Evidence for an Anomalous Like-Sign Dimuon Charge Asymmetry

We measure the charge asymmetry $A\ensuremath{\equiv}({N}^{++}\ensuremath{-}{N}^{--})/({N}^{++}+{N}^{--})$ of like-sign dimuon events in $6.1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of $p\overline{p}$ collisions recorded with the D0 detector at a center-of-mass energy $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$ at the Fermilab Tevatron collider. From $A$ we extract the like-sign dimuon charge asymmetry in semileptonic $b$-hadron decays: ${A}_{\mathrm{sl}}^{b}=\ensuremath{-}0.009\text{ }57\ifmmode\pm\else\textpm\fi{}0.002\text{ }51(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.001\text{ }46(\mathrm{sys})$. It differs by 3.2 standard deviations from the standard model prediction ${A}_{\mathrm{sl}}^{b}(\mathrm{SM})=(\ensuremath{-}{2.3}_{\ensuremath{-}0.6}^{+0.5})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, and provides first evidence of anomalous $CP$ violation in the mixing of neutral $B$ mesons.