With today’s announcement of what could be the first evidence of the discovery of a Higgs boson, I thought it would be a good idea to write out some things that I’ve noticed the media has missed in its coverage of what happened and how.
Why do particles have mass?
It’s been reported that the Higgs boson is the particle that gives all other particles their mass. This is not quite true. What gives particles their mass is the Higgs mechanism. As it turns out, the vacuum (or empty space) isn’t so empty; even if no matter (particles) exist in a certain region of space, all kinds of energy fields do. In fact, it’s physically impossible for “nothing” to exist anywhere, except where space doesn’t exist. (There’s something to talk about the next time you have a Deep Discussion with your philosophically-leaning friends.) As a particle travels through “empty space,” it interacts with these fields. The degree to which a particle interacts with these fields is called “charge.” This is nothing new: particles with electric charge interact with electric fields in a predictable way. So mass can be considered a charge, so that must mean particles interact with some field based on how strongly they interact with it. The field is called the Higgs field. So what we call “mass” of a particle is really just how strongly it interacts with this field.
So where does the Higgs boson come in?
The Higgs field permeates all space. That means there is a local Higgs symmetry everywhere (best summed up as, “wherever you go, there you are”). However, the symmetry is spontaneously broken. (One way to understand spontaneously broken symmetry: perfectly balance a pencil vertically on a table. Now shake the table. The pencil will choose a direction in which to fall; the directional symmetry has been broken. This isn’t a very good analogy, since we could probably figure out from the way the table was shaken which direction the pencil would choose. For truly spontaneous symmetry breaking, it’s truly random.) One of my favorite theories in physics says that whenever there is a spontaneously broken quantum local symmetry in Nature, there is a particle called a Goldstone boson associated with it. It’s really a fascinating theorem with a clever proof. If the symmetry is broken not just spontaneously but also explicitly, as in the case of the Higgs field, the Goldstone boson is not massless. The Goldstone boson associated with the Higgs field is called the Higgs boson.
So we’re just now finding it?
Yes. Because we know how badly the Higgs symmetry is explicitly broken, we’ve had a good idea how much its mass should be. And it’s big by particle standards. Until the LHC came online a couple of years ago, no lab in the world had any hope of detecting it.
How do you detect a particle, anyway?
It’s very, very difficult. It’s not like there’s only one of them, and it’s hiding in the back corner of the laundry room. Higgs bosons must be created, and can only be so if two particles going really, really fast smash into each other. When that happens, the particles exchange a great deal of energy. From that energy, a menagerie of particles can be spontaneously created out of thin air, thanks to Einstein’s famous equation E=mc2, based on the created particles’ masses. Since the Higgs is so heavy, it needs a lot of energy to create.
So how do we know we’ve found it? Take a look at this graph, published by the CDS collaboration at the LHC. They’ve taken the data they’ve collected (and it’s taken a long time just to get a little), and plotted it with the black dots connected by the red line. Then they traced a yellow line of what the data should look like if there was no Higgs boson. That bump around 125 GeV? That’s probably a Higgs boson. The big announcement today is that the confidence they have in this result is 5-sigma, the standard for announcing a discovery. It’s my opinion that even with that result, much more study is needed to verify the claim. After all, let’s not forget arsenic-based life and FTL neutrinos.
Why should I care?
Short answer: you shouldn’t. This will not make your power bill go down or lead to flying cars. What it does do is place the final piece in the puzzle that is the strong nuclear force, which keeps protons and neutrons together. The theory was proposed in the 1960′s, and every part of it has (perhaps) now been experimentally verified. If the Higgs discovery holds, we can say with certainty that this is really it. However, that doesn’t mean the field is dead. Unlike the study of the electromagnetic force, in which all problems can be answered, there’s still much we don’t know about the strong nuclear force. For one, we have the theory, but we can’t solve it. It’s like in Hitchhiker’s Guide to the Galaxy: we’ve now satisfied our Answer, we just need to find the Question. That, in fact, is part of my research. I’m helping to find the Question by asking lots of other smaller, related questions about the strong nuclear force. And the more we know about what it is, the closer we come to solving it. That’s the day that will get me truly excited.