The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World (44 page)

The only remaining fundamental diagram is the Higgs interacting with itself; it can split into two or three copies. Clearly this would violate energy conservation unless it were embedded in a bigger diagram.

The real fun comes from combining these fundamental diagrams to make bigger ones. All we have to do is join lines describing matching particles: We join an electron to an electron, and so forth. Starting from the diagrams above, we might have to flip some lines from right to left and turn particles into antiparticles to make it work.

For example, let’s say we want to ask how a muon can decay. We see that there is a diagram where a muon emits a W
^-
and turns into a muon neutrino; but that can’t happen by itself, since the W is heavier than the muon. Never fear; all is okay as long as the W remains virtual, and decays into something lighter than the muon, such as an electron and its neutrino. All we have to do is glue together the W
^-
lines from two of the previous diagrams in a consistent way.

We can also bend lines back on themselves to form loops. Here is a diagram that contributes in an important way to the search for the Higgs at the LHC: a Higgs decaying into two photons. The loop of virtual particles in the middle could contain any particle that couples both to the Higgs (so that the vertex on the left exists) and to photons (so that the vertices on the right exist). Particles with stronger couplings will contribute the most; in this case, that would be the top quark, which is the most massive particle in the Standard Model, and therefore the one with the strongest coupling to the Higgs.

Finally, here are some of the important ways that Higgs bosons are actually produced at the LHC before they decay. There is “gluon fusion,” where two gluons come together to make a Higgs; because gluons are massless, they must proceed through a virtual massive particle that feels the strong force, namely a quark.

There is also “vector boson fusion,” referring to the fact that the W and Z bosons are sometimes called “vector bosons.” Since they are massive, they can combine directly into a Higgs.

At last there are two different kinds of “associated production,” where the Higgs is made along with something else: either a
W
or
Z
boson, or a quark-antiquark pair.

The take-home lesson here isn’t the ins and outs of all the different processes that contribute to Higgs production and decay; it’s simply that both processes are complicated, arising from a collection of different possibilities, and we have definite rules that allow us to figure out what they are. It’s amazing to think that these little cartoons capture something deeply true about the microscopic behavior of the natural world.

JoAnne Hewett, rapping about dark matter at a physics slam in Eugene, Oregon, in 2011.

© JACK LIU

At CERN on July 4, 2012, Fabiola Gianotti, Rolf Heuer, and Joe Incandela, preparing for the big announcement.

© CERN