Search for a vector gauge boson in phi meson decays with the KLOE detector

"The existence of a light dark force mediator has been tested with the KLOE detector at DA��NE. This particle, called U, is searched for using the decay chain φ → ηU, η → π+π−π0, U → e+e−. No evidence is found in 1.5 fb−1 of data. The resulting exclusion plot covers the mass range 5 < MU < 470 MeV, setting an upper limit on the ratio between the U boson coupling constant and the fine structure constant, α′\/α, of ��2×10−5 at 90% C.L. for 50<MU <420 MeV."