How the Standard Model of Particle Physics Explains Reality as We Know It

From the outside, the high-speed collisions of atomic nuclei inside particle accelerators like CERN’s Large Hadron Collider (LHC) may seem like they have very little in common with more mundane objects like your morning coffee or fluffy slippers. However, on a subatomic level, your favorite mug is made up of exactly the same stuff that’s being shattered at the LHC, and it can all fit within a neat framework that physicists call the Standard Model of particle physics.

Solidified in the 1970s, the Standard Model consists of 17 fundamental particles that make up a big chunk of (but not quite all) matter in the universe. There are two main camps these 17 particles can be sorted into: “fermions” and “bosons.” Roughly speaking, you can think of fermions as the “stuff” of matter and bosons as the forces moving that stuff around. Within the fermion family there are six “leptons,” which includes electrons, and six particles called “quarks.”

Conceptual illustration showing the Standard Model particles with their heavier superpartners introduced by the supersymmetry (SUSY) principle. In supersymmetry, force and matter are treated identically. Using supersymmetry, physicists may find solutions for a bunch of problems like the weakness of gravity, the low mass of the Higgs boson, the unification of forces, or even dark matter.

While we’re taught in school that matter is made up of protons, neutrons, and electrons, only one of these particles is considered “fundamental,” meaning it cannot be broken into smaller pieces. Because of this, only electrons can be classed as a fundamental lepton particle, and protons and neutrons are instead represented by their respective quarks. In particular, protons and neutrons are both a mix of “up” and “down” quarks.

In the wild, it’s these up and down quarks that physicists observe most often, but there are also four other variations of these quarks that are increasingly heavier and less stable. Related to up, you also have “charm” and “top” quarks, and for down, you have “strange” and “bottom” quarks.

The lepton family also includes a kind of “superlight” particle, called a “neutrino,” that comes in three flavors associated with the other non-quark leptons: tau neutrino, muon neutrino, and electron neutrino. (“Flavor” is the name that physicists give to different versions of the same kind of particle.) Neutrinos are often referred to as a “ghost” particle because they rarely interact with other matter and can only be spotted through the tracks they leave behind.

Together, leptons and quarks make up all matter we interact with in our universe. However, these particles would be nothing without bosons to ferry them around or stick them together. For all 12 fermions, there are only five known bosons:

1. Photons, which carry the electromagnetic force

2. Gluons, which hold quarks to each other with the strong force to help create atoms

3. W and Z bosons, which are responsible for the weak force and radioactive decay

4. Higgs, the most recent addition to the group, which gives mass to other particles

Altogether, these bosons create four out of five fundamental forces, with gravity being a glaring exception. Because gravity’s effect at the subatomic level is so tiny, it cannot easily fit into the framework of the Standard Model—despite physicists’ best efforts.

Gravity’s omission from this family picture is just one of several problems with the Standard Model, leading more and more physicists to believe that its reign as the ultimate physics theory may be waning. In addition to failing to incorporate gravity, the Standard Model also doesn’t offer up an explanation for the massive amounts of dark energy and dark matter that make up 95 percent of the universe, according to NASA.

There are also rumblings through other sectors of particle physics, like neutrino research, of observations of particle behavior not quite adding up with the Standard Model’s predictions. Does this mean the whole model should be thrown away? Probably not. However, it does mean that physicists are becoming more interested in moving “beyond” Standard Model physics—that is, looking to uncover which kinds of unknown forces may also be tugging on these particles. In its third run, which began earlier this month, the LHC will be looking for some of these incongruities.

Depending on what physicists find in the years to come, our understanding of the subatomic world, and the universe itself, may be about to change forever.