It’s the biggest discovery in particle physics of the last 30 years! Or not . . . But probably it is, barring the 1 in 3.5 million chance that it is a statistical fluke. Such are the vagaries of the careful statistical analysis of Higgs boson search being done by physicists at the Large Hadron Collider (LHC) in Switzerland.
At 9:00 AM Switzerland time on the 4th of July, at universities and laboratories around the world, physicists hunched in front of computer screens and gathered in lecture halls to see the much anticipated release and report of data from the LHC. The data was conclusive for finding “something Higgs-like,” but the scientists were circumspect of what exactly they’d found. The experiment favored the existence of a Higgs-like boson with a mass of approximately 125 billion electron volts. What does this mean? What is the Higgs boson? How are people looking for it? Why is it important?
The Higgs boson is the only missing particle in the so-called “Standard Model” of particle physics. This model has been established over the last 40 years and has been exceedingly (perhaps unreasonably) effective at accounting for the results of almost every elementary particle physics experiment ever done. The Higgs Boson is an essential part of it.
In the early 1960’s, the existing framework for particle physics was very successful in some respects. However, in contradiction to experiments and everyday experience, it predicted that elementary particles had no mass. Then a series of 3 separate papers, known as BEHGHK for its authors (Brout and Englert; Peter Higgs; Guralnik, Hagen, and Kibble) proposed what is now called the “Higgs mechanism,” which explains how a certain class of elementary particles could acquire mass. Elementary particles are the most basic particles in the universe.
This was a huge deal because it contributed to a well-defined theory as to why the world around is as it is. Even despite the lack of experimental verification, the Higgs mechanism quickly became most accepted game in town.
To understand the prediction, we need a few elementary facts about quantum mechanics. First, energy in quantum mechanics generally comes in discrete chunks that we call “quanta.” For example, light is produced or absorbed in discrete amounts. We call these pieces of light or particles of light, photons. Establishing this fact is what got Albert Einstein his Nobel prize. Second, we believe that all the universe is pervaded by certain background media that we call “fields,” which encode certain essential qualities of particles that are known to exist. The electric/magnetic field is an example, and it corresponds to these photon particles of light. The third essential fact is that wha
t we call particles are actually the quantized “ripples” in these fields, much as ripples exist on a pond into which a pebble is tossed.
What was proposed in the three landmark papers of BEHGHK is that the universe is pervaded by a certain field, which we now call the Higgs field. And through interactions with the Higgs field, these certain classes of particles will acquire their mass. The physical mechanism works something as follows: We recognize physical mass as inertia, which is the quantity that tells us how hard it is to accelerate or move an object. In the Higgs mechanism, the principle of mass generation is much the same as a ping-pong ball being dragged across the surface of this pond (the Higgs field). The ping-pong ball is very light, but it can nevertheless experience a strong drag force that impedes any attempt to accelerate it. This is the “Higgs mechanism” for mass generation, and was proposed independently in these three papers. Different particles have different masses depending on how strongly they interact with the Higgs field.
The Higgs boson is the smallest quanta of energy of the Higgs field. In other words, it is the ripples of water on the pond. And the discovery of this ripple is a clue that the Higgs field exists. Essentially, the experiments in Switzerland are dropping pebbles in the Higgs field and observing its excited state as a particle. Similar to a pond’s surface on a still day, we see only the ripples on the pond when a stone is thrown in, but not the still surface.
So what was found and how was it found?
The Higgs boson cannot be directly observed like a photon or electron. Instead, scientists can observe its decay products through collider experiments. This is where the Large Hadron Collider is needed. The LHC is the most complex machine ever built. It is 26 kilometers of tunnel that straddle the borders of Switzerland and France near Geneva. Inside are two counter-propagating streams of protons moving at more than 99.9999% the speed of light. The tremendous energy in the colliding proton beams is concentrated at a small point and, through Einstein’s famous E=mc^2, their energy can be converted into the mass of new, possibly unstable particles. The Higgs boson will be a very unstable particle and probably lives for only millionth of a second before it deca
ys into other particles. These are the decay products scientists are trying to measure.
But there are many ways to make similar decay products without a Higgs boson. So the point of the experiment is to observe a small excess of decay products that would be slightly enhanced if a Higgs boson exists. In recent analysis, scientists detected a particle wih a mass equivalent (through E=mc^2) again of 125 billion “electron volts.” That is one of the heaviest subatomic particles discovered, with a mass approximately equivalent to an iodine atom, and right in the middle of the expected mass range for the Higgs boson. Furthermore, scientist can tell that this particle is a boson, which is to say that it is not a fermion which are the particles that make up everyday matter. But the signal (the bump) is very, very small. To the uninitiated eye it is almost indistinguishable from just random noise (See: Figure 1). But through advanced statistical analysis, scientists have shown that the bump they see is likely to be real, with odds that it is not 1:3,5000,000.
So what now?
The next few months are going to be very interesting. In principle, a Higgs boson can decay into a variety of different particles. Researchers are going to study exactly how the particle they have found decays and see if it differs from expectations. If not, it will confirm that this “Higgs-like” particle is actually THE Higgs.
In my opinion, it is. The scientists conducting these experiments and analyzing the data are very, very careful and generally prone to 
understatement. But what they find may also be just the tip of the iceberg; numerous models predict not just one Higgs boson, but many. There could be more bumps. And moreover, we know from observations of Dark Matter and Dark Energy among other sources, that the Standard Model is ultimately incomplete. There HAVE to be new particles out there waiting to be discovered.
Is this the only surprise waiting for us? Is it just the first member of a larger family of such particles, or is it something else entirely? The LHC is going to extend this year’s data collection period for another 2-3 months before it shuts down for a two-year machine upgrade. This is the science discovery of the decade, but is it the science discovery of the century? Stay tuned!