A top quark cautionary tale

Today’s measurement shows top quarks have a discernible preference for the direction in which they are created.

The top quark, discovered at Fermilab in 1995, is the heaviest elementary particle known to date. By colliding protons and antiprotons to annihilate quarks and antiquarks in the beams, physicists can produce a top-antitop quark pair. Collisions similar to these produce light particles that are carefully studied, and their characteristics agree with the Standard Model. Does the Standard Model also accurately describe in detail collisions that produce top quarks?

On July 25, DZero announced the results of a study that examined the direction in which the top quark is produced. The specific question is whether or not the top quark favors production in the direction of the proton or antiproton beams.

DZero and CDF measured this quantity in the past and found disagreements with the Standard Model, but not enough to claim a discovery. Mostly, the problem is a small data set. In a small data set, things can happen that look odd, but really aren’t, like flipping a coin and getting four heads in a row. In this result, DZero analyzed five times as much data as earlier studies, and the scientists still found disagreements with the Standard Model. This supports the previous measurements and raises the possibility of a discovery.

Now we need to be careful. We compare our data to a calculation that begins with the Standard Model theory. However, the Standard Model equations are very difficult, and nobody knows how to use them to make exact calculations. So we make a series of approximations. The hope is that subsequent approximations add small and, eventually, negligible corrections. This technique is used in all particle physics calculations.

For this measurement, the first approximation predicts there should not be a preferred direction for this scattering, while subsequent approximations of the Standard Model do predict a favored direction. Several levels of approximations were made, however the most recent approximation still noticeably affects the prediction. It is possible that any discrepancies between data and theory reflect limitations in the approximations, rather than in the Standard Model itself.

This measurement is an excellent example of why scientists are cautious with their announcements. It also explains why progress is sometimes slow. Sometimes things aren’t what they appear to be. Still, this very challenging measurement could be an important hint of new physics, and ongoing work will continue to try to sort it all out.

Don Lincoln

These physicists from the University of Rochester performed this very delicate analysis.
Since top quarks decay essentially instantaneously, we detect the presence of their decay products. Top quarks decay 100 percent of the time into bottom quarks, so this measurement requires excellent bottom quark detection capabilities. These scientists head this effort for DZero.