In principle, a hadron-hadron collision can produce top quarks both in particle-antiparticle pairs, or singly. However, the single particle production processes are less frequent, as they take place via electroweak interactions: the weakness of the production mechanism more than counterbalances the fact that in energetic terms producing a single top quark is easier than producing two of them together. After all is accounted for, the production of a single top is three times less frequent than the production of a tpp pair at the Tevatron; this ratio is not too different in higher-energy LHC collisions.
After the observation of top, single top production had to wait until 2009 before CDF and DZERO could conclusively claim to have observed it. The difficulty in observing the process did not have to do with the three times smaller signal rate, as much as with the much less distinctive features of single top events. The products of a single top quark decay - a W boson and a b-quark jet - can be easily mistaken for the incoherent production of those same bodies by background processes; instead, a top-antitop pair produces much more striking signatures.
Now that the Tevatron has been shut down, we can only study single top production at the LHC, where the center-of-mass energy is several times higher. It has consequently become easier to spot single top production events, and this has in turn made more meaningful to study the detailed properties of these processes. A recent CMS analysis, using 8 TeV data, has isolated a sample of single top events large and clean enough that a measurement of the polarization of the top quark has become possible.
For an elementary particle, polarization corresponds to the angle that the spin vector of the particle makes with the direction of motion. Being an electroweak process, single top production is expected to yield a very distinct polarization to the top quark. It is then quite interesting to measure the polarization structure of these events, to verify the predictions of the standard model.
The CMS analysis extracts a very clean signal of single top quarks from events with a muon, missing transverse energy, and jets by using a boosted-decision trees multivariate technique. The result is shown in the graph below, where the signal is in red and backgrounds (mostly from top pair production, in yellow, and W plus jet production, in green) are stacked below it. The variable shown, cos theta_μ, is the cosine of the angle between the muon direction and the direction of the quark recoiling against the top, in the top quark rest frame. As you can see, the signal exhibits a peak at a large value of this angle.
The final quantity which is extracted from the data is the top quark spin asymmetry, measured as 0.26+-0.10. This value is smaller than the standard model prediction of 0.44; the compatibility is at the two-sigma level. As new physics theories might predict a depolarization of single tops, this measurement will be an interesting one to do at higher center-of-mass energy and with more data.