Background check for anomalous like-sign dimuon charge asymmetry

The D0 Collaboration has reported an excess of roughly one percent of $\mu^- \mu^-$ pairs over $\mu^+ \mu^+$ pairs in $\bar p p$ collisions at a center-of-mass energy $\sqrt{s} = 1.96$ GeV at the Fermilab Tevatron, when known backgrounds are subtracted. This excess, if ascribed to CP violation in meson-antimeson mixing of non-strange or strange neutral $B$ mesons, is about 40 times that expected in the Standard Model (SM). We propose a null test, based on a tight restriction on the muon impact parameter $b$, to confirm that this excess is indeed due to $B$ mesons. If the asymmetry is due to anomalous CP violation in $B_s$-$\bar B_s$ mixing then a tight restriction on $b$ would increase by a factor two the net asymmetry from neutral $B$ mixing, while the sample of dimuons from neutral $B$ decays will be reduced significantly relative to background events.