Search for light vector boson production in $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}\gamma$ 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$\mathrm{\Phi }$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$\mu^+ \mu^-$\text{gam},correspondingtothedecay$U\to\mu^+\mu^-$. 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 $520<m_{\mathrm{U}}<980$ MeV.