Background check for anomalous like-sign dimuon charge asymmetry

The D0 Collaboration has reported an excess of roughly 1% of {mu}{sup -{mu}-} pairs over {mu}{sup +{mu}+} pairs in pp collisions at a center-of-mass energy {radical}(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 nonstrange or strange neutral B mesons, is about 40 times that expected in the standard model. 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{sub s}-B{sub s} mixing, then a tight restriction on b would increase by a factor 2 the net asymmetry from neutral B mixing, while the sample of dimuons from neutral B decays will be reduced significantly relative to background events.