Limit on the production of a new vector boson in e(+)e(-) -> U gamma, U -> pi(+)pi(-) 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 gamma, U -> pi(+)pi(-) based on 28 million e(+)e(-) -> pi(+)pi(-) gamma events collected at DA Phi NE by the KLOE experiment. The pi(+) pi(-) production by initial-state radiation compensates for a loss of sensitivity of previous KLOE U -> e(+) e(-), mu(+) mu(-) searches due to the small branching ratios in the rho-omega 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, epsilon(2), in the U mass range between 527 and 987 MeV. Above 700 MeV this new limit is more stringent than previous ones. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP(3).