Evidence for an anomalous like-sign dimuon charge asymmetry

We measure the charge asymmetry $A$ of like-sign dimuon events in 6.1 fb${}^{-1}$ of $p\bar{p}$ collisions recorded with the D0 detector at a center-of-mass energy $\sqrt{s}=1.96$ TeV at the Fermilab Tevatron collider. From $A$, we extract the like-sign dimuon charge asymmetry in semileptonic $b$-hadron decays: $\aslb = -0.00957 \pm 0.00251 ({\rm stat}) \pm 0.00146 ({\rm syst})$. This result differs by 3.2 standard deviations from the standard model prediction $\text{aslb}(SM)=(-2.3_{-0.6}^{+0.5})\times {10}^{-4}$ and provides first evidence of anomalous CP-violation in the mixing of neutral $B$ mesons.