Mixing tops

This Feynman diagram shows the mixing of a b (bottom) quark into a t (top) quark. Time flows from left to right. A b enters as one of the ephemeral quarks of the “quark sea” that is the proton or antiproton. It interacts with a W boson emanating from a quark or antiquark in the other colliding particle.

In fiction, a common plot device is the police’s confusion about the identity of the perpetrator of some crime. In physics, a common situation is a subatomic particle being confused about its own identity.

Mixing is a term we use to describe this kind of situation. It actually happens in a number of different situations. For example, neutrino mixing and kaon mixing are different processes at a fundamental level. But in both cases a particle changes its nature — its identity — between when it is created and decayed.

Mixing can also occur among quarks, of which there are six flavors arranged in three so-called generations. Quark mixing occurs when a quark of a given flavor makes a transition into a quark of a different flavor. This happens every time a quark interacts with a W boson — when the quark decays into a W and another quark, or perhaps when a W splits into a quark and an antiquark.

Most of the time, the two quarks have different flavors but are still within the same generation of quarks. In particular, quarks of the top flavor can become quarks of different flavors and vice versa. Quark mixing is a very common process. There is a well-established theory to describe this; it is called the Cabbibo-Kobayashi-Maskawa (CKM) matrix. Notwithstanding, there is still some possibility that the top quark could mix into some other, as yet undiscovered, particle or particles.

To gain insight into the mixing properties of this heaviest of all quarks, we study events in which only a single top was produced and subsequently decayed into a different quark type.

From earlier measurements, we know that the top quark mixes much more with the bottom quark than with the other quarks in the Standard Model. The question is, what is the absolute scale of this, and how likely is it exactly? More intriguingly, are there other quarks that are yet to be discovered that the top mixes with?

Single-top quark events are hard to find. While events with two top quarks were seen in 1995 during Tevatron Run I, observation of events with a single top took until 2009. In such a case it is very advantageous for the two Tevatron experiments to combine their results; one then effectively has twice as much data to work with.

CDF and DZero have recently combined their measurements of single-top production at the Tevatron. Explicitly allowing that there might be a new, undiscovered particle that mixes with top quarks, DZero and CDF found that a bottom quark produces a top quark through the weak interaction 104 +12/-10 percent of the time. Now, 104 percent isn’t actually possible; the number must be 100 percent or less, and the only reason it came out over 100 percent is that the measurement has some margin of error. If the fact that it cannot be over 100 percent is included, bottom produces top at least 84 percent of the time, which is to say top decays into bottom pretty much all of the time.

So there isn’t any evidence for a new possible particle for the top quark to decay into. The rate of the production of single top quark also matches theoretical predictions well, further supporting the idea that we have only CKM style mixing in top quarks.

Leo Bellantoni, with a little help from his friends

These physicists are the primary analysts for this result. Top row, from left: Matteo Cremonesi (INFN Pisa, Italy) and Tom Junk (Fermilab). Bottom row, from left: Christian Schwanenberger (Manchester University, England) and Reinhard Schwienhorst (Michigan State University)
Ken Herner (left) and Bo Jayatilaka have led the DZero and CDF efforts, respectively, on long-term data preservation in the Scientific Computing Division.