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We present a search for a new light vector boson, carrier of a "dark force"between WIMPs, with the KLOE detector at \DA$\Phi$NE. We analyzed $e^+ e^- \to\mu^+ \mu^- \gamma$ ISR events corresponding to an integrated luminosity of$239$ pb$^{-1}$ to find evidence for the $e^+ e^- \to U\gamma ,\,\,U\to\mu^+\mu^-$ process. We found no $U$ vector boson signal and set a 90% CLupper limit on the ratio of the U boson and photon coupling constants between1.6$\times10^{-5}$ to 8.6$\times10^{-7}$ in the mass region $520<M_{\rm U}<980$MeV. A projection of the KLOE sensitivity for the $\mu\mu\gamma$ and$\pi\pi\gamma$ channels at full statistics and extended muon acceptance is alsopresented.

Search for the U-boson in the process $e^ +e^- \to \mu^+ \mu^- \gamma, \,\, U \to \mu^+\mu^-$ with the KLOE detector