Feynman diagrams depicting the different (decay-)channels for...

Feynman diagrams depicting the different (decay-)channels for single-top-production mechanisms. The letters refer to top quarks (t), bottom quarks (b), light quarks (q: u, d, s), W bosons (W), and gluons (g). In each case, the incoming particles are shown on the left and the outgoing particles on the right of the diagram. The circles indicate the W-t-b vertex (interaction), whose coupling strength is governed by the CKM matrix element Vtb.

Source: Top Quarks Go Solo in Rare Events (American Physical Society-Physics)

Paper at Phys. Rev. Lett. 112, 231802 (2014) - PDF (Observation of the Associated Production of a Single Top Quark and a W Boson in pp Collisions at √s = 8 TeV)