Prof. Gerber presented the DZero experiment results at Fermilab.

Fourteen years ago this month, the DZero and CDF experiments jointly announced the discovery of top quarks. In those observations, top quarks were always produced in pairs. A quark and antiquark from the proton and antiprotons in the beam annihilated into a gluon that then became a top quark and an antimatter top quark. By capturing just a couple of dozen collisions in which these quarks were made, scientists were able to definitively say top quarks existed.

Pairs of top quarks are relatively easy to see since they have a distinct signature, but it has long been assumed that it is also possible to create collisions producing a single top quark. As an example, in these types of collisions, a quark and antimatter quark from the beams combine and create the weak-force carrying W boson. This W boson then decays into a top quark and a bottom antimatter quark. Because the W boson only has half the mass of a top quark, this kind of event could not occur without the principles of quantum mechanics. The W boson in these events briefly has about double the regular mass, allowing the reaction to proceed.

Physicists have been combing the data from billions of collisions for years, looking for events in which single top quarks have been made. Even though the Tevatron produces about the same amount of top quark pairs as single tops, these events are trickier to isolate. The false positive rate is much higher. It’s much harder to mistakenly find two top quarks than just one.

However DZero and CDF jointly presented yesterday to a packed house at Fermilab their newest results: a discovery of the single top quark. A celebration in the Fermilab User’s Center followed the presentation. Yet another piece of the Standard Model puzzle has clicked neatly into place.