“We knew that we had indeed done something that was very different and very exciting, but we still didn’t expect it to have something to do with physical reality.” –Gerald Guralnik, co-developer of the Higgs mechanism
Might as well make this entire week “Higgs week” here on Starts With A Bang, given how important yesterday’s discovery/announcement was! It isn’t every day, after all, that you see a theoretical physicist on the 7PM news. (Video here.)
(So proud of Portland, OR’s local TV station, KGW NewsChannel 8, for being willing to promote science to the whole city and have me on last night!)
Let’s just take a little time today — now that we’ve confidently announced the discovery of the Higgs — to recap what we now know.
All the matter we know of is made out of quarks and leptons of various types, with the most common being the up-and-down quarks (that bind together to make protons and neutrons) and electrons (that bind with nuclei to make atoms). There are heavier fundamental quarks and leptons that will decay into the light ones with very short lifetimes, but they’re just as fundamental to the Universe as the ones that make us up. All quarks and leptons have an intrinsic angular momentum — known as spin — equal to Planck’s constant (ħ) divided by 2.
There are also the gauge bosons: the fundamental particles responsible for the fundamental forces. There are the gluons, responsible for the strong force holding atomic nuclei and individual nucleons together, the weak bosons, responsible for radioactive decay and neutrino interactions, and the photon, responsible for electromagnetic interactions, radiation, and light. All of these gauge bosons have a quantum mechanical spin of Planck’s constant (ħ), double the value for quarks and leptons.
On the level of individual particles, we do not have a complete understanding of how gravity works. Our best theory for that is Einstein’s general relativity, which treats space-time as a fabric, and where matter and energy are responsible for curving this fabric. The matter and energy in the Universe determine the shape of the fabric, and then the particles in the Universe follow the path determined by that fabric.
The shape of the fabric is also important for quantum field theory; all of these particles we know of exist and interact in this curved spacetime, and the shape of this spacetime must be taken into account to get the correct predictions for the behavior of particles in the Universe. That’s our best understanding of gravitation.
The first and only fundamental particle with no spin. The particle that comes from the mechanism responsible for the masses of all the other fundamental particles, including the Higgs boson itself. The final piece of the standard model puzzle required to explain the strong, weak, and electromagnetic forces and all of the particles therein.
It took us decades of recreating temperatures and energies here at particle accelerators on Earth not found anyplace else to figure this out. The conditions we create in our most powerful accelerators are not found in the center of the Sun, nor in the central core of the Milky Way galaxy, nor around neutron stars and black holes, nor in the cosmic supernova explosions that give rise to all the heavy elements.
These conditions have not existed* in the Universe, in fact, since the very early stages of the Big Bang, when the Universe was less than a microsecond old!
But yet, here we are, having successfully accelerated protons up to a record 299,792,450 m/s, just 8 m/s shy of the true speed of light, and collided them with protons moving the same speed in the opposite direction. Do this billions upon billions of times with a giant particle detector around the collision point, and on very rare occasions, you’ll be fortunate enough to create a Higgs boson, whose decay remnants we can detect.
And now, at long last, the Higgs boson — the last undetected particle from the standard model — has been discovered. We’ve measured its mass and its spin, but not its width or its lifetime, yet, and there are still a bunch of unanswered questions. But for now, at least, I’m still celebrating the one question we did answer: the Higgs mechanism is correct, the Higgs boson (a fundamental, spinless scalar particle) does exist, it has a mass of 125-126 GeV, and the standard model is now complete!
We’ll take a look at the unanswered questions — and what’s next for physics and the LHC — very soon, but in the meantime, enjoy the continue Higgs-celebration, and if you’ve made it this far, enjoy my appearance on yesterday’s evening news!
* — Okay, so occasionally there are ultra-high-energy cosmic rays that are energetic enough to produce a particle such as this. But they’re extremely rare and their origin is not understood, so for all intents and purposes, these conditions don’t exist in the Universe today.