Search for a muonic dark force at<i>BaBar</i>

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 ${Z}^{\ensuremath{'}}$ coupling only to the second and third generations of leptons in the reaction ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}{Z}^{\ensuremath{'}},{Z}^{\ensuremath{'}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ using $514\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data collected by the BABAR experiment. No significant signal is observed for ${Z}^{\ensuremath{'}}$ masses in the range 0.212--10 GeV. Limits on the coupling parameter ${g}^{\ensuremath{'}}$ as low as $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ are derived, leading to improvements in the bounds compared to those previously derived from neutrino experiments.