July 4th, 2012 will go down in history as one of the most significant days in science history, and certainly of the last few decades. A new particle consistent with the long-sought Higgs Boson has been discovered at the Large Hadron Collider. Leading up to this discovery, I had been keeping up with various particle physics blogs, and the rumors I were reading pointed to a major announcement. Excited, I stayed up late on the 3rd just to watch the live webcast from CERN. I have to say, watching the excitement in the eyes and voices of the physicists giving the announcement of a 5-sigma detection of this new particle sent shivers down my spine.
Almost 50 years ago, a theoretical physicist by the name of Peter Higgs (who is still alive and kicking and was present in person for the announcement!) wrote a paper (initially rejected, no less!) describing a mechanism that would allow fundamental particles to acquire mass by virtue of interacting with the “Higgs field”, a field analogous to the gravitational and electromagnetic field, that permeates all space. (This field was only later called the Higgs field; Peter Higgs was far too modest to name the field after himself!). In this landmark 1964 paper, he pointed out, almost as an afterthought, that this field would have its own particle, which came to be called the Higgs Boson (again, he didn’t name it such himself!). As an aside, Higgs was not the only player in this game, there were several other scientists who contributed to our theoretical understanding of the Higgs mechanism and Higgs Boson. Rather than going into that here, I recommend you peruse the relevant Wikipedia articles, which discuss these issues in depth.
To drastically oversimplify matters, this mechanism was incorporated into the so-called Standard Model of particle physics and quickly became an essential feature of it. Up until now, it was the last puzzle piece of the Standard Model that had not been put into place. Just like the electromagnetic field has its corresponding “quantum”, or particle, namely, the familiar photon, so the Higgs field has its corresponding particle, the Higgs Boson. Unlike the photon, however, which each of us detects everyday with one’s retinas (assuming one is not blind, of course!), the Higgs Boson has been devilishly difficult to pin down. For, while the Standard Model predicts that it should exist, it inconveniently doesn’t tell us what its mass is. And, the details of how it behaves and how easy it is to detect depend crucially on its mass. That’s one of the reasons it has been so hard to find. Another is the fact that, whatever mass it has, it’s unstable, and once formed, immediately (within an incredibly small fraction of a second) decays into other particles. Thus, it cannot be observed directly, but only by observing the products of its decay and working backwards to infer its existence, sort of like inferring the existence of a long-extinct dinosaur by digging up its fossilized footprints.
Why is this important? Up to now, particle physicists strongly suspected that something like a Higgs field exists in nature, because otherwise, it is very difficult to account for the fact that many observed particles (such as quarks, and the W and Z bosons that mediate the weak force) have a non-zero mass. Theoretical calculations of the behavior of these particles work best when such particles are massless. The fact that they aren’t (we wouldn’t be here if they were!) obviously required an explanation, which was the motivation for a series of papers introducing the Higgs field. As stated before, the Higgs Boson is a byproduct of this additional field (you can think of it as an “excitation” of the field, sort of like a breaking wave of water on the beach is an “excitation” of the ocean it comes from). And particle physicists have been searching for it ever since, because the only way they know of confirming the existence of the Higgs field is to detect the particle associated with it.
Now, it appears they’ve finally found it. The fact that such a particle was predicted almost 50 years ago, and experimentally confirmed just this past year is as much a testament to the power of scientific theory as one will ever come across. I for one am excited, and I’m not even a particle physicist! On the other hand, many theories that have been posited in the interim to attempt to explain the mass of fundamental particles apart from the Higgs field and the Higgs boson have now fallen flat on their face with this new discovery. Such is the nature of science: you never know when a theory is going to be dramatically upheld, or completely ruled out, by a new discovery. And we scientists wouldn’t (or shouldn’t!) have it any other way!
So, what’s next? Now that this new particle has been discovered, there are several things that particle physicists are going to follow up on. The first thing is to continue collecting more data. The particle was originally discovered by sifting through the debris of trillions of proton collisions to look for the unique signature it would produce above the noisy background of all the other particles produced; the problem is orders of magnitude worse than looking for a needle in a haystack. More collisions that produce the particle are needed to determine its properties. The Standard Model predicts that the particle should have very specific decay modes into other particles. For example, it should most often decay into a pair of bottom quarks, and much less often into a pair of gamma ray photons. If the decay rates of the Higgs Boson differ from the Standard Model, even a little bit, then it would be a signpost pointing to new physics (what particle physicists refer to as anything beyond what the Standard Model already accounts for) just around the corner.
Many physicists strongly suspect that the Standard Model is not the whole story, for various reasons. One is its inability to explain why the various particles have the particular masses they do, which seem to follow no discernable pattern. Another is why there are so many different kinds of particles to begin with. Another big one is the fact that it doesn’t account for gravity at all. But, to date there have been only a handful of observations that have been at odds with its predictions, which have been confirmed to astounding precision time and time again. The LHC was built in part to probe new frontiers that might tell us more about how the Universe works at its most fundamental level and give us clues to how we might solve some of the Standard Model’s problems. Now that we’ve found the last missing piece of the Standard Model, what this piece tells us could be the bridge to a fundamental new understanding of the physical Universe, and that’s something to get excited about!
