Limit on the production of a new vector boson in $mathrm{e^+ e^-}rightarrow {rm U}gamma$, U$rightarrow pi^+pi^-$ 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$rightarrow pi^+pi^-$ based on 28 million $mathrm{e^+ e^-} rightarrow pi^+ pi^-gamma$ events collected at DA$Phi$NE by the KLOE experiment. The $pi^+ pi^-$ production by initial-state radiation compensates for a loss of sensitivity of previous KLOE ${rm U} rightarrow mathrm{e^+ e^-}$, $mu^+mu^-$ 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, $varepsilon^2$, in the U mass range between $527$ and $987$~MeV. Above 700 MeV this new limit is more stringent than previous ones.