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.