The Standard Model is remarkably successful at explaining all the variety of particles we see in nature in terms of 16 fundamental particles. But there was one mystery yet remaining: why do some of those particles have mass but not others?
It seemed odd that photons and gluons (the force carriers of the electromagnetic and strong forces) were completely massless, and yet the W and Z particles (the force carriers of the weak force) weighed more than an entire atom of iron! It was clear that there had to be some kind of new mechanism that could give mass to some particles and none to others.
The solution came in 1964, when six physicists in three different groups (including Peter Higgs and François Englert, the 2012 noble prize winners) postulated that a field filling the entire universe, the Higgs field, is what gives fundamental particles their mass. The smoking gun signature of the existence of such a field would be a seventeenth fundamental particle: the Higgs boson.
So how does it work? Imagine a field (the kind with grass and sheep) coated in snow. A skier passes by, gliding easily over the snow. Then a woman wearing snowshoes shuffles by, being slowed down by the snow. Next, a man in heavy boots struggles onwards, at each step being slowed by the snow. Finally, imagine a bird flying overhead, completely unaffected by the snow. In this analogy, the field of snow is the Higgs field and each character is a different fundamental particle. The bird is a massless particle like a photon, passing by without interacting with the field. The skier is a really light particle such as an electron, with very little mass at all. The woman in the snowshoes is a slightly heavier particle, such as a quark, and finally the man with the heavy boots is a truly massive particle like the W and Z particles, slowed by the Higgs field at every opportunity. So we see that the mass of a particle depends on how strongly it interacts with the Higgs field.
(Next time: Discovery of the Higgs)