Heralded in headlines as “the God particle,” the discovery of Higgs
boson generated an almost electric media buzz last July. We're told repeatedly in news reports that the Higgs boson "gives particles mass," but what does that mean? Like all short, pithy statements in
physics there is some truth contained therein, but the reality is more nuanced and subtle than befits a sound bite.
To understand the significance of the Higgs particle we have to learn about the basic particles and forces that make up the universe according to the current and highly successful “standard model” of elementary particles. I’m sure you recall that all atoms are made up of neutrons, protons and electrons, even if you’re a little vague on what those words mean. The standard model tells us that neutrons and protons are made up of more basic particles called “quarks.” There are two different types of quarks involved, called “up” and “down,” a neutron contains one up and two down quarks while a proton contains two up quarks and a down quark. The electron is fundamental in the standard model and is not made of more basic parts. There is one more fundamental particle that is common in our universe, called the “electron neutrino.” Neutrinos are ghostly things, with no electric charge and very, very small mass— neutrinos were thought to have zero mass until the mid 1980’s. Electrons and neutrinos are examples of particles called “leptons,” just like up and down are examples of quarks. So all stable matter observed in the universe is composed of up or down quarks or electrons or neutrinos.
“...Three of the four known forces in the universe are described by the standard model.”
The story does not end there: it turns out that there are quarks called “charm,” “strange,” “top” and “bottom.” These quarks are heavier than the up and down, and will rapidly disappear and are replaced by up and down quarks. (Actually, in the right circumstances a down quark will spontaneously disappear and be replaced by an up quark, which is why an isolated neutron will turn into a proton in a process called “beta decay.”) We know about these heavier quarks because we can create them in particle accelerators such as Fermilab in Illinois and the Large Hadron Collider in Europe. Similarly to the heavy quarks there are also heavier leptons, called the “muon” and “tau.” These particles are identical to electrons except they have higher mass: the tau is more than three thousand times heavier than the electron. The muon and tau are paired with the “muon neutrino” and “tau neutrino.” Like the heavy quarks the muon and tau rapidly disappear and become electrons, though the muon and tau neutrinos possibly last forever. All of these heavier quarks and leptons have been observed in particle accelerators.
Do you notice the pairs in this collection of particles? The quark pairs are (up, down), (charm, strange) and (top, bottom). The lepton pairs are (electron, electron neutrino), (muon, muon neutrino) and (tau, tau neutrino). This pairing is one of the foundations of the standard model.
In addition to the particles, three of the four known forces in the universe are described by the standard model: the electromagnetic force familiar from magnets and electricity; the strong nuclear force that holds quarks inside protons and neutrons as well as holding protons and neutrons in the nuclei of atoms; and the weak nuclear force which changes particles from one kind into another. It is the weak nuclear force that changes the heavy particles like the strange quark or muon into lighter particles like the up quark or electron.
The weak force does not change any particle into any other particle. It can only change one element of the pairs described above into the other element. For example a down quark can change into an up quark or a muon into a muon neutrino (I’m glossing over a technical issue: in reality the quarks are mixed so a down almost always changes into an up but rarely changes into a charm and even more rarely into a top). Because this is the quantum world, the forces are “carried” by so-called “force-carrying particles” The electromagnetic force is carried by one type of particle called the photon. The weak nuclear force is carried by three types of particles called “W-plus”, “W-minus” and “Z-zero.” The strong nuclear force is carried by eight types of particles collectively called “gluons.”
When the weak nuclear force turns one particle in a pair into the other particle in the pair, it emits one of the W particles, and the W rapidly becomes another pair. So, for example, the muon will turn into a muon neutrino, and emit a W particle that turns into an electron— electron neutrino pair (actually an electron anti-neutrino for those who know about anti-matter). This is how a muon ultimately turns into an electron and some neutrinos. You can see that the pairing structure is at the foundations of the standard model description of the weak force.
The fourth force, not described by the standard model of particle physics is gravity. Gravity is described by Einstein’s General Theory of Relativity.
These particles and forces cover everything we’ve observed in the universe. So where is the Higgs particle and why is it important? The brief answer is that the Higgs is the conceptual glue that holds the weak and electromagnetic portions of the standard model together. The Higgs does so by unifying the electromagnetic and weak forces into a single force, called the “electroweak force,” and in so doing causes the W and Z force carrying particles to have mass as well as giving mass to the quarks and leptons. In fact, without the Higgs the theoretical structure of the standard model would completely fail to describe quarks, leptons and the three forces described above. We can see that the Higgs is enormously important to the standard model, which is why it has been referred to as the “God particle.” Let me tell you a little about why the Higgs is so important.
The quantum description of matter was complete by the late 1920s, but it was unable to fully describe how light (the electromagnetic force) and matter interacts. This problem was solved in 1949 by the development of “quantum electrodynamics” (QED). QED is a wonderfully elegant theory that describes the interaction of electrons (or any particle with electromagnetic charge) with photons, the particle that carries the electromagnetic force. QED is also the most precisely tested theory in history, with predictions matching observation to better than one part in a trillion. QED is unquestionably a smashing success, and it was hoped that QED could be extended to the weak and strong nuclear forces. The problem was that QED only works for force-carrying particles that have no mass. This is true for the photon, but lots of evidence indicated that whatever particles carry the weak nuclear force, they have mass. So at first sight a theory like QED cannot describe the weak nuclear force.
Enter the Higgs particle (which was actually theorized by several people including Peter Higgs). The Higgs is a very strange beast, with the property that when there are no Higgs particles there is more energy than when there are a few Higgs. Because the universe will always try to be in its lowest energy state there will always be Higgs particles around. But these Higgs particles interact with leptons and quarks in a way that gives them mass. In addition the Higgs gives the W and Z force-carrying particles their mass. We cannot, however, observe the Higgs particles directly.
But here is the amazing thing: when there is lots of energy, say because things are very hot such as just after the big bang, then the “lowest energy” is above the energy of zero Higgs particles. Then the Higgs disappears, quarks, leptons and the W and Z particles lose their mass, and the electromagnetic and weak forces unify into the same force. Because in this state the W and Z have no mass, a theory much like QED works!
The Higgs is specially hand-designed so that a QED-like theory still works even when energy is low, there are Higgs in abundance, and force-carrying particles are massive. This hand-design looks, at first sight, unnatural and somewhat arbitrary. But it works spectacularly well, predicting the masses of the W and Z particles before they were observed, and allowing a coherent theoretical description of the weak and electromagnetic forces. The standard model with the Higgs is the only theory we know of that is capable of describing the weak force.
Which brings us to the “discovery” of the Higgs itself. It takes a lot of energy to create the circumstances where the Higgs itself changes into something we can directly observe like electrons and up and down quarks (in protons and neutrons). This is why it took us until this year to finally “see” the Higgs particle itself. Sure enough, the observations that were announced in July matched exactly those predicted by the Higgs portion of the standard model.
The Higgs particle explains much in the standard model, such as the mass of the W and Z, and provides the only mechanism we know of that allows the quarks and leptons to have mass. But there is much in the standard model that is not explained, instead being put in “by hand.” The fact that there are three “generations” of quark and lepton pairs, the quark mixing alluded to above, the relative strength of the three forces, the relative masses of the particles, the relationship between the strong and electroweak forces, and the Higgs particle itself. While successful, the standard model is clearly not the whole story.
Discovery of the Higgs particle tells us that even the seemingly arbitrary and hand-designed features of the standard model are on the right track. By carefully measuring and studying the Higgs we hope to discover hints of where that track is leading in ways that discovering another quark or lepton could not. We hope that track leads to a deeper theory, which will explain what the current standard model cannot. That’s why the Higgs is so important; it represents our ability to move our understanding of the universe forward.
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Steve Bryson
is the lead mathematician on the Kepler mission at NASA Ames Research Center in Mountain View. His previous work at NASA has included numerical methods for non-linear evolution equations, and pioneering applications of virtual reality for scientific visualization.