|This diagram shows the s-channel single-top quark production mechanism sought in this analysis, which selects signal events in which the lepton is not identified.|
In collisions of high-energy beams, new, heavy particles are usually produced in pairs: a matching antiparticle for every particle. That’s the case for the bottom quark and the top quark, both discovered at Fermilab. But every now and then, the weak force is responsible for the collision, and so particles can be transmuted from one kind into another. The weak interaction can also produce a pair of dissimilar particles, a bit like giving birth to fraternal twins rather than identical ones. Studying the rate of such production tells us volumes about the secrets of the weak force and possible new interactions that may mimic it, especially if they favor heavy particles such as the top quark.
This article describes a search for such a process: A top quark and an associated anti-bottom quark are produced via s-channel W boson exchange (see top figure). It is similar to an earlier reported study on evidence for the s-channel process used in a search in what is known as the lepton-plus-jets mode.
The search described in this column is performed in the so-called missing-energy-plus-jets mode. The production mechanism is the same as the earlier CDF search — the top quark decays to a W boson and a bottom quark, and the W boson subsequently decays to a lepton and a neutrino. The two bottom quarks, circled in green in the above figure, produce jets that have long-lived, heavy B hadrons in them. If the lepton is not identified, we use it in the missing-energy-plus-jets analysis; otherwise the earlier analysis makes use of it.
One challenge of analyzing data with jets and missing energy is that they can be mimicked by events with only jets in them. These jets-only events can be mismeasured, resulting in large amounts of fake missing energy. Some of these mismeasured events then contaminate the sample of events used to search for the single-top signal. Scientists use sophisticated algorithms to reduce the amount of contamination from these events, and then use the rejected data to estimate the amount that remains. Other algorithms reduce the contamination from other sources.
The measured cross section in this analysis is 1.12 +0.61/-0.57 picobarns. When combined with the earlier lepton-plus-jets result, the cross section is 1.36 +0.37/-0.32 picobarns. The addition of the missing-energy analysis increases the sensitivity of the combination by more than 10 percent compared with the lepton-plus-jets result alone. This analysis forms the CDF contribution to the Tevatron combined observation of the s-channel single top quark process.