4 July 2012

Happy Higgs of July!

Posted by Ryan Anderson


I don’t normally talk about particle physics here. Planets are really more my thing, but today I need to make an exception for some big news. Scientists at CERN and Fermilab are reporting that they have found the Higgs boson, the missing link in the leading theory of how our universe works at the subatomic scale.

The new particle was detected at a mass of 125 to 126 GeV at a “5 sigma” confidence level by the CMS and ATLAS detectors on the Large Hadron Collider, and at Fermilab they have detected a bump at 115-135 GeV with a 2.9 sigma confidence. “5 sigma” means that scientists are 99.9999427% confident that the detection is not a statistical fluke. GeV stands for “giga-electron volts”. One electron volt is the amount of energy a particle with the charge of on electron gains by being accelerated through an electric potential of one volt, and is the standard way of expressing the mass of high-energy particles. In the press release, the spokesperson for the CMS experiment, Joe Incandela said: “The results are preliminary but the 5 sigma signal at around 125 GeV we’re seeing is dramatic. This is indeed a new particle. We know it must be a boson and it’s the heaviest boson ever found

That bump is the signature of the Higgs, as seen by CMS. Image Source: CERN

The remarkable thing about the Higgs boson is that it is being detected right where our current best model of particle physics – the Standard Model – said it should be. This is a Big Deal because it confirms that the standard model is correct, and also because the Higgs is the particle responsible for giving all other particles mass. This New Scientist article does a nice job of explaining:

The Higgs boson gives all elementary particles mass, allowing for the existence of matter. It is the fundamental unit, or quantum, of the Higgs field, an all pervading entity that all particles must pass through.

Some, like the photon, slip through unhindered – they are massless. Others, though, must struggle like a fly trapped in treacle. The Higgs and its field are required by the standard model, but had never been conclusively detected before today’s report.

If the Higgs didn’t exist, it would have required a major rethinking of how the universe works, but instead it looks like we’re on the right track.

Of course, there are caveats. The mass of the new particle has been measured, but to be really sure it is the Higgs, other properties such as its quantum spin need to be measured. And then, even if the particle is confirmed to be the Higgs, the Standard Model is still not perfect because it doesn’t explain how gravity works. Einstein’s theory of general relativity does a fantastic job of explaining gravity on large scales, but it breaks down when you get strong gravity fields at subatomic scales (such as in a black hole). The Standard Model does a great job of explaining all the other forces in the universe, but in all particle physics on earth, gravity is not a big deal so it is not explained by the model.

The physics results are really important, but my favorite part of the story is that Peter Higgs, the physicist who first suggested the presence of the Higgs field, was present at today’s announcement to hear his life’s work vindicated. Now that’s what I call job satisfaction!

Update: You should go check out this live-blog of the announcement from Cosmic Variance to get a taste of what proper theoretical physicists think of the announcement. Most interesting was their hope that the detected particle doesn’t fit the standard model exactly because that would mean there is new physics going on.