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.
But if this is the Higgs, then it completes the Standard Model, right?
Not necessarily. The thing is, just because this is a Higgs doesn't mean that this is the Higgs, the one that fills in the missing blank in the Standard Model. ("Completes" is really too strong a word, as our own Dr. Dave Goldberg explained at the end of his post yesterday.) But that's not the only kind of possible Higgs — depending on how this new particle behaves, it might point to more exotic physics that go beyond what's predicted by the Standard Model. That's an exciting possibility, as it might mean the Higgs can open the door up for supersymmetry, or perhaps improve our understanding of the mysterious dark matter and even more mysterious dark energy that make up 96% of the universe.
The particle's decay paths, or the way in which the boson decays into other particles in the Large Hadron Collider, should clarify whether this particle actually fits the Standard Model as we currently know it. As ATLAS team member Dr. Pippa Wells told BBC News, several of the observed decay paths already show apparent departures from what would be expected. These could well just be statistical flukes that will get ironed out with the addition of more data, but they might also point to something significant.
So just what are we left with? Well, we almost certainly have ourselves a new elementary particle, the first such particle found in nearly twenty years. All the data announced today is still preliminary - the first formal analyses of the CMS and ATLAS data should start rolling out towards the end of the month.
And the Large Hadron Collider is still set to run through the end of the year, meaning there's still many months more of data still left to be gathered. All that should help confirm once and for all whether what was announced today is really the Higgs boson, and whether it's really the Higgs boson predicted by the Standard Model. This is unquestionably a massive breakthrough in our understanding of physics — but there's every reason to think today's announcement will be dwarfed by all the breakthroughs still to come.
For more from the CERN physicists themselves, check out a fairly technical webcast of the results and subsequent press conference from today's annoucement. For some more general background on just what the Higgs boson is, try here and here.
Image of Kai Schulte's Visual Higgs Boson sculpture by Michael J. Linden on Flickr.