© David Kaplan.
The three Feynman diagrams above represent parts of the calculation of the probability of two W particles scattering. The first, and others like it, together represent the calculation including all known particles. When calculated for very high energy W particles, the result is nonsense. The way out is if there are unknown diagrams which, when included, yield sensible answers. The yellow blob in the middle picture represents the unknown part of the diagram. It turns out that simplest way to make sense of this calculation is to add a single new particle—the Higgs boson—and include all possible new diagrams, including the one on the right. (Unit: 2)