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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$\Phi$NEby the KLOE experiment. The $\pi^+ \pi^-$ production by initial-state radiationcompensates for a loss of sensitivity of previous KLOE ${\rm U} \rightarrow\mathrm{e^+ e^-}$, $\mu^+\mu^-$ searches due to the small branching ratios inthe $\rho-\omega$ resonance region. We found no evidence for a signal and set alimit 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 MeVthis new limit is more stringent than previous ones.

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