The Standard Model of Particle Physics

Via Wikimedia Commons.

In the most recent entry of De Omnibus, we considered the Copenhagen Interpretation of quantum mechanics. Quantum mechanics is a heavy topic, to say the least, and it is not isolated merely to quantum mechanics; many other scientific phenomena are notoriously difficult to understand.

In this entry, we will consider an aspect of particle physics, which ties into quantum mechanics. Even knowing the Copenhagen Interpretation and quantum field theory, there is a lot we have yet to learn. We have only begun to dip our toe on the surface of the quantum mechanical waters, but now that toe shall protrude deeper as we begin to explore another model in physics: the Standard Model of particle physics.

History of the development of the Standard Model

The Standard Model of particle physics is the basis of quantum mechanics and quantum field theory. It is also important in astronomy, as it is used to determine neutrino oscillations and the existence of dark matter and energy. 

The Standard Model is quite new–formulated in 1954–and evolves as new particles are discovered. Two physicists, Chen Ning Yang and Robert Mills, extended a concept known as gauge theory–a field theory that combines doctrines of Einsteinian relativity and quantum field theory in order to describe subatomic particles and their wave fields–so that they could describe the strong interactions, which are responsible for the strong nuclear force. In 1961, Sheldon Glashow applied gauge theory to the interactions responsible for two of the three other fundamental forces–the weak and electromagnetic forces. The application led to the electroweak interaction–a unified description of both the electromagnetic and weak nuclear interactions–and and the inclusion of elementary particles; as the Standard Model continued to develop and the elementary particles became more accepted as legitimate objects, it gained the scientific consensus.

The model

The Standard Model seeks to classify all the elementary particles and describe three of the four fundamental forces in the known universe; one of the four fundamental forces is not described in this model: gravity (trust me, we will get to that). Let us first consider the fundamental forces described in the model: the strong nuclear force, the weak nuclear force and the electromagnetic force.

Strong nuclear force

Otherwise considered the strong force, the strong nuclear force is responsible for protons and neutrons–the subatomic particles constituting the nucleus. The strong force combines quarks–the constituents of protons and neutrons–together. It also holds atomic nuclei together, and keeps them from breaking apart. 

Any elementary particle that can experience strong interactions will have a color charge, which is neither analogous with color nor with charge; the color charge merely classifies the strong interactions of a particle that can experience them, like quarks and gluons. Some fermions, like electrons and other leptons, have no color–they do not experience strong interactions. The particles that do, specifically the quarks, are important in the formation of and constitute neutrons and protons. Protons and neutrons each are composed of three quarks: the proton of 2 up quarks–which have charges of +⅔–and one down quark–which has a charge of negative -⅓. The neutron, on the other hand, has two down quarks and one up quark. The strong nuclear force maintains the strong interaction and binds the quarks together through gluon (the exchange particle for the strong nuclear force) exchange between the various quarks.

Weak nuclear force

The weak nuclear force is another of the four fundamental forces; it is primarily responsible for some radioactivity, the subatomic decay of mesons, and nuclear fusion. Particles interact with one another through the weak interaction by exchanging carrier particles, or gauge bosons known as W and Z particles (similar to the gluon). W and Z particles each are one hundred times heavier than protons, whereas the gluon is massless; the weak force, therefore, produces a weaker force (hence the name) at longer distances and, because massive particles cannot travel at the speed of light, the process is much slower.

Electromagnetic force

The last of the three fundamental forces described by the Standard Model of particle physics is the electromagnetic force. The electromagnetic force is primarily responsible for many phenomena applicable to our lives, like friction, tension, elasticity, air drag, bonds between atoms that form molecules, and many others. The electromagnetic force is specific to charged particles–particles with electric charges. 

There are two forms of the electromagnetic force: the electric force and the magnetic force. The electric force acts upon particles regardless of whether they are moving or not, whereas the magnetic force acts only upon moving particles. The carriers of the electromagnetic force are photons, which can be anything to 10-16m long to longer than the diameter of Earth itself, occupying a continuum known as the electromagnetic spectrum. 

Elementary particles

Let us now consider the types of elementary particles classified and described in the Standard Model. 

Scalar Boson

A boson, first off, is any elementary particle that has an integer-valued spin. Let us first consider the Higgs boson, or the God Particle. The Higgs boson is responsible for supplying matter to particles, specifically quarks; the Higgs boson, therefore, is responsible for at least all the baryonic mass in the universe. Without the Higgs Boson, mass–and likely the universe itself–would not exist. Scalar bosons–like the Higgs boson–have integer value spins of zero.

Gauge Boson

While scalar bosons supply mass, gauge bosons supply force. Earlier, we considered gluons, W and Z particles, and photons, as the suppliers and transporters of the strong nuclear force, the weak nuclear force and the electromagnetic force; all these particles are vector bosons–a group of bosons with non-zero spins (whose spins can, therefore, be described with non-zero vectors).

Gauge bosons are not characterized by mass, as the gluons and the photons are massless, while the W and Z particles do have mass. Gauge bosons supply all of the fundamental forces of nature, perhaps excluding the gravitational force. Some speculation exists regarding a graviton–a hypothetical particle that would act as the carrier of the gravitational force.


Quarks, the building blocks of the hadrons–like the proton and neutron–have partial charges of either +⅔ or -⅓. They combine to form protons and neutrons, and also intertwine and pull towards their respective quarks through the strong nuclear force, facilitated by the gluons. Depending on which flavors of quarks are pulled together, a large set of hadrons could exist: it could result in a proton, a neutron, an antiproton, a λ-particle (lambda), to name a few. The quarks are, simply put, the building blocks of matter.


The leptons–a group of elementary particles that include electrons, muons, taus, and neutrinos–cannot experience strong interactions. In the group are the electron-like leptons–the electrons, muons, and the taus–which are charged and combine with other elementary particles to form composite particles, like atoms and positronium–an exotic atom, an otherwise normal atom with a cloud of muons or taus, rather than electrons. Charged leptons react via the electromagnetic, gravitational, and weak forces, but not the strong force. There are also the neutral-charged leptons–the electron neutrinos, muon and tau neutrinos–which interact via only the gravitational and weak forces. Neutrinos have an almost negligible mass, and for decades they were believed to be massless. 

Where’s gravity?

Gravity, too, is a fundamental force and is known to exist, which leads one to question why it is not included in the Standard Model. 

Although it is obvious that gravity does exist, we still do not know how it works. Classical mechanics cannot describe gravity on greater–or faster–scales, and particle physics also has yet to correctly describe gravity conceptually. The Standard Model attempts to explain the fundamental forces through particles, but the graviton–the hypothetical facilitator of the gravitational force–is much more hypothetical than other gauge bosons, and is even inconsistent with observational evidence. For now, gravity is a major hole in particle physics and even all of science. 

Wrapping it up

The Standard Model represents a still ongoing attempt to categorize and enumerate the elementary particles and the fundamental forces; it is, by nature, an incomplete and still not fully coherent model, and thus will become more cumbersome as our understanding of elementary particles and the fundamental forces grows stronger. As always, take care and stay curious, everyone.

If you have any questions, comments, or corrections, please comment on this post or email with your concerns. Thank you.


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