Synopsis: Producing top quarks one at a time

Two collaborations at Fermilab observe the production of single top quarks.
Synopsis figure
Illustration: Alan Stonebraker

Top quarks are most often produced in pairs via gluons, the carriers of the strong force. Such pairs were first observed at Fermilab in 1995. But single top quarks can also be produced via an electroweak process involving a W boson and a bottom quark. This process is sensitive to the top-bottom quark mixing parameter known as Vtb and the rate at which it occurs provides new information about the top and bottom quarks. (The top and bottom quarks form a third generation of quarks, added to the first generation, up and down quarks, and the second generation, charm and strange quarks.) Measurements of single top quark production can therefore constrain extensions of the standard model, such as those with a fourth generation of quarks.

Single top quark events are more difficult than top quark pair events to pick out of the vast number of background events. The first evidence for single top quark production was reported only recently by the Fermilab experiments, in 2006 for the D0 experiment and 2008 for the CDF experiment, with a statistical significance of 3.4 and 3.7 standard deviations, respectively. Since that time, both D0 and CDF have taken more data and improved their multivariate analyses. In a pair of papers appearing in Physical Review Letters, they now claim to have definitively observed single top quark production, each at the 5.0 standard deviation level. They also put constraints on Vtb, which agree with what the standard model predicts.

In the future, Fermilab, and later the LHC, will accrue more single top quark events and constrain Vtb further, which will either help to confirm the quark mixing picture of the standard model, or point to new physics. – Robert Garisto


More Announcements »

Subject Areas

Particles and Fields

Previous Synopsis

Next Synopsis


An event on the horizon

Read More »

Related Articles

Viewpoint: Connecting the Higgs Mass with Cosmic History

Viewpoint: Connecting the Higgs Mass with Cosmic History

A theoretical proposal ties the puzzling light mass of the Higgs particle to a hypothetical new particle that plays an important role during the big bang. Read More »

Synopsis: LHC Data Might Reveal Nature of Neutrinos
Particles and Fields

Synopsis: LHC Data Might Reveal Nature of Neutrinos

A long-standing question over whether the neutrino is its own antiparticle might be answered by looking at decays of W bosons. Read More »

Viewpoint: Are We on the Brink of the Higgs Abyss?
Particles and Fields

Viewpoint: Are We on the Brink of the Higgs Abyss?

The probability that the Universe exists in a stable state may be greater than previously thought. Read More »

More Articles