Glossary

antibaryon

A subatomic particle composed of three antiquarks. The antimatter counterpart of a baryon. A type of hadron. Examples include the antiproton and antineutron.

antimatter

Every type of matter particle has a corresponding type of antimatter particle which has the same mass but opposite other properties, such as electric charge.

baryon

A subatomic particle composed of three quarks. The matter counterpart of an antibaryon. A type of hadron. Examples include the proton and neutron.

bottom

(b) A type of quark - a fundamental particle with electric charge –e/3. (The bottom quark is sometimes alternatively referred to as 'beauty'.)

charm

(c) A type of quark - a fundamental particle with electric charge +2e/3.

colour charge

A property possessed by quarks (and antiquarks) and gluons. It plays a role in QCD equivalent to that of electric charge in QED. Quarks can possess any one of red, green or blue colour charge.

confinement

The process by which quarks and antiquarks remain locked up inside hadrons.

down

(d) A type of quark - a fundamental particle with electric charge –e/3. One of the constituent particles of both the proton and the neutron.

electron

(e^-) One of the component particles from which an atom is made. Electrons have a negative electric charge, and they surround the atom's positively charged nucleus. Electrons carry the electric current in metals. An electron is a fundamental particle (a lepton) with electric charge –e and mass energy about 500 keV. Electrons are produced in beta-minus decay processes.

electron neutrino

(_e) A fundamental particle (a lepton) with zero electric charge. It is produced in beta-plus decay along with a positron. Its antiparticle is the electron antineutrino.

electronvolt

(eV) The unit of energy corresponding to the energy converted when an electron moves through a voltage difference of 1V. The electronvolt is abbreviated to eV and 1 eV = 1.602 × 10^-19 J.

flavour

Somewhat whimsical name used to describe the different types of lepton (i.e. electron, electron neutrino, muon, muon neutrino, tauon, tauon neutrino) and the different types of quark (up, down, strange, charm, top, bottom).

generations

(of fundamental particles) The electron and electron neutrino are referred to as first-generation leptons, whereas the up and down quarks are referred to as first-generation quarks. The muon, muon neutrino, strange and charm quarks belong to the second generation; the tauon, tauon neutrino, bottom and top quarks belong to the third generation.

gluon

The quantum of energy associated with the strong interaction. It plays a role in QCD analogous to that of photons in QED. Unlike photons, gluons themselves experience the strong interaction (photons do not experience the electromagnetic interaction). This is because gluons possess colour charge. (Photons do not possess conventional electric charge.) Consequently, gluons have a very short range and are never observed in isolation.

hadron

A composite particle composed of quarks and/or antiquarks. Baryons are hadrons consisting of three quarks.

meson

A subatomic particle composed of a quark and an antiquark. A type of hadron. Examples of mesons include the pions.

muon

(μ^-) A fundamental particle (a lepton) with electric charge –e which is similar to an electron but with a mass about 200 times heavier. Its antiparticle is called the antimuon (µ⁺)

muon neutrino

() A fundamental particle (a lepton) with zero electric charge. Its corresponding antimatter particle is called the muon antineutrino.

neutron

(n) One of the two types of particle found in the nucleus of an atom. The neutron has zero electric charge and a mass very close to that of the proton. As with the proton therefore, the relative atomic mass of a neutron is very close to unity on the scale of relative atomic masses. Neutrons are baryons, with the quark composition (udd). They have an electric charge of zero, and a mass energy of about 1 GeV.

photon

A particle of light or other electromagnetic radiation. Monochromatic light consists of photons that each have exactly the same amount of energy, called a quantum of energy. Therefore also, the quantum of energy associated with the electromagnetic interaction. Photons have no mass or electric charge and do not experience the electromagnetic interaction. (Compare this behaviour with gravitons and gluons, which also have no mass or electric charge.

positron

(e⁺) (antielectron) The antimatter counterpart to the electron. It has an electric charge of +e, but the same mass as the electron. It is produced in beta-plus decay processes.

proton

(p) The nucleus of the hydrogen atom and one of two constituents of all other nuclei; the particle has a relative mass very close to unity on the scale of relative atomic masses. Protons are baryons, with the quark composition (uud). They have an electric charge of +e and a mass energy of about 1 GeV.

quantum chromodynamics

(QCD) The theory that describes the strong interaction. It explains these interactions as arising due to the exchange of gluons between particles that possess colour charge. Compare with quantum electrodynamics.

strange

(s) A type of quark - a fundamental particle with electric charge –e/3.

strong interaction

The fundamental interaction between quarks and antiquarks responsible for binding them together as triplets inside baryons and antibaryons, or as quark–antiquark pairs inside mesons. A residual effect of the strong interaction is responsible for binding protons and neutrons together as nuclei. One of the four fundamental interactions, described by the modern theory of QCD. It describes interactions between particles that possess colour charge in terms of the exchange of quanta called gluons. Strong interactions get weaker with increasing energy of interaction. Compare with electromagnetic interaction.

tauon

(^-) A fundamental particle (a lepton) with electric charge –e which is similar to an electron but with a mass about 3500 times heavier. Its antiparticle is called the antitauon (⁺).

tauon neutrino

() A fundamental particle (a lepton) with zero electric charge. Its antiparticle is called the tauon antineutrino.

top

(t) A type of quark - a fundamental particle with electric charge +2e/3. (The top quark is sometimes alternatively referred to as 'truth'.)

up

(u) A type of quark - a fundamental particle with electric charge +2e/3. One of the constituent particles of both the proton and the neutron.

virtual photon

A photon involved in carrying the electromagnetic force from one part of an interaction to another. Virtual photons do not escape from processes and are never detectable directly.

W boson

One of the quanta associated with the weak interaction. W bosons come in two varieties labelled W⁺ and W^-, where the superscripts denote the electric charge of the particles. Both W bosons have mass energies of about 80 GeV.

weak interaction

Fundamental interaction involving both quarks and leptons. One of the four fundamental interactions. It describes interactions between particles in terms of the exchange of quanta called W bosons and Z bosons. There is no structure that's bound together by a 'weak force'. Weak interactions are responsible for processes such as beta-decay in which quarks change flavour and lepton–antilepton pairs are produced.

Z boson

One of the quanta associated with the weak interaction. Z bosons are often labelled Z⁰, where the superscript denotes that it has zero electric charge. Z bosons have mass energies of about 90 GeV.