Earlier today, scientists from the ATLAS and CMS experiments at CERN's Large Hadron Collider announced the discovery of a subatomic particle that's consistent with the Higgs Boson. So, have physicists finally found the elusive particle? Short answer? Yes. Longer answer? Well...
If you were to say July 4, 2012 was the day the Higgs boson was officially discovered, you wouldn't really be wrong. As CERN Director General Rolf Heuer put it, "As a layman, I think we have it." That's about as unequivocal a statement as you're likely to get from CERN, which is understandably conservative when it comes to announcing possible physics-shattering discoveries. But then, there's a second half to Heuer's quote, one that probably won't get as much attention: "But as a scientist, I have to say, 'What do we have?'" That's a huge question, one that won't be answered today, maybe not this year. Today's announcement is just the beginning of the Higgs story, definitely not the end.
So what, exactly, did we discover?
Rolf Heuer offered as good a summation as any when he said, "We have observed a new particle that is consistent with a Higgs boson." Certainly, the two sets of independent experiments have a tentatively confirmed discovery of a new subatomic particle, and it's in the correct mass range and is the right type of particle to be the Higgs. That's a huge deal in and of itself — this is the first new elementary particle discovered since the top quark in 1995. But we can't say just yet that what CERN has discovered is definitely the Higgs boson, let alone the one predicted by the Standard Model.
So then, here's what we know. The CMS experiment detected a particle at 125.3±0.6 gigaelectronvolts, meaning its mass is about 133 times that of a proton. The ATLAS experiment, which works independently of CMS, has found a particle at 126.5±0.6 GeV. Depending on how you combine the data, both experiments are hovering right around a five-sigma level of certainty.
This means both teams are about 99.9999% that the signals they have detected really do belong to a new particle, as opposed to being random statistical noise. Five-sigma is the accepted threshold for a discovery, and the fact that both experiments are at five-sigma — or 4.9-sigma, depending on the exact data, but the figure is only expected to climb as the team analyzes more data — means we can be very confident that a new particle has indeed been discovered.
But is this the Higgs boson?
The honest answer is: We don't know yet. We know that the particle in question is a boson, one of the two fundamental classes of elementary particles and the type generally associated with carrying force. We know this, because of the newly discovered particle's diphotonic decay — simply meaning that it decays into a pair of photons, which is something only bosons do. And of course, the mass range of 125-126 GeV is in line with what we expected for the Higgs boson — after all, that's why ATLAS and CMS were looking there in the first place. That, as CMS spokesperson Joe Incandela pointed out today, makes this particle the heaviest boson ever found.
But the Higgs should have a bunch of unique properties that have yet to be confirmed. The most important of these is that its spin value is 0, unlike any known particle. We don't yet know the spin of this new particle. If it's 0, then that goes a long way to confirming that this is indeed the Higgs. If its spin is some other value, then there's a near endless supply of other hypothetical particles it could be. The odds are probably still pretty good that we're looking at the Higgs here, but it certainly can't be taken for granted.