Many models of physics beyond the standard model predict the existence of new Abelian forces with new gauge bosons mediating interactions between "dark sectors" and the standard model. We report a search for a dark boson Z′ coupling only to the second and third generations of leptons in the reaction e+e-→μ+μ-Z′,Z′→μ+μ- using 514 fb-1 of data collected by the BABAR experiment. No significant signal is observed for Z′ masses in the range 0.212-10 GeV. Limits on the coupling parameter g′ as low as 7×10-4 are derived, leading to improvements in the bounds compared to those previously derived from neutrino experiments.
