  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- -> mu+mu- Z', Z' -> mu+mu- 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 7x10^-4 are derived,
leading to improvements in the bounds compared to those previously derived from
neutrino experiments.
