We report searches for a new muonic dark force mediated by a gauge boson (Z 0 ) coupling only to
the second and third lepton families. The existence of the Z 0 boson is probed in e + e − → μ + μ − Z 0 ,
Z 0 → μ + μ − events based on the full data sample collected with the BABAR detector at the PEP-
II e + e − collider. No significant signal is observed. Limits on dark-sector coupling constants are
derived; these improve upon current bounds, and further constrain the allowed parameter space.