Evidence for an anomalous like-sign dimuon charge asymmetry

We present a measurement of the like-sign dimuon asymmetry in semileptonic $b$-hadron decays, performed using 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. This measured value is $A^b_{\rm sl} = [-0.957\pm0.251\thinspace({\rm stat})\pm0.146\thinspace({\rm syst})]$ %, which disagrees with the Standard Model prediction at a statistical level of 3.2 $\sigma$, and provides the first evidence of anomalous CP violation in the mixing of neutral $B$ mesons.