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

We present a search for a new light vector boson, carrier of a “dark force” between WIMPs, with the KLOE detector at \DA$\mathrm{\Phi }$NE. We analyzed $e^+ e^- \to \mu^+ \mu^- \gamma$ISReventscorrespondingtoanintegratedluminosityof$239$pb$^{-1}$tofindevidenceforthe$e^+ e^- \to U\gamma ,,, U\to\mu^+\mu^-$process\mathrm{.}Wefoundno$U$ vector boson signal and set a 90% CL upper limit on the ratio of the U boson and photon coupling constants between 1.6$\times {10}^{-5}$ to 8.6$\times {10}^{-7}$ in the mass region $520<M_{\mathrm{U}}<980$ 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 also presented.