Evidence for an Anomalous Like-Sign Dimuon Charge Asymmetry

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

Figure 1

The difference in the number of events for the $K^{+}{\pi }^{-}$ and $K^{-}{\pi }^{+}$ mass distributions of $K^{*0}$ candidates in the inclusive muon sample. The solid line represents the result of the fit, while the dashed line shows the background contribution.

Figure 2

The observed and expected like-sign dimuon charge asymmetry $A$ as a function of dimuon invariant mass. The expected asymmetry is shown for $A_{\mathrm{sl}}^b=0$ and $A_{\mathrm{sl}}^b=-0.00957$.

Figure 3

Comparison of $A_{\mathrm{sl}}^b$ in data with the SM prediction for $a_{\mathrm{sl}}^d$ and $a_{\mathrm{sl}}^s$. Also shown are other measurements of $a_{\mathrm{sl}}^d=-0.0047\pm 0.0046$ [16, 17, 18] and $a_{\mathrm{sl}}^s=-0.0017\pm 0.0091$ [19]. The bands represent the $\pm 1$ standard deviation uncertainties on each measurement.