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We present a new limit on the production of a light dark-force mediator withthe KLOE detector at DAPHNE. This boson, called U, has been searched for in thedecay phi --> eta U, U --> e+ e-, analyzing the decay eta --> pi0 pi0 pi0 in adata sample of 1.7 fb-1. No structures are observed in the e+e- invariant massdistribution over the background. This search is combined with a previousresult obtained from the decay eta --> pi+ pi- pi0, increasing the sensitivity.We set an upper limit at 90% C.L. on the ratio between the U boson couplingconstant and the fine structure constant of alpha'/alpha < 1.7x10^-5 for30<M_U<400 MeV and alpha'/alpha < 8x10^-6 for the sub-region 50<M_U<210 MeV.This result assumes the Vector Meson Dominance expectations for thephietagamma^* transition form factor. The dependence of this limit on thetransition form factor has also been studied.

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