  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.
