Limit on the production of a light vector gauge boson in ϕ meson decays with the KLOE detector

We present a new limit on the production of a light dark-force mediator with the KLOE detector at DAϕNE. This boson, called U, has been searched for in the decay ϕ →ηU, U →e+e−, analyzing the decay η → π0π0π0 in a data sample of 1.7 fb−1. No structures are observed in the e+e− invariant mass distribution over the background. This search is combined with a previous result obtained from the decay η→π+π−π0, increasing the sensitivity. We set an upper limit at 90% C.L. on the ratio between the U boson coupling constant and the fine structure constant of α′ /α < 1.7 × 10−5 for 30 < MU < 400 MeV and α′ /α ≤ 8 × 10−6 for the sub-region 50 < MU < 210 MeV. This result assumes the Vector Meson Dominance expectations for the ϕηγ ∗ transition form factor. The dependence of this limit on the transition form factor has also been studied.