Scientists using the Large Hadron Collider in Geneva have announced the discovery of a new subatomic particle to very high confidence that is consistent with what we expect the Higgs particle to look like.
This plot shows the discovery as seen in one of the LHC detectors. Hang tight, and I’ll explain it!
OK, the quick version. The Higgs particle is extremely important, because the Standard Model of particle physics – the basic idea of how all particles behave – predicts it exists and is what (indirectly) gives many other particles mass. In other words, the reason electrons, protons, and neutrons have mass is because of this Higgs beastie. Last year, the Guardian put up a nice article explaining this. A more technical discussion is on Discover Magazine’s Cosmic Variance blog from 2007. Sean Carroll has been live-blogging the announcement, and has lots of good info as well.
This particle is very hard to detect, because it doesn’t live long. Once it forms it decays in a burst of energy and other particles (think of them as shrapnel) extremely rapidly. The only way to make them is to smash other particles together at incredibly high energies, and look at the resulting collisions. If the Higgs exists, then it will decay and give off a characteristic bit of energy. The problem is, lots of things give off that much energy, so you have to see the Higgs signal on top of all that noise.
So, you have to collide particles over and over again, countless times, to build up that tiny signal from the Higgs decay. The more you do it, the bigger the signal gets, and the more confident you can be that the detection is real. I described all this in detail last December, when preliminary results from LHC were announced. I strongly urge you to read that first!
Back now? Good. So last year, an excess signal was seen at an energy around 125 GeV – that’s a unit of energy physicists use, and it also indicates the mass of the particle decaying. Because energy and mass are interchangeable at some level, detecting the energy emitted when a particle decays tells you its mass.
A proton has a mass of about 1 GeV, so this excess found is about 125 times that much. Last year’s results were tantalizing, but the strength of the signal only led to a confidence level of about 90% that it was real. Nice, but not enough to claim a discovery.
Today that all changed. Two different detectors at the LHC both independently found a strong signal between 125 and 126 GeV at about the 5 sigma level – that means they can claim a 99.9999% confidence this signal is real! This means they found a previously undiscovered particle which, as it happens, is within the range of mass the Standard Model predicts for the Higgs particle! That’s what that plot above shows: a bump in the energies detected, and it’s seen so strongly that it can be called a discovery.
Now technically, that’s all the physicists can say: the particle is definitely there. But is it the Higgs? Well, to be fair, they can’t actually say that. But if it walks like a Higgs, looks like a Higgs, and quacks like a Higgs… yeah.
So there you have it. A new fundamental particle has been found, and if it’s the Higgs – which it really really really looks like it is – is the first step to our truly understanding such basic concepts as mass and gravity in the Universe. It’s technical, and it’s complicated, and it’s the result of a vast amount of time, money, and effort by thousands upon thousands of people… but it’s real.
And it’s only the first step. There’s much work to be done. But oh, what a step. The Universe has once again done something wonderful — let us peek behind the curtain and get a glimpse of its inner workings.
Never forget this either: we humans did this. The discovery of this new particle, and the vast potential it has, was all because we’re curious. This huge machine, the LHC, was built solely because we wanted to find things out, and some people had the vision to fund it and build it. When we wish to explore, when we wish to see what’s over the next hill, wonders unfold before us.
All we have to do is want it enough.
Image credit: CERN
Today, scientists at CERN in Geneva announced their results for their search for the Higgs boson, a subatomic particle that, if it exists, is thought to be responsible for giving other particles mass. It’s no exaggeration to call it a keystone in quantum mechanics, and finding it for sure will be a huge accomplishment for particle physicists.
So, did they find it?
Maybe. Then again, maybe not.
Um, what? OK, this’ll take a wee bit of explaining.
Last things first
|I said Higgs, Magnum. HIGGS.|
First, the conclusion, so at least you have that in mind as you read the rest. There are two experiments running at CERN looking for the Higgs particle. They don’t smash particles together, look around with magnifying glasses and tweezers, and then yell “AHA!” when they find one. Instead, they build up a picture of it after doing gazillions of particle collisions. After a year of runs, both experiments see something that might be Higgs, but they’re not 100% sure. One sees something at about the 94% confidence level, the other at 98%. That’s pretty good, but it’s not enough to be completely sure. It seems likely they’ve found something, but it’s like a fuzzy picture: it looks like Higgs, but it still might be something else.
So why can’t they be sure one way or another?
Basically, what the Large Hadron Collider at CERN does is whip protons around at nearly the speed of light, then smashes them into each other. At that speed they have huge energies, and when they collide that energy gets converted into matter: other particles. Like shrapnel, these new particles explode away from the collision site. Many of these new particles aren’t stable; they decay into yet lower energy particles after incredibly short time intervals. For example, electron and protons are almost certainly stable over long times (like the lifetime of the Universe), but neutrons decay after only a few minutes, turning into a proton, and electron, and a particle called an antineutrino.
So these daughter particles from the proton collisions in LHC decay, and they have daughter particles, and some of those decay, and so on. At the LHC there are two ginormous detectors called ATLAS and CMS. Both of these, in essence, measure the energy of the particles that hit them; like forensics team, they look at the aftermath of the collision and try to work backwards to figure out what happened.
We know to some extent how much energy is expected from these collisions due to all the particles that are currently known, so those can be accounted for. But if there’s some excess of energy, that could very well indicate a new particle. And we have theories as to how much energy the Higgs particle should have. So the energies are measured, calibrated for known particles, and the excesses are examined.
What both experiments found is an excess of energy — a bump in the graph — indicating a particle that has an energy* about 125 times that of a proton — right in the expected range for the Higgs particle. That’s exciting! But what they’re doing is counting up things statistically, so they can’t be 100% sure. The bump in the graph is still fuzzy.