Forward-backward asymmetries in the production of quark-antiquark pairs, especially bottom and top quark pairs, are now of great interest to the particle physics community.
At the Tevatron, protons and antiprotons collided with each other, and the resulting outgoing particles would go roughly in the direction of the incoming proton (forward) or of the incoming antiproton (backward). Forward-backward asymmetry refers to a preference of one direction over the other. We express it as a difference between the number of quarks and antiquarks produced in the forward direction.
The phenomenon is currently of interest mainly for two reasons. First, in the Standard Model, the asymmetry comes from higher-order corrections, and the present theory describing forward-backward asymmetry is now able to calculate them. Second, different non-Standard Model processes can influence the size of the asymmetry. For example, the existence of an axigluon — a massive hypothetical partner of the carrier of the strong force — could increase the asymmetry. Hence, the asymmetry is a good place to search for new physics.
Study of the top quark production asymmetry led to measurements that showed a sizable discrepancy between the experimental results and Standard Model expectations. The recent Standard Model estimates of the asymmetry based on next-to-next-to-leading-order calculations describing the strong force have reduced the discrepancy.
The asymmetry in bottom quark production can provide additional constraints on non-Standard Model physics scenarios. Using high-mass bottom quark pairs, CDF found no evidence for non-Standard Model physics and was able to exclude the non-Standard Model production of axigluons with a mass of 200 GeV/c2.
The latest CDF measurement is performed in events with two jets with low invariant mass. Both the jets are produced by the strong force, where one originates from a bottom quark and the other from an antibottom quark. To distinguish between b jets from bottom or antibottom quarks, we use the charge of the muon from the decay of a b quark. A negative muon indicates a b quark; a positive muon indicates an anti-b quark.
The asymmetry is then measured in four intervals of the reconstructed b quark pair mass (see figure). To compare this with the Standard Model prediction, we corrected for effects due to the CDF detector acceptance and resolution. Using the full data set (6.9 inverse femtobarns), we observed a tendency for the asymmetry to increase with the reconstructed b quark pair mass.
As the b quark pairs can be also produced in the decay of a Z boson — one of the particles that mediates weak interactions — the Standard Model predicts a local increase of the asymmetry around Z boson mass of 91 GeV/c2.
The data are consistent with the Standard Model expectations, and they disfavor the axigluon model with a mass of 100 GeV/c2.