Search for the U-boson in the process $e^ +e^- \to μ^+ μ^- γ, \,\, U \to μ^+μ^-$ 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$Φ$NE. We analyzed $e^+ e^- \to μ^+ μ^- γ$ ISR events corresponding to an integrated luminosity of $239$ pb$^{-1}$ to find evidence for the $e^+ e^- \to Uγ,\,\, U\toμ^+μ^-$ process. We found no $U$ vector boson signal and set a 90% CL upper limit on the ratio of the U boson and photon coupling constants between 1.6$\times10^{-5}$ to 8.6$\times10^{-7}$ in the mass region $520<M_{\rm U}<980$ MeV. A projection of the KLOE sensitivity for the $μμγ$ and $ππγ$ channels at full statistics and extended muon acceptance is also presented.

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