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.
Categories: hep-ex