One at a time

While top quarks have been more commonly found produced in pairs, this analysis looked for top quarks created one at a time. The time between observation of pair production and single quark production was very short and is a strong demonstration of the performance of the CMS detector.

Last week, the CMS Result described the measurement of the mass of the top quark at the LHC, as well as how frequently it is produced. However, what you might not have realized was that article described the production of top quark pairs, rather than a single top quark. Making or observing two of something sounds harder than making just one, but it is actually more difficult to produce a single quark. Today’s article describes the recent observation of the single top by the CMS experiment.

By any measure, top quarks are extremely rare, even at the LHC. Top quark always decay into bottom quarks and W bosons, and the W bosons decay either into two quarks or a lepton and a neutrino. The problem is that (a) bottom quarks and W bosons are produced in more ordinary ways far more often than top quarks and (b) due to an imperfect detector and particle identification algorithms, it is possible to misidentify an event as having a bottom quark and a W boson, when the event actually contains neither. This second issue is very difficult to overcome. Even though it is quite rare for ordinary events to mimic top quarks, there are so many ordinary events that these fake top quark events swamp the real ones.

What makes top quark pair production easier to identify are statistics and probability. If it is rare to make a fake top quark, faking two is extremely rare. Because of this, events that seem to contain two top quarks are likely to actually contain top quarks. This is mainly the reason why single top quark production was observed at the Tevatron about 15 years after the observation of top quark pair production.

CMS’s recent announcement of the observation of single top is therefore quite an achievement. By employing the extraordinary performance of the detector, algorithms and the teams that made them possible, the rate at which ordinary events are mistaken for ones containing a single top quark has been suppressed enough to make observation of single top relatively straightforward. This achievement is only possible because of the exceptional performance of the CMS detector and bodes well for our searches for truly new phenomena.

—Don Lincoln

It is impossible to isolate events containing single top quarks without identifying the bottom quark into which they decay. This group has contributed to the CMS bottom quark identification effort.