The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (43 page)

The Higgs boson is much like the Z: Any particle that feels the weak interaction can emit one.

Now we turn to bosons coming in. They can emit another boson, or they can split into two fermions. However, since a fermion line can never end, a boson has to split into one fermion and one antifermion; that way the total number of fermions at the end is zero, just like it was at the beginning. Here we have a multitude of examples. Note that these are all related to diagrams we’ve already drawn, just by moving lines around and flipping particles to antiparticles where appropriate. If the entering boson is massless, we once again know that it can only be used as part of a bigger diagram, since massless particles can’t decay into massive ones while satisfying conservation of energy. (One way to see that is that the combination of two massive particles must have a “rest frame” in which the total momentum is zero, while a single massless particle has no state of rest.)