|This shows a Feynman diagram for s-channel single top quark production. The top quark decays into a W boson and a b-jet.|
This week, a combined effort from the CDF and DZero experiments reported the observation of a new top quark process. This article discusses the contribution to that result from the CDF experiment.
The top quark was discovered almost 20 years ago by the CDF and DZero experiments through a process in which it is produced in pairs. Events with two top quarks are quite distinctive and can be identified relatively easily. Top quarks could also be produced singly at the Tevatron, but at a lower rate than events with two top quarks. They are also harder to identify because they produce less distinctive signals in our detectors. In fact, because of these challenges, it took 14 years after the top quark was discovered for the Tevatron experiments to observe single-top-quark events.
Measurements of single-top-quark production are important because they can be used to constrain our Standard Model theory and could provide a window to new physics. At the Tevatron, top quarks could be produced singly through two different processes, and new physics could affect these two processes differently, thereby providing clues about the nature of the new physics.
The single-top process for which theory predicts the highest rate occurs when two quarks exchange a W boson to produce a top quark and another quark. This is called t-channel production.
A second process occurs in which two quarks fuse into a W boson that then splits into a top quark and another quark. This s-channel production occurs when an up quark and an anti-down quark combine to form a W boson that then breaks apart, leaving a top quark and an anti-bottom quark (see top figure).
When scientists discovered single-top events five years ago, it was primarily through the events produced in the t-channel process. In fact, until recently, no experiment had seen evidence of the s-channel process. We can separate the s-channel events from those of the t-channel because they are more centrally produced and because the events typically contain two b-jets rather than one.
Because of subtle differences between the proton-antiproton collisions of the Tevatron and the proton-proton collisions of the LHC, the Tevatron has the better chance to measure the s-channel single top. In fact, last summer the DZero experiment reported first evidence for s-channel single top production.
The result of our experiment is that CDF confirms the evidence previously reported by DZero and measures the s-channel cross section to be 1.41 +0.44/-0.42 picobarns using events with an imbalance in the transverse momentum, b-tagged jets and one lepton. CDF also recently submitted a paper that looks for the s-channel single top production by using events with an imbalance in the total transverse momentum, b-tagged jets and no identified leptons. Even more recently, CDF and DZero scientists have combined their results for this difficult process.