Limit on the production of a new vector boson in e+e− → Uγ, U → π+π− with the KLOE experiment

The recent interest in a light gauge boson in the framework of an extra U(1) symmetry motivates searches in the mass range below 1 GeV. We present a search for such a particle, the dark photon, in e+e−→Uγe+e−→Uγ, U→π+π−U→π+π− based on 28 million e+e−→π+π−γe+e−→π+π−γ events collected at DAΦNE by the KLOE experiment. The π+π+π−π− production by initial-state radiation compensates for a loss of sensitivity of previous KLOE U→e+e−, μ+μ− searches due to the small branching ratios in the ρ–ωρ–ω resonance region. We found no evidence for a signal and set a limit at 90% CL on the mixing strength between the photon and the dark photon, ε2ε2, in the U mass range between 527 and 987MeV. Above 700 MeV this new limit is more stringent than previous ones.