How the Standard Model of Particle Physics Explains Our World


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

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Solidified in the 1970s, the Standard Model consists of 17 fundamental particles that make up much (but not quite) of the matter in the universe. There are two main camps into which these 17 particles can be classified: “fermions” and “bosons”. Roughly speaking, you can think of fermions as the “stuff” of matter and bosons as the forces that move that stuff. In the fermion family, there are six “leptons”, which include electrons, and six particles called “quarks”.

artwork conceptualizing supersymmetry
Conceptual illustration showing Standard Model particles with their heavier superpartners introduced by the principle of supersymmetry (SUSY). In supersymmetry, force and matter are treated identically. Using supersymmetry, physicists can find solutions to a host of problems such as weak gravity, the low mass of the Higgs boson, the unification of forces, and dark matter.


While we are taught in school that matter is made up of protons, neutrons and electrons, only one of these particles is considered “fundamental”, which means that it cannot be further broken down. small pieces. For this reason, only electrons can be classified as a fundamental lepton particle, and protons and neutrons are instead represented by their respective quarks. In particular, both protons and neutrons are a mixture of “up” and “down” quarks.

In nature, it is these up and down quarks that physicists most often observe, 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 “super light” particle, called a “neutrino”, which comes in three flavors associated with the other non-quark leptons: the tau neutrino, the muon neutrino and the electron neutrino. (“Flavor” is the name physicists give to different versions of the same type of particle.) Neutrinos are often referred to as “ghost” particles because they rarely interact with other matter and can only be spotted through traces they leave behind. .

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

  • photonswhich carry the electromagnetic force
  • Gluonswhich hold quarks together with the strong force to help create atoms
  • W and Z bosonsresponsible for the weak force and radioactive decay
  • Higgthe most recent addition to the group, which gives mass to other particles

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

    The omission of gravity from this family picture is just one of many problems with the Standard Model, leading more and more physicists to believe that its reign as the ultimate physical theory may be waning. decline. In addition to not incorporating gravity, the Standard Model also offers no explanation for the massive amounts of dark energy and dark matter that make up 95 percent of the universeaccording to NASA.

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      There are also rumblings in other areas of particle physics, such as neutrino research, observations of particle behavior that do not quite match the predictions of the Standard Model. Does that mean the whole model has to be discarded? Probably not. However, this means that physicists are increasingly interested in going “beyond” Standard Model physics, i.e. in seeking to discover what kinds of unknown forces may also be pulling on these particles. . In its third run, which began earlier this month, the LHC to be looking for some of these incongruities.

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

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