Due to an article in the local newspaper this morning, I have received a few questions and comments from friends and family on the nature of the Higgs boson, the search for it and the theory behind it. Although I have commented on this topic several times in the past, the time seemed right to review the Higgs model one more time.

For the last forty years or so, particle physicists have been constructing and refining the Standard Model which contains everything that we know about subatomic particles and forces. At present it contains about 30 particles (depending on whether gravity is included in the theory or not), with 29 of them observed directly or indirectly in experiments. The lone particle which is as yet undetected is the Higgs boson.

The Standard Model includes three basic forces (gravity is the fourth, but it isn't well understood yet on this scale) which are the electromagnetic force that holds matter together and is responsible for electricity and magnetism, the weak nuclear force which is responsible for certain types of radioactive decay, and the strong nuclear force which holds the nuclei of atoms together. Each of these three forces can be predicted by requiring particles to obey certain laws of symmetry called gauge symmetries.

But there is a problem with the weak nuclear force. Gauge symmetry only works if the forces are transmitted by massless particles, which is valid for the other two gauge forces. But weak bosons (called W+,W-, and Z) are in fact three of the four most massive particles known. They should not obey a gauge symmetry at all, and yet they seem to do exactly that based on their interactions with other particles.

The solution to this puzzle was discovered in the 1960s by several physicists almost simultaneously. The answer seems to be that the weak bosons are in fact massless, and as such obey the symmetry laws. But in this model, there is another particle called the Higgs boson, and it has the odd property that it can be spontaneously produced by the vacuum of space until a certain density is reached, and so our Universe is filled with a thick soup of these invisible particles. When the weak bosons pass through them, they interact and are effectively slowed down. In the experiments, this slowing effect appears to physicists as an effective mass for the weak bosons, while not violating the fundamental symmetry laws. (This mechanism is called spontaneous symmetry breaking for anyone who is interested)

So everything works out fine from a theoretical viewpoint. The three fundamental forces of the Standard Model can be predicted from symmetry laws, and the large mass of the weak bosons explained away as an effect of the Higgs boson. In fact this method of adding masses works so well that it is believed to be responsible for the mass of all known particles in the Universe!

But then when the model started to be tested in experiments, new problems arose.

Huge particle accelerators and colliders were built over the last fifty years to search for the Higgs boson (and other more exotic particles of course) but no sign of them has ever been discovered. The Higgs model predicts that at some energy level, we can either increase the density of the Higgs bosons and detect the extra particles, or decrease the density and detect the deficit. But it has never happened.

And the situation is starting to get tense for the physics community, because there are strong arguments that if the Higgs boson is not discovered below a certain energy level, then it cannot exist. The newest collider, called the Large Hadron Collider, can easily reach these energies and exceed them, so if it isn't seen in the near future then the Standard Model has a large flaw in it.

Of course there are always options. Already the theoretical physics community has generated hundreds of academic papers outlining modifications of the model or alternative mechanisms for mass generation. But all of those require a more complicated theory than the standard Higgs model, and so would leave questions as to why nature chose a more difficult mechanism.

For now, the scientific community continues to watch and wait, and to hope for a discovery in the coming year.  One way or another, this search will be resolved in the next few years and whatever the outcome it will lead to some very interesting new science...