Top quark continues to pique physicists’ interest

The asymmetry of the leptons from top decays as a function of the lepton rapidity from the CDF data. The fit (red curve) is in good agreement with the data and provides for a measurement of the inclusive asymmetry.

Since the discovery of the top quark in 1995, physicists at the Tevatron have been probing the properties of this mysterious elementary particle to see whether it behaves the way we expect it to. One of these properties is called forward-backward asymmetry.

The Tevatron collided protons with antiprotons to produce other particles, including top quark pairs. Forward-backward asymmetry refers to the preference of top quarks to follow the proton direction, forward, and antitops to follow the opposite direction, backward. The asymmetry is the difference between the fraction of top quarks going forward and the fraction of them going backward: the larger its value is, the larger the positive asymmetry.

Simple theoretical estimates of this asymmetry turned out to be small, but experiment shows otherwise. Larger-than-expected forward-backward asymmetry measurements made at the Tevatron have triggered substantially better theoretical Standard Model predictions and new physics models. These results have pushed scientists for better measurements and a better understanding of top quark physics.

A powerful independent piece of evidence to help determine whether the observations are in tension with the Standard Model predictions comes from measurements of just the leptons arising from top quark decays. Physicists expect the leptons (electrons or muons) from the top quark decay to follow the directions of their parents. Thus these leptons should inherit the forward-backward asymmetry properties of their respective quarks or antiquarks.

CDF has just completed a new measurement of the lepton asymmetry using the sample of the full Tevatron Run II data with two charged leptons from top pair decays. The above figure uses a hyperbolic tangent function to recover the total lepton asymmetry both in the detector and in the undetected region. The resultant asymmetry in the two-lepton mode is 7.2 ± 6.0 percent.

After combining this with the previous measurement, which used data with only one charged lepton from top pair decays, the final CDF measurement of this asymmetry comes to 9.0 +2.8/-2.6 percent (see lower figure). While there are several competing theoretical predictions, most currently predict an asymmetry of about 4 percent, so this result is in moderate disagreement with predictions.

This new result continues to pique the scientific community’s interest in top quark production. The effort at CDF continues, working on the asymmetry of the top quark pairs in the two-lepton mode. Measurements of the asymmetry of the bottom quark pairs that probe the same physics question are also on the way. It may take a long time before we know if the observed asymmetry is consistent with the Standard Model.

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edited by Andy Beretvas

The figure shows a comparison of forward-backward asymmetry measured in this experiment (DIL, dilepton mode), in an earlier CDF measurement in one-lepton mode (L+J, lepton + jets), and in their combination.
These CDF physicists contributed to this data analysis. From left: Ziqing Hong, Jason Nett and Dave Toback, all from Texas A&M University.