9.8.E - Particle Physics

The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, The Standard Model is truly "a tapestry woven by many hands", sometimes driven forward by new experimental discoveries, sometimes by theoretical advances. It was a collaborative effort in the largest sense, spanning continents and decades. The current formulation was finalized in the mid 1970s upon experimental confirmation of the existence of quarks. Since then, discoveries of the bottom quark (1977), the top quark (1995), and the tau neutrino (2000) have given further credence to the Standard Model. More recently, (2011-2012) the apparent detection of the Higgs boson completes the set of predicted particles. Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything".

In this unit you will:

  • Classify fundamental particles according to their propetries
  • Describe the composition of composite particles
  • Outline the mediation of composite particles by the fundamental forces

Classification of Particles

The Standard Model is an attempt to describe the behaviour of all known subatomic particles within a single unified framework. This model incorporates all particles with mass as well as their interactions through the massless particles that mediate the strong, weak and electromagnetic forces. Gravity alone remains outside the standard model.

The highest level of particle classification. Note that mesons are bosons
and hadrons; and baryons are hadrons and fermions.

There are 61 elementary particles in the Standard model and they are broadly classified as fermions, hadrons and bosons.

Fermions and they are classified as such because they have half-integer spin. Fermions are further classified into two families: baryons that are made of of quarks and held together by the strong nuclear force and leptons that are mediated by the weak nuclear force.

Hadrons are classified as such because they are mediated by the strong nuclear force and made up of quarks. Hadrons are categorised into two families: baryons (made of three quarks) and mesons (made of one quark and one antiquark).

Bosons are classified as such because they have a have a whole number integer spin. Bosons are also contrasted with fermions because they do not have to obey the Pauli Exclusion Principle. There are two families of bosons: the mesons which are compositie particles made up of quarks and the gauge bosons which are elementary particles that mediate the strong, weak and electromagnetic forces.


Gauge Bosons

There are four fundamental forces in nature whose properties are outlined in the table below. In the Standard Model, these forces are mediated by the Gauge bosons.

Force Range (m) Relative Strength Examples of Effects
Strong nuclear 10-15 1 Binds protons and neutrons together
Electromagnetic infinite 10-2 Binds charged particles, atoms and molecules together
Weak nuclear 10-17 10-5 Interacts with nuclear particles to change them into other particles
Gravitational infinite 10-39 Draws masses together


(a) Two skaters exert repulsive forces on
each other by tossing a ball back and forth.
(b) Two skaters exert attractive forces on
each other when one tries to grab the ball
out of the other's hands.

In classical physics we describe the interaction of charged particles in terms of Coulomb's law forces. In quantum mechanics, we can describe this interaction in therms of the emission or absorption of photons. Two electrons repel each other as one emits a photon and the other absorbs it, just as two skaters can push each other apart by tossing a heavy ball back and forth between them. For an electron and a proton, in which the charges are opposite and the force is attractive, we imagine the skaters trying to graph the ball away from each other. The electromagnetic interaction is mediated in this way by the Gauge boson known as the photon.

Interactions in physics are the ways that particles influence other particles. At a macroscopic level, electromagnetism allows particles to interact with one another via electric and magnetic fields, and gravitation allows particles with mass to attract one another in accordance with Einstein's theory of general relativity. The Standard Model explains such forces as resulting from matter particles exchanging other particles, known as force mediating particles. When a force-mediating particle is exchanged, at a microscopic level the effect is equivalent to a force influencing both of them, and the particle is therefore said to have mediated (been the agent of) that force.

The gauge bosons of the Standard Model all have spin (as do matter particles). The value of the spin is 1, making them bosons. The different types of gauge bosons are described below.

  • Photons mediate the electromagnetic force between electrically charged particles. The photon is massless and is well-described by the theory of quantum electrodynamics.

  • The W+, W-, and Z gauge bosons mediate the weaknuclear force interactions between particles of different flavors (all quarks and leptons).

  • The eight massless gluons mediate the strong nuclear force interactions between the quarks.



In particle physics, mesons belong to the hadron family subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometre, which is about two-thirds the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Charged mesons decay (sometimes through intermediate particles) to form electrons and neutrinos. Uncharged mesons may decay to photons.

Mesons are not produced by radioactive decay, but appear in nature only as short-lived products of very high-energy interactions in matter, between particles made of quarks. In cosmic ray interactions, for example, such particles are ordinary protons and neutrons. Mesons are also frequently produced artificially in high-energy particle accelerators that collide protons, anti-protons, or other particles containing quarks. In nature, the importance of lighter mesons is that they are the associated quantum-field particles that transmit the nuclear force, in the same way that photons are the particles that transmit the electromagnetic force. The higher energy (more massive) mesons were created momentarily in the Big Bang but are not thought to play a role in nature today. However, such particles are regularly created in experiments, in order to understand the nature of the heavier types of quark which compose the heavier mesons.

Each type of meson has a corresponding antiparticle (antimeson) in which quarks are replaced by their corresponding antiquarks and vice-versa. Since mesons are composed of quarks, they participate in both the weak and strong interactions. Mesons with net electric charge also participate in the electromagnetic interaction. They are classified according to their quark content, total angular momentum, parity, and various other properties such as C-parity and G-parity. While no meson is stable, those of lower mass are nonetheless more stable than the most massive mesons, and are easier to observe and study in particle accelerators or in cosmic ray experiments.



The quark composition of protons and neutrons.

A baryon is a composite subatomic particle made up of three quarks (as distinct from mesons, which comprise one quark and one antiquark). Baryons and mesons belong to the hadron family, which are the quark-based particles. The name "baryon" comes from the Greek word for "heavy" (barys), because, at the time of their naming, most known elementary particles had lower masses than the baryons. As quark-based particles, baryons participate in the strong interaction, whereas leptons, which are not quark-based, do not.

The most familiar baryons are the protons and neutrons that make up most of the mass of the visible matter in the universe. Each baryon has a corresponding antiparticle (antibaryon) where quarks are replaced by their corresponding antiquarks. For example, a proton is made of two up quarks and one down quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark. A neutron is made up of one up quark and two down quarks and the antineutron would be made of one up antiquark and two down antiquarks.

The Standard Model's 12 fundamental fermions and their
corresponding four fundamental bosons. Six of the particles
in the Standard Model are quarks (shown in purple).

There are six types of quarks, known as flavors: up, down, strange, charm, bottom, and top. Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, top, and bottom quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators).

Quarks have various intrinsic properties, including electric charge, color charge, mass, and spin. Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge. For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties have equal magnitude but opposite sign.



A lepton is an elementary particle and a fundamental constituent of matter. Leptons have a half-integer spin which also makes them fermions but the are distinct from hadrons because they are mediated by the weak nuclear force rather than the strong nuclear force. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties.

Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed. There are six types of leptons, known as flavours, forming three generations. The first generation is the electronic leptons, comprising the electron and electron neutrino; the second is the muonic leptons, comprising the muon and muon neutrino; and the third is the tauonic leptons, comprising the tau and the tau neutrino. Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).

Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, electromagnetism (excluding neutrinos, which are electrically neutral), and the weak interaction. For every lepton flavor there is a corresponding type of antiparticle, known as antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign.

The Standard Model Of Particle Physics. This film was produced as part of the CERN/ATLAS multimedia contest internship.