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−e+e− collider, motivated by astrophysical evidence for the presence of dark matter in the Universe. Using e+e−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−γe+e−→μ+μ−γ, corresponding to the decay U→μ+μ−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 1.6×10−51.6×10−5 to 8.6×10−78.6×10−7 for the mass region 520<mU<980 MeV520<mU<980 MeV.