Search for a muonic dark force at BABAR

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.