On July 4 two independent collaborations announced the observation of a new particle which has properties consistent with what is known as the Higgs boson. European physicists at CERN — the European Center for Nuclear Research — began waiting outside of the auditorium at 10 p.m. the night before to get a seat for the historic seminar. By 7 a.m. the next morning there were hundreds of people in line. Those physicists who couldn't attend in-person gathered in groups at their respective institutions to watch the broadcast as it streamed live to the rest of the world. Each time spokespeople from the two organizations which ran the experiments — ATLAS and CMS — announced the significance of their findings, the room erupted in applause.
As two of the physicists who woke up early enough to get seats in the room, we can say that for us it was well worth waking up at 3 a.m., sitting outside until 7 a.m. and then barely making it through the door!
So why are physicists so excited?
The Higgs boson is the final piece of the Standard Model of particle physics, the most successful and accurate theory of particles and forces. Like the periodic table of elements, the Standard Model attempts to classify all the constituents of matter in the universe. It includes leptons (for example the electron) and quarks (which form protons and neutrons). It also includes particles which mediate forces between the quarks and leptons (like electromagnetism). The Higgs boson is thought to give mass to some of these particles while leaving others, like the photon, massless.
At the LHC, we smash protons together at very high energies because we want to produce and measure the properties of heavy particles which we wouldn't otherwise observe. These particles only exist fleetingly before decaying. The higher the energy of the accelerator, the heavier the particles we can produce. Since the LHC is the highest energy particle accelerator ever built, we can produce many more massive particles than any previous experiment.
The Higgs boson has been predicted by theorists because it solves the problem of “electroweak symmetry breaking.” Broken symmetry is actually something you have encountered before, just not by this name. For example, you have a ball at the top of a mountain. From the ball's point of view, the mountain is symmetric and all directions are the same. However, when you drop the ball it will roll in a particular direction. Once it has chosen a direction, the symmetry has been broken. The mountain is still symmetric by itself, but not from the ball's point of view. This is an example of a broken symmetry.
So what does a ball rolling down a mountain have to do with the Higgs? To answer that question, let's look at a very basic property of matter in our universe: mass.
Why do particles in our universe have mass? The Higgs is like the ball in our example. When it rolls to one side of the “mountain”, it breaks a different kind of symmetry between two forces of nature, the weak force responsible for radioactive decay and the electromagnetic force. The particles which mediate the weak force become massive, and the photon which mediates the electromagnetic force does not.
The Higgs is one of the main reasons that the LHC was built, and finding it is something thousands of physicists have been working towards for decades. We do not yet know if the new particle we have observed is actually the Higgs or something more exotic with similar properties. Many other theories which may explain the shortcomings of the Standard Model (including phenomena such as dark matter) predict a similar particle. Much more work still needs to be done, but we have taken a large step towards understanding the fundamental structure of our world. It is a major accomplishment and reason for celebration.