A window to new physics from top quark studies?

Precision measurements of top quark behavior could lead to a glimpse of new physics.

The first sign of new physics may not be the direct observation of a new particle, but a glimpse of something new through its effect on known particles. Certain models of new physics would alter the behavior of top quark pair production from what the Standard Model predicts, which opens a window to look for new physics through precision studies of top quarks. Calculations based on the Standard Model predict a slight preference for the top quark to be produced in the direction of the proton beam, while the antitop quark will rather follow the antiproton beam at the Tevatron. Certain models of new physics would dramatically enhance these behaviors. Previous studies by CDF and DZero indicated deviations between the Standard Model predictions and the data, but no definitive evidence for new physics has yet been observed. Two new results from DZero take another look through this window to new physics.

A top quark nearly always decays into a W boson and a bottom quark. The new analyses from DZero focus on top quark pair events where one or two W bosons decay into leptons, including either an electron or a muon in addition to a neutrino. In events with one charged lepton, the analyzers build a special discriminant out of a number of variables that characterize the event to separate top quark pair events from background while simultaneously measuring the top quark direction preference. In events with two charged leptons, two measurements are considered to extract the top quark behavior: the angle from the proton beam to each lepton, adjusted for their charge sign, and the angle between the positive and negatively charged leptons with respect to the proton beam.

After examining the data, the analyzers find that these new measurements of top quark direction preference are consistent with the Standard Model prediction. While this peek through the top quark window does not show us new physics, the precision measurement of top quark behavior is still being performed at the Tevatron and may yet give us a glimpse of something new.

Mike Cooke

These physicists made major contributions to this analysis.
Analyses that involve electrons, like the result above, rely on the crucial efforts of the electron identification group, who optimize and characterize the performance of algorithms that find electrons based on activity in the DZero detector.