The Standard Model of Elementary Particles

The Standard Model of Elementary Particles
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The Elementary Particles of Matter

The atom is the smallest, most basic unit of a chemical element. It cannot be divided farther and still be that element. But atoms are made up of smaller particles called protons, neutrons, and electrons. Protons and neutrons form a cluster at the center of the atom, called a nucleus, and the total number of protons is what defines the element. To complete the structure of the atom, a number of electrons equal to the number of protons orbit the nucleus.

Protons and neutrons, in turn, are made up of even smaller particles known as quarks. There are six types, or “flavors,” of quarks: up, down, charm, strange, top, and bottom. A proton is two up quarks and a down quark, and a neutron is two down quarks and an up quark.

Quarks and the Strong Force

Quarks are half of the fundamental matter particles in the Universe. According to the Standard Model of particle physics, each elementary particle comes with an opposite antiparticle, so there are six quarks and also six antiquarks. Quarks are never found in nature as individual, isolated particles because they bind very strongly to each other via the strong force.

The strong force is one of the four fundamental forces of physics. Like the more familiar electric force, it has positive and negative charges. Unlike the electric force, there are three of them rather than just one. Physicists find it convenient to think of them as if they were three primary colors - red, green, and blue - because the way they work is just like the RGB color system.

Each quark has a color charge, and each anti-quark has an anti-color charge (cyan, magenta, yellow). Quarks and anti-quarks must combine with each other in such a way that the total color charge is “white.” Quarks therefore combine in triplets - one red, one green, one blue - called baryons, or pairs of one quark and one anti-quark (e.g. red, cyan) called mesons. Combinations of quarks are called hadrons. Protons and neutrons, both baryons, are the smallest types of hadrons.

The Leptons

The other half of the elementary particles in the universe are called leptons. There are six of these: electron, electron neutrino, muon, muon neutrino, tau, and tau neutrino. Each comes with an antiparticle version as well. The anti-electron has its own name, the positron.

The biggest difference between leptons and quarks is that leptons don’t have color charges, and therefore aren’t affected by the strong force. In addition, neutrinos also don’t have electric charges, so they aren’t affected by electromagnetism either. They have weak charges, but the range of the weak force is extremely small - much smaller than that of the strong force. All of this makes it challenging to get neutrinos to interact with anything else for scientific study.

Collectively, quarks and leptons are called fermions. Everything else in the universe is made up of combinations of fermions.

The Elementary Particles of the Forces

Fermions interact with each other via the four fundamental forces - gravity, electromagnetism, strong force, and weak force. Each fermion can have a charge from each force, though in some cases the value is zero.

Another 12 elementary particles mediate the latter three forces. These are called gauge bosons. 8 gluons mediate the strong force, the photon mediates the electric force, and the W+, W-, and Z bosons mediate the weak force.

The main difference between fermions and bosons is that fermions obey the Pauli exclusion principle, while bosons do not. The Pauli exclusion principle states that no two identical particles can hold identical quantum states at the same time - roughly, no two fermions can be in the same place at the same time. Bosons, on the other hand, can overlap on top of each other as much as they want.

The Color Force Bosons

The Standard Model of Particle Physics

Eight types of gluons mediate the strong force, which is also known as the color force. Quarks within a hadron stick together by passing gluons to each other. They emit more gluons as they get farther away from other quarks - which means that the strength of the strong force actually goes up with distance. Within an atomic nucleus, all the gluon passing also causes protons and neutrons to bind together, which in turn gives rise to all the different chemical elements.

Unlike the other gauge bosons, gluons have their own charges for the force they mediate. There are eight types of gluons because there are eight possible pairs of color-anticolor they can have. As the quarks pass color-charged gluons to each other, they swap out their own color charges according to the color charges of the gluons. The mathematical details of how all this works is known as quantum chromodynamics (QCD).

The Electroweak Bosons

Three gauge bosons mediate the weak force: the W+, W-, and Z bosons. These are roughly a hundred times the mass of a proton - very high for a subatomic particle. The high mass makes them much shorter-lived than they would be if they had no mass. Since they can’t move very far during their extremely short lifespans, the range of the weak force is very small.

One gauge boson, called the photon, mediates the electric force. It has no mass, an infinite lifespan, and unlimited range.

According to electroweak theory, the weak force and electric force are actually facets of the same force. At high enough energies, such as those found shortly after the universe began, they merge into a single force - an electroweak force - mediated by four massless gauge bosons and with unlimited range.

The Higgs Boson

According to the electroweak theory, the W+, W-, and Z bosons should be massless even at low energies. The Higgs mechanism was invented to explain why they have mass. Basically it calls for mass to operate somewhat like a force, with its own carrier particle. The Higgs boson is the only particle in the Standard Model of particle physics that has not yet been discovered.


Gravity is not presently incorporated into the Standard Model. In particle physics where everything is a particle, there should be a boson involved, and it would be called a graviton. Beyond that, no one is quite sure yet exactly how gravity actually works.

References and Photo Credits

Giancoli, Douglas C. Physics: Principles with Applications 3rd ed. Prentice Hall 1991

Standard model of particle physics - Quarks and leptons, Interactions: gauge bosons, The Higgs particle, Electro-weak unification

Proton with quarks picture made by Arpad Horvath, used under CC-A-SA 2.5 license

Standard Model diagram by TriTertButoxy, public domain

This post is part of the series: Particle Physics in Cosmology

The Standard Model of particle physics identifies the elementary particles of matter and provides an explanation for three of the four fundamental forces of nature - electromagnetism and the strong and weak forces, using force mediating gauge bosons and a mass mediating Higgs boson particle.

  1. Elementary Particles of the Standard Model
  2. The Four Forces of Physics
  3. Information About the Higgs Boson Particle