(Shakespeare was a particle physicist. Didn’t you know that? For other Higgs Shakespearean quotes, see this Twitter stream.)
Many of you heard there is a big announcement today about the Higgs boson, one of the missing pieces of the Standard Model of particle physics. Said announcement has come and gone, and I fear today’s post is yet another “let’s not get carried away” entry. (See also “I Don’t Want to Write About Neutrinos” and “What Does Habitable Mean?“) In many ways, I hate writing these because it is fair to get excited over possible discoveries, and I don’t want to always place big announcements into the “boy who cried wolf” category. Because today’s announcement is potentially exciting, but we won’t know if it’s actually exciting until next year.
What is the Higgs Boson?
The nuclear forces (which come in weak and strong varieties) are short-ranged: they cut off after a very tiny distance, which accounts in part for the tiny size of atomic nuclei. Contrast this with the electromagnetic and gravitational forces, which extend a very long way (technically infinitely far). That means the particles associated with electromagnetism and gravity—the photon and the graviton, respectively—have no mass and travel at light-speed, while the particles associated with the nuclear forces have mass and decay if let loose outside the nucleus. (For nitpickers: this is a huge oversimplification, but I don’t want to write 5000 words on particle physics before I get to today’s announcement.)
The Higgs boson is an extra particle in the Standard Model of particles and interactions that helps explain why the particles carrying the weak force (called the W and Z gauge bosons—particle physicists ran out of cute names by that time) have mass, while the photon does not. (This is a case of symmetry breaking; my post on Emmy Noether has a little more information.) However, the mass of the Higgs itself is not part of the Standard Model: only a range of possible masses. To make matters worse, the Higgs boson decays rapidly into other particles, so direct detection is challenging: instead, it’s easier to look for what it decays into (W and Z bosons, or high-energy gamma ray photons).
Actually, it’s worse than that: other models exist that predict more than one Higgs field, so searching for the Higgs may not be the right way to think about it at all. Nobody seriously thinks the Standard Model is the end of the line for physics, since it fails to predict the specific masses of particles and doesn’t include gravity, so there’s no reason to think the Standard Model version of the Higgs is the only one possible. Having said all that, let’s turn to today’s Higgs announcement.
A Resounding “Maybe”
Today’s announcement from CERN (the European particle physics laboratory) is in two parts, from two major detectors at the center. One result sounds somewhat promising, the other doesn’t (see Matt Strassler’s blog for details); it won’t be until 2012 that CERN will have enough data to say yea or nay. The better result has found a possible particle at an energy of 126 GeV, which makes it about 134 times more massive than a proton. That mass is well within the range allowed by the Standard Model, while not ruling out all other possibilities.
Now for the cold water: the result is a peak in a noisy set of data. The technical size of the certainty is 2.8 sigma, which means over 99% certainty there is something there. Nevertheless, because the data is messy in particle physics and despite the huge amount of it, a lot of results that are less than 3 sigma end up being false positives: background fluctuations can sometimes make you think there’s a particle where there is none. That’s tough, but that’s particle physics for you. Only a result of 5 sigma (which again should appear in 2012 if all goes well) is good enough for scientists to say definitively “Yes, we’ve discovered the Higgs!”
So don’t be sad that we haven’t discovered the Higgs yet: we may have done. We just won’t know for sure until next year.
For more information, here are some links from people who know a lot more about the subject than I do.
- Why the Higgs Boson Matters is an award-winning post by Kelly Oakes, explaining (wow!) why the Higgs is important.
- Matt Strassler has a bunch of posts about the Higgs boson: The Standard Model Higgs and How to Search for the Higgs are good starting points.
- Eugenie Reich, writing for Nature News, goes into some detail about today’s announced results.
- Sean Carroll explains the statistics involved in particle physics, which in turn may help you understand why 99% confidence in this case ain’t good enough.
- Update: Here’s Phil Plait’s take, which is similar to mine, but with a bit more detail.