Through a glass uncertainly

Top quarks or the particles involved in their creation can be indirectly studied through symmetries — or lack thereof.

Experimental physicists spend their lives thinking about uncertainty. Although the subject of their experiments may be heady stuff like the Higgs field, dark matter and gravity waves from the beginning of time, they spend most of their time wondering, “How can I be sure my instrument isn’t lying to me?” Combating potential sources of error is easily 90 percent of the work on a typical analysis.

There are many techniques for dealing with uncertainties, and one of them is to exploit symmetries. Imagine that you have a picture of a butterfly on a piece of thin paper and you want to know if the left wing is exactly like the right wing. Measuring each of its spots with a ruler and compass is error-prone, but folding the paper and holding it over a light reveals all of the differences quickly and accurately. If a spot on the right wing is slightly larger than the corresponding spot on the left, it won’t line up exactly.

One of the last big discoveries of the Tevatron was a forward-backward asymmetry in pairs of top and antitop quarks. When protons and antiprotons collided to produce top-antitop pairs, the top quarks flew out of the collision in the direction of the original proton more often than in the direction of the antiproton (and vice-versa). Just like folding butterfly wings, this result is robust against uncertainties in the total collision rate because such an error would cancel in the forward-backward comparison. For three years now, physicists have been trying to explain this asymmetry.

It is natural to ask if the LHC sees the same asymmetry. Unfortunately, the experiment can’t be exactly repeated because the LHC collides protons and protons — no antiprotons. However, there’s another potential asymmetry that could reveal the underlying cause. Protons are made of two energetic up quarks, an energetic down quark and a froth of low-energy quark-antiquark pairs. Antiprotons are the opposite. When the Tevatron saw an excess of backward-moving antitop quarks, it might have been because the antiproton’s antiquarks were, on average, more energetic than the proton’s antiquarks. At the LHC, this asymmetry would show up in top-antitop pair trajectory angles relative to the beamline: parallel to the line of colliding protons or perpendicular to it.

In a recent experiment, CMS scientists studied exactly that. They measured this top-antitop ratio, but found no asymmetry. Clearly, something else is going on here. Like a rear-view and a side-view mirror, these two views together give us a more complete picture of the mystery.

Jim Pivarski

The U.S. physicists pictured above performed this measurement of top-antitop asymmetries with CMS data.
These U.S. physicists are actively studying jet substructure, a key component of complex analyses.