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 - U, U + -based on 28 million e + e - + - events collected at DA NE by the KLOE experiment. The + -production by initial-state radiation compensates for a loss of sensitivity of previous KLOE U e + e -, + -searches due to the small branching ratios in the - 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