set a limit at 90% CL on the mixing strength between the photon and the dark photon, ${\epsilon }^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV this new limit is more stringent than previous ones.