Search for light vector boson production in $e^+e^- \rightarrow \mu^+   \mu^- \gamma$ interactions with the KLOE experiment
  We have searched for a light vector boson $U$, the possible carrier of a "dark force", with the KLOE detector at the DA$\Phi$NE \epm\ collider, motivated by astrophysical evidence for the presence of dark matter in the universe. Using \epm\ collisions collected with an integrated luminosity of $239.3$~pb$^{-1}$, we look for a dimuon mass peak in the reaction \epm\to$\mu^+ \mu^-$\gam, corresponding to the decay $U\to\mu^+\mu^-$. We find no evidence for a $U$ vector boson signal. We set a 90\% CL upper limit for the mixing parameter squared between the photon and the $U$ boson of 1.6$\times$10$^{-5}$ to 8.6$\times$10$^{-7}$ for the mass region $520<m_{\rm U}<980$ MeV. 
D. Babusci, I. Balwierz-Pytko, G. Bencivenni, C. Bloise, F. Bossi, P.   Branchini, A. Budano, L. Caldeira Balkest\r{a}hl, F....
hep-ex