Search for light vector boson production in e+e-→μ+μ-γ 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 DAΦNE e+e- collider, motivated by astrophysical evidence for the presence of dark matter in the Universe. Using e+e- collisions collected with an integrated luminosity of 239.3 pb-1, we look for a dimuon mass peak in the reaction e+e-→μ+μ-γ, corresponding to the decay U→μ+μ-. 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 1.6×10-5 to 8.6×10-7 for the mass region 520