“…the publisher wouldn’t let us call it the Goddamn Particle, though that might be a more appropriate title, given its villainous nature and the expense it is causing.”
–Leon Lederman, author of The God Particle
The Higgs Boson: you know the deal. It’s the last undiscovered particle in our current picture of all the fundamental particles in the Universe.
If we can find it, we’ll either have a big clue as to what the next step to take in physics will be, or we’ll be forced to admit that physics works too well, and many of the great hypotheses (supersymmetry, large extra dimensions, etc.) are highly unlikely to be within our reach. It all depends — if we find it — on what its properties are.
So how do we go about finding it? We accelerate particles to the highest energies we’ve ever reached on Earth, smash them into one another at strategic collision points, and observe the debris that results.
These collisions are so frequent at the Large Hadron Collider, numbering about 600 million per second, that we couldn’t possibly record them all. Instead, what we do is look for exotic signals, or signals that have quickly measurable indicators that something interesting may be going on, and only record those. This is vital, because each collision shows up looking like this in one of the two main detectors.
So what particle physicists looking at this do is try to reconstruct, based on what’s showing up in the detector, what was created when these collisions occurred. The difficulty of this is mind-boggling; it makes blood-spatter analysis look like child’s play.
We can actually do this, and determine what particles came from what locations with a specific energy at a specific time, for pretty much everything (except neutrinos) that are produced! This is important, because depending on what mass of the Higgs Boson is, and whether it’s a normal, standard-model Higgs or something more exciting, the Higgs should decay into different particles of a given energy.
So when we reconstruct that there were bottom-antibottom quarks leaving a collision, that’s interesting. When we see two high-energy photons, that’s interesting. When we see a tau-antitau pair, that’s interesting, too. And so on.
But the Higgs isn’t the only thing that produces those particles. In fact, many other things produce those particles. The big question — and the reason finding the Higgs is so difficult — is that we have to figure out how much. It’s what makes physics so powerful, the fact that we’re a quantitative science. And you may have seen the ATLAS results here just a few months ago, where their combined data provided some very suggestive evidence of a Higgs boson at about 126 GeV/c2.
The best signal, as you can see in red, comes from looking at what appears to be a Higgs Boson decaying into two photons.
But what does that mean? Where does that graph come from? Well, I don’t need to describe it, when the cover of one of last month’s issues of Physical Review Letters can show you!
What this graph is showing you is, with the black data points, the data observed by the ATLAS experiment. This is contrasted with the (red line) theoretical prediction of all the known particles and interactions of the standard model, excepting the Higgs. A deviation from that red line indicates either an experimental fluctuation or some type of new physics.
Let’s go in for a closer inspection, with some annotations (in blue) by me.
A visual inspection clearly shows the excess of data peaked at around 125 GeV/c2, but that’s hardly an incredibly convincing graph! It should clearly show you why we say we need to take more data before we have successfully convinced ourselves that this is new physics and not simply a fluctuation. The degree of statistical significance we require in this discipline to announce a discovery is five standard deviations; on its own, this study — the best individual channel searching for the Higgs Boson — doesn’t even reach three.
But this is why we’re increasing the energy of the beam, taking more data, and trying to establish exactly what is and what isn’t a fluctuation.
And if we can get the data to say that with some degree of certainty, “there is some new physics here,” the next step is to ask whether it’s a standard model Higgs Boson or not. Because it might be simply be a standard model Higgs Boson at ~125 GeV/c2, and there might not be any new physics beyond that — in the world-case scenario — all the way up to the Planck scale! (Of 1019 GeV!)
If this is the case, we should see a specific excess of signals at an energy corresponding to the Higgs Boson’s mass in each channel: bottom-antibottom quarks, two photons, a tau-antitau pair, W-bosons, Z-bosons, etc.
If you take a look at the error bars on the graph, above, you’ll see that we are way more likely to wind up on the green line (Standard Model Higgs) than the red line (no new physics), but we may also wind up in… well… a weird place! What do I mean by weird? I mean that the thing we find may not be the Higgs Boson we’re looking for, or it may not be a Higgs Boson at all.
What we see, so far, is consistent with a Standard Model Higgs Boson, but that is by no means the only (or, arguably, even the best) interpretation. If I were a betting man, the Standard Model Higgs Boson is what *I* would bet on, but it isn’t the only possibility, and we need to take more data to be able to decide.
So be patient. This is how we do science, and this is what it takes to get it right. Above all else, we should all be ecstatic to see that the science is being done properly here; we owe it to everyone doing their job correctly to give them the time to do it right.