The DØ collaboration at Fermilab has reported evidence of a violation of matter–antimatter symmetry (“CP symmetry”) in the behaviour of neutral mesons containing b quarks. Studying collisions where B mesons decay semi-leptonically into muons, the team finds about 1% more collisions when two negatively charged muons are produced than collisions with two positively charged muons. The collisions in the DØ detector occur through a symmetric proton–antiproton state and the expected CP asymmetry from the Standard Model is predicted to be much smaller than what has been observed. An asymmetry of 1% is therefore completely unexpected.
The properties of B mesons, created in collisions where a bb quark pair is produced, are assumed to be responsible for this asymmetry. Mesons containing b quarks are known to oscillate between their particle (B=bd or bs) and anti-particle (B=bd or bs) state before they decay into a positively charged muon (for the B) or a negatively charged muon (for the B). If a B meson oscillates before its decay, its decay muon has the “wrong sign”, i.e. its charge is identical to the charge of the muon from the other b decay. Having 1% more negatively charged muon pairs therefore implies that the B meson decays slightly more often into its matter state than into its antimatter state.
The DØ detector has two magnets, a central solenoid and a muon-system toroid, which determine the curvature and charge of muons. By regularly reversing the polarities of these magnets the collaboration can eliminate most effects coming from asymmetries in the detection of positively and negatively charged muons. This feature is crucial for reducing systematic effects in this measurement.
Another known source of asymmetry arises in muons produced in the decays of charged kaons. Kaons contain strange quarks and the interaction cross-sections of positively and negatively charged kaons with the matter making up the DØ detector differ significantly: more interactions are open to the K–, which contains strange quarks, than to the K+, which contain strange antiquarks. In detailed studies the collaboration has derived the contribution of this effect almost entirely from the data, making the measurement of the asymmetry in B-meson decays largely independent of external assumptions and simulation.
The final result is 3.2 σ from the Standard Model expectation, corresponding to a probability of less than 0.1 per cent that this measurement is consistent with any known effect. The analysis was based on an integrated luminosity of 6.1 fb–1 and the plan is to increase the accuracy of this measurement by adding significantly more data and improving future analysis methods.