|This shows the asymmetry of the leptons from top decays as a function of the lepton rapidity. This is fit to a functional form, allowing the recovery of the inclusive lepton asymmetry.|
For several years, physicists at the Tevatron — both at CDF and DZero — have been studying a mystery in the production of top quarks: Outgoing top quarks prefer to go in the direction of the incoming proton beam (forward), while anti-top quarks prefer the opposite direction (backward). This “forward-backward asymmetry” disagrees with the Standard Model calculations, which suggest it should be much smaller than is actually observed.
A new measurement from CDF aims to shed some additional light on this discrepancy by examining the asymmetry of leptons — electrons or their heavier cousins, muons — that are produced by the decaying top quarks. Since the leptons inherit momentum from their parent tops, they also should inherit the asymmetry. Observation of a lepton asymmetry would confirm the earlier top measurements in a very direct way. In addition, the lepton direction is sensitive to the spin direction of the top quark, and when compared to the top asymmetry, can test theories of the top asymmetry that call for polarized, or spin-aligned, top quarks.
In the practical detection of top quarks, many of the decay leptons escape through the ends of the detector and are never seen. The CDF analysis invents a technique to infer the details of the full complement of leptons from the fraction that are observed.
Using the full sample of 3,864 leptonic top events found in the full Run II Tevatron data set, the asymmetry AlFB(qyl) of the leptons in top pair events is measured to be 9.4+3.2/-2.9 percent. This is larger than the Standard Model prediction of 3.8±0.3 percent but consistent with what would be expected from the observed top forward-backward asymmetry in the case of an unpolarized top quark. This supports the prior observations of top asymmetry and further indicates that models that predict large top polarizations are unlikely to be the correct explanation for the anomalous top asymmetry.
As the catalog of Tevatron top asymmetry measurements moves to completion, the LHC will become the new hunting ground for further information on this puzzling effect.
—edited by Andy Beretvas