The Standard Model



The standard model is the name given in the 1970s to a theory of fundamental particles and how they interact. It incorporated all that was known about subatomic particles at the time and predicted the existence of additional particles as well.

There are seventeen named particles in the standard model, organized into the chart shown below. The last particles discovered were the W and Z bosons in 1983, the top quark in 1995, the tau neutrino in 2000, and the higgs boson in 2012.

Particles of The Standard Model

particle families

Fermions and Bosons

Fundamental particles are either the building blocks of matter, called fermions, or the carriers of forces, called bosons. There are twelve named fermions and five named bosons in the standard model.

Fermions obey a statistical rule described by Enrico Fermi (1901–1954) of Italy, Paul Dirac (1902–1984) of England, and Wolfgang Pauli (1900–1958) of Austria called the exclusion principle. Simply stated, fermions cannot occupy the same place at the same time. (More formally, no two fermions may be described by the same quantum numbers.) Leptons and quarks are fermions, but so are things made from them like protons, neutrons, atoms, molecules, people, and walls. This agrees with our macroscopic observations of matter in everyday life. People cannot walk through walls unless the wall gets out of the way.

Bosons, in contrast, are have no problem occupying the same place at the same time. (More formally, two or more bosons may be described by the same quantum numbers.) The statistical rules that bosons obey were first described by Satyendra Bose (1894–1974) of India and Albert Einstein (1879–1955) of Germany. Gluons, photons, and the W, Z and higgs are all bosons. As the particles that make up light and other forms of electromagnetic radiation, photons are the bosons we have the most direct experience with. In our everyday experience, we never see beams of light crash into one another. Photons are like phantoms. One may pass through the other with no effect.



Quarks and Leptons

Fermions are divided into two groups of six: those that must bind together are called quarks and those that can exist independently are called leptons.

The word "quark" originally appeared in a single line of the the novel Finnegans Wake written by the Irish author James Joyce (1882–1941). The protagonist of the book is a publican named Humphrey Chimpden Earwicker who dreams that he is serving beer to a drunken seagull (no joke). Instead of asking for "three quarts for Mister Mark" the inebriated bird says "three quarks for Muster Mark". Since the pre-standard model theory was complete with only three quarks, the name made some sense. The full standard model today needs six quarks. That hasn't made the word any less fun to say. Quark!

Quarks are known to bind into triplets and doublets. The triplets are called baryons, a term derived from the Greek word βαρύς (varys) meaning "heavy". The doublets are called mesons, a term derived from the Greek word μέσος (mesos) meaning "medium". Collectively baryons (the heavy triplets), mesons (the middleweight doublets), and quarks (the fundamental particles) are known as hadrons, from the Greek word χοντρός (hadros) meaning "thick". This name alludes to the ability of the point-like quarks to bind together and form particles with size, volume, bulk, or thickness (to pick but a few synonyms).

Baryons found in the nucleus (the proton and neutron) are called nucleons. The Latin word for kernel is nucleus. Nucleons are found in the metaphorical "kernel" of the atom. All the other baryons are called hyperons. The Greek word for beyond is υπέρ (yper), which morphed into the English prefix hyper- over the years. Hyperons are particles that are "way out" in a certain sense.

The other fermions are called leptons, a name derived from the Greek word λεπτός (leptos) meaning "thin". These particles don't need to bind to each other, which keeps them "thin" in a certain sense.

The Neutrinos

The neutrinos are an important subgroup within the leptons. They come in three flavors named for their partner leptons: the electron, muon, and tau. Neutrinos have very little mass (even for leptons) and interact so weakly with the rest of the particles that they are exceptionally difficult to detect. The name is a play on words. The Italian word for neutron (neutrone) sounds like the word neutral (neutro) with an augmentative suffix (-one) tacked on the end. That is, it sounds something like "big neutral" to Italian ears. Replace the augmentative suffix -one with the diminutive suffix -ino and you have a "little neutral", which is a good description of what a neutrino is — a diminutive neutral particle. Aaaaaw, so small and precious.

Fermion Generations

Fermions belong to one of three known generations from ordinary (I), to exotic (II), to very exotic (III). (These are adjectives I selected to describe the generations.) Generation I particles can combine to form hadrons with effectively infinite life spans (stable atoms made of electrons, protons, and neutrons for example). Generation II particles always form unstable hadrons. The longest lived hadron containing a generation II quark is the lambda particle (made of an up, down, and strange quark). It has a mean lifetime less than a billionth of a second, which is long-lived for an unstable hadron. Generation III particles are divided in their behavior. The bottom quark isn't much stranger than a strange quark, but the top quark is so short-lived that it doesn't exist long enough to do anything. It falls apart before anything knows it exists. Top quarks are only known from their decay products.

particle interactions


Charge is the property of matter that gives rise to electric and magnetic phenomena (known collectively as electromagnetism). Charge is quantized, which means it can only exist in discrete amounts with restricted values — multiples and fractions of the elementary charge. Particles that exist independently (the electron, muon, and tau) carry multiples of the elementary charge, while quarks carry fractions of the elementary charge. Quarks always bind together in groups whose total charge is an integral multiple of the elementary charge, which is why no one has ever directly measured a fractional charge. In addition, since opposite charges attract, electrons tend to bind to protons to form atoms that are neutral overall. We don't normally notice the electrical nature of matter because of this.

The Electromagnetic Force

Charged particles interact by the exchange of photons — the carrier of the electromagnetic force. Whenever an electron repels another electron or an electron orbits a nucleus, a photon is responsible. Photons are massless, uncharged, and have an unlimited range. The mathematical model used to describe the interaction of charged particles through the exchange of photons is known as quantum electrodynamics (QED).


Quarks stick to other quarks because they possess a characteristic known as color (or color charge). Quarks come in one of three colors: red, green, and blue. Don't let the word mislead you. Quarks are much too small to to be visible and thus could never have a perceptual property like color. The names were chosen because of a wonderful analogy. The colors of quarks in the standard model combine like the colors of light in human vision.

Red light plus green light plus blue light appears to us humans as "colorless" white light. A baryon is a triplet of one red, one green, and one blue quark. Put them together and you get a color neutral particle. A color plus its opposite color also gives white light. Red light plus cyan light looks the same to humans as white light, for example. A meson is a doublet of one colored quark and one anticolored antiquark. Put them together and you get another color neutral particle.

There's something about color that makes it want to hide itself from anything bigger than a nucleus. Quarks can't stand being apart from one another. They just have to join up and always do so in a way that hides their color from the outside world. One color is never favored over another when quarks get together. Matter is color neutral down to the very small scale.

The Strong Force

Colored particles are bound together by the appropriately named gluons. Gluons are also colored, but in a more complicated way than the quarks are. Six of the eight gluons have two colors, one has four, and another has six. Gluons glue quarks together, but they also stick to themselves. One consequence of this is that they they can't reach out and do much beyond the nucleus.

The mathematical model used to describe the interaction of colored particles through the exchange of gluons is known as quantum chromodynamics (QCD). The whole sticky mess is called the strong force or the strong interaction since it results in forces in the nucleus that are stronger than the electromagnetic force. Without the strong force, every nucleus would blow itself to smithereens.


There are twelve named elementary fermions. The difference between them is one of flavor. The word "flavor" is used here to mean "type" and it applies only to fermions. Don't let the word mislead you. Subatomic particles are much too small to have any characteristics that could be directly observed by human senses.

The Weak Force

Flavored particles interact weakly through the exchange of W or Z bosons — the carriers of the weak force (also known as intermediate vector bosons). When a neutron decays into a proton, a W boson is responsible. When a neutron captures a neutrino, a W boson did it. The mathematical model used to describe the interaction of flavored particles through the exchange of intermediate vector bosons is known as quantum flavordynamics (QFD), but this is a term that is rarely used by practicing particle physicists. At higher energies, the weak and electromagnetic forces are indistinguishable. The mathematical model that describes both of these fundamental forces is known as electroweak theory (EWT). This is the conventional name for the theory of the weak force.


All fermions are thought to have mass. Particles in generation I are less massive than those in generation II, which are less massive than those in generation III. Within the generations, quarks are more massive than leptons and neutrinos are less massive than the other leptons. Bosons are divided when it comes to mass. Gluons and photons are massless. The W, Z, and higgs bosons are massive. This

The Higgs Mechanism


particles mass (kg) mass (u) mass (MeV/c2)
quarks u up 4.1 × 10−30 0.0025 2.3
d down 8.6 × 10−30 0.0052 4.8
c charm 2.273 × 10−27 1.369 1,275
s strange 1.7 × 10−28 0.102 95
t top 3.093 × 10−25 186.3 173,070
b bottom 7.88 × 10−27 4.75 4,420
leptons e electron 9.10938291 × 10−31 0.00054857990946 0.510998928
μ muon 1.883531475 × 10−28 0.1134289267 105.6583715
τ tau 3.16747 × 10−27 1.90749 1776.82
neutrinos νe electron neutrino < 10−35 < 10−8 < 10−5
νμ muon neutrino < 10−35 < 10−8 < 10−5
ντ tau neutrino < 10−35 < 10−8 < 10−5
bosons g gluons   0 0 0
γ photon   0 0 0
W   1.4330 × 10−25 86.312 80,385
Z   1.62557 × 10−25 97.8393 91,187.6
H higgs 2.25 × 10−25 135 125,900
nucleons p proton 1.672621777 × 10−27 1.00727646681 938.272046
n neutron 1.674927351 × 10−27 1.00866491600 939.565379
Standard Model particle masses (nucleons included for comparison) Source: Particle Data Group

Gravity is the force between objects due to their mass. The mathematical model that would describe gravity on the particle level is sometimes called quantum geometrodynamics (QGD), but is more usually referred to as quantum gravitation. The standard model of particle physics does not include gravity (nor could it ever) and there currently is no quantum theory of gravitation. If there was, it would have to include a force carying particle. The proposed name for this particle is the graviton. It is hoped that gravity will be taken care of beyond the standard model in what is often referred to as a theory of everything.

names, names, names

The theme of this topic seems to be "names, names, names".

group statistics eponyms
fermions fermi – dirac Enrico Fermi
(1901–1954) Italy
Paul Dirac
(1902–1984) England
bosons bose – einstein Satyendra Bose
(1894–1974) India
Albert Einstein
(1879–1955) Germany
maxwell – boltzmann James Clerk Maxwell
(1831–1879) Scotland
Ludwig Boltzmann
(1844–1906) Austria
higgs mechanism Peter Higgs
(1929–0000 ) England
Particle groups named after physicists
Classical particles (the molecules of an ideal gas, for example) are not a part of the Standard Model, but are included for comparison.
group greek root meaning
hadrons χοντρός (hodros) thick
leptons λεπτός (leptos) thin
baryons βαρύς (varys) heavy
mesons μέσος (mesos) medium
hyperons υπέρ (yper) over
Particle groups with names of Greek origin
group latin root meaning
nucleons nucleus kernel
Particle groups with names of Latin origin
group source explanation
neutrinos Enrico Fermi
Italian diminutive form of neutron (neutrone). Neutrino could be translated as "little neutral" in contrast with neutrone, which is the "big neutral".
quarks Murray Gell-Mann
United States
An arbitrary utterance later associated with a passage in Finnegans Wake — a novel by Irish modernist author James Joyce. Sounds like a drunken seagull ordering quarts of beer.
Particle groups with names of miscellaneous origin