At CERN today, home of the Large Hadron Collider, particle physicists announced the most recent (and most tantalizing) results in their search for the Higgs Boson. They haven't nailed down the elusive particle quite yet, but they're closer than ever before, and they may now know just exactly where it's hiding.
What is a Higgs Boson?
The Higgs Boson, if it exists, would be the particle that explains why other particles (and by extension everything else) have mass. And as far as Standard Model elementary particles go, it's the only one that we haven't managed to directly (experimentally) observe. Things like that really niggle at scientists, which is why we spent like ten billion dollars building the Large Hadron Collider.
Theoretical models and previous experiments have suggested some good places to look for the Higgs: it should have a mass in a range between 115 and 141 GeV, where "GeV" stands for "gigaelectronvolt," a unit of energy. At such small scales, mass and energy are thought of as equivalent, but converting this to something understandable isn't too bad: a hydrogen atom is about 1 GeV of mass, so you might expect a Higgs Boson to have about the same amount of mass as an atom of cesium. Of course, the Higgs won't be made up of protons, neutrons, and electrons like a regular atom: instead, it's just a piece (a quantum) of this hypothetical "Higgs Field," which is what other particles (and the Higgs itself) get their mass from.
How are we looking for it?
So with the mass of the Higgs Boson narrowed down to a general range, the Large Hadron Collider at CERN has been chucking super-energetic protons at each other and watching what happens as they collide. Every time a proton smashes into another proton, they blow apart into a subatomic soup* of all kinds of weird stuff that we can't make any other way, like quarks and gluons. And if we get very, very lucky, some of these things (like two gluons or a couple of W or Z bosons) might combine with each other to make a Higgs Boson.
Unfortunately, Higgs Bosons themselves don't stick around around long enough for us to detect them. What we can detect are what they leave behind when they decay into other particles. The tricky bit, though, is that a Higgs Boson can decay in a bunch of different ways, called decay channels. For example, a Higgs might turn into some leptons. Or some gamma rays. Or some electrons. But lots of other things decay in these ways too, so the only way to figure out if we've really seen a Higgs decay (and not something else) is to run so many experiments that we start to see a statistical blip, where we're getting more of these potentially Higgs-type decay events than we'd otherwise be able to explain. And it's going to take a lot of experiments to conclusively prove that we've spotted traces of a Higgs Boson and not something less cool.
What has CERN found?
Two separate LHC detectors have gotten in on the Higgs-hunting action: ATLAS, and CMS. Both of these systems are general purpose particle detectors with slightly different designs; ATLAS is a bit better at measuring hadrons (protons and neutrons), while CMS is a bit better at measuring electrons and photons. Today's announcement was a joint statement of results from both ATLAS and CMS, and it's probably easiest to break it up into two parts:
- We're more sure of where the Higgs Boson definitely isn't. ATLAS is 95% certain that they've seen no signs of the Higgs at energies higher than 130 GeV or lower than 116 GeV. CMS is pretty sure that the Higgs must be lighter than 127 GeV, but heavier than 115 GeV. So, the target energies where the Higgs might be hiding have been reduced to a narrow range between about 115 and 130 Gev.
- There are a few hints of where the Higgs Boson might be. Both ATLAS and CMS reported a few statistically anomalous readings that might point to Higgs detection. ATLAS may have spotted something at 127 GeV, and CMS saw an unusual peak at 124 GeV. Statistically speaking, however, these anomalies (while they definitely exist) are fairly weak and don't provide much in the way of conclusive evidence. Just hints.
So what's the takeaway from all this? Well, progress is being made, that's for sure. Narrowing the search area is a significant accomplishment, perhaps an even more important one than the vague and statistically ambiguous hints at the existence of the Higgs Boson itself. That said, the Higgs Boson is what we're excited about, and it's okay to forget about statistics and probabilities for a second and just think about how cool it would be if we finally (finally!) seen traces of the particle that gives everything mass right around that 125 GeV sweet spot. But either way, as the LHC continues to collect data through 2012, at this point it's not unrealistic to imagine that we might have proof of the Higgs Boson within just a few years.
*At high enough collision energy, you get pure energy that's released, not subatomic particles. The energy rapidly turns into particles, and the more energy that's released, the heavier (more exotic) particles you can get. This is why particle physicists keep trying to crank up the energy of their particle colliders.