Background check for anomalous like-sign dimuon charge asymmetry

TL;DR

This paper proposes a null test using muon impact parameter restrictions to verify if the observed dimuon charge asymmetry at Fermilab is due to CP violation in B meson mixing, aiming to distinguish signal from background.

Contribution

It introduces a new method based on muon impact parameter cuts to confirm the origin of the dimuon asymmetry as B meson CP violation.

Findings

A tight impact parameter cut can double the observed asymmetry if due to B_s mixing.

The method significantly reduces background events, enhancing the signal's clarity.

Provides a practical test to validate the CP violation hypothesis in meson mixing.

Abstract

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.