Mega. A prefix denoting million. A mega electron volt (MeV) is a million electron volts, 106 eV or 1,000,000 eV.
Meson. From the Greek mésos, meaning ‘middle’. Mesons are a sub-class of hadrons. They experience the strong nuclear force and are composed of quarks and anti-quarks.
MIT. Acronym for the Massachusetts Institute of Technology.
Mole. A standard unit for the amount of a chemical substance, equal to its atomic or molecular weight in grams. A mole contains 6.022×1023 particles. The name is derived from ‘molecule’.
Molecule. A fundamental unit of chemical substance formed from two or more atoms. A molecule of oxygen consists of two oxygen atoms, O2. A molecule of water consists of two hydrogen atoms and one oxygen atom, H2O.
MSSM. Acronym for the Minimum Supersymmetric Standard Model, the minimal extension of the conventional Standard Model of particle physics which accommodates supersymmetry, developed in 1981 by Howard Georgi and Savas Dimopoulos.
Muon. A second-generation lepton equivalent to the electron, with a charge –1, spin ½ (fermion), and mass 106 MeV. First discovered in 1936 by Carl Anderson and Seth Neddermeyer.
NAL. Acronym for the National Accelerator Laboratory in Chicago. Renamed the Fermi National Accelerator Laboratory, or ‘Fermilab’, in 1974.
Nambu–Goldstone boson. A massless, spin-0 particle created as a consequence of spontaneous symmetry-breaking, first discovered by Yoichiro Nambu in 1960 and elaborated by Jeffrey Goldstone in 1961. In the Higgs mechanism, the Nambu–Goldstone bosons become a third ‘degree of freedom’ of quantum particles that would otherwise be massless (see Figure 14, p. 89).
Neutral currents (weak force). Interactions between elementary particles involving no change in electric charge. These may involve exchange of a virtual Z0 particle or simultaneous exchange of both W+ and W− particles (see Figures 15 and 16, pp. 100 and 127).
Neutrino. From Italian, meaning ‘small neutral one’. Neutrinos are the chargeless, spin ½ (fermion) companions to the negatively charged electron, muon, and tau. The neutrinos are believed to possess very small masses, necessary to explain the phenomenon of neutrino oscillation, the quantum-mechanical mixing of neutrino flavours such that the flavour may change over time. Neutrino oscillation solves the solar neutrino problem – that the numbers of neutrinos measured to pass through the earth are inconsistent with the numbers of electron neutrinos expected from nuclear reactions occurring in the sun’s core. It was determined in 2001 that only 35 per cent of the neutrinos from the sun are electron neutrinos – the balance are muon and tau neutrinos, indicating that the neutrino flavours oscillate as they travel from the sun to the earth.
Neutron. An electrically neutral sub-atomic particle, first discovered in 1932 by James Chadwick. The neutron is a baryon consisting of one up- and two down-quarks with spin ½ and mass 940 MeV.
Noether’s theorem. Developed by Amalie Emmy Noether in 1918, the theorem connects the laws of conservation with specific continuous symmetries of physical systems and the theories that describe them, used as a tool in the development of new theories. The conservation of energy reflects the fact that the laws governing energy are invariant to continuous changes or ‘translations’ in time. For linear momentum, the laws are invariant to continuous translations in space. For angular momentum, the laws are invariant to the angle of direction measured from the centre of the rotation.
Nucleus. The dense region at the core of an atom in which most of the atom’s mass is concentrated. Atomic nuclei consist of varying numbers of protons and neutrons. The nucleus of a hydrogen atom consists of a single proton.
Parton. A name coined by Richard Feynman in 1968 to describe the point-like constituent ‘parts’ of protons and neutrons. Partons were subsequently shown to be quarks and gluons.
Pauli exclusion principle. Discovered by Wolfgang Pauli in 1925. The exclusion principle states that no two fermions may occupy the same quantum state (i.e. possess the same set of quantum numbers) simultaneously. For electrons, this means that only two electrons can occupy a single atomic orbital provided that they possess opposite spins.
Perturbation theory. A mathematical method used to find approximate solutions to equations that cannot be solved exactly. The offending equation is recast as a perturbation expansion – the sum of a potentially infinite series of terms which starts with a ‘zeroth-order’ expression which can be solved exactly. To this are added additional (or perturbation) terms representing corrections to first-order, second-order, third-order, etc. In principle, each term in the expansion provides a smaller and smaller correction to the zeroth-order result, gradually bringing the calculation closer and closer to the actual result. The accuracy of the final result then depends simply on the number of perturbation terms included in the calculation. Although it is structurally very different, we can get some idea of how the perturbation expansion is supposed to work by looking at the power series expansion for a simple trigonometric function such as sin x. The first few terms in the expansion are: sin x = x – x3/3! + x5/5! – x7/7! +…For x = 45° (0.785398 radians), the first term gives 0.785398 from which we subtract 0.080745, then add 0.002490, then subtract 0.000037. Each successive term gives a smaller correction, and after just four terms we have the result 0.707106, which should be compared with sin (45°) = 0.707107.
Photon. The elementary particle underlying all forms of electromagnetic radiation, including light. The photon is a massless, spin 1 boson which acts as the carrier of the electromagnetic force.
Pion. A group of spin-0 mesons formed from up- and down-quarks and their anti-quarks. These are π+ (up-anti-down), π– (down-anti-up), and π0 (a mixture of up-anti-up and down-anti-down), with masses 140 MeV (π±) and 135 MeV (π0).
Planck constant. Denoted h. Discovered by Max Planck in 1900. The Planck constant is a fundamental physical constant which reflects the magnitudes of quanta in quantum theory. For example, the energies of photons are determined by their radiation frequencies according to the relation E = hv, i.e. energy equals Planck’s constant multiplied by the radiation frequency. Planck’s constant has the value 6.626×10–34 joule seconds.
Positron. The anti-particle of the electron, denoted e+, with a charge +1, spin ½ (fermion), and mass 0.51 MeV. The positron was the first anti-particle to be discovered, by Carl Anderson in 1932.
Proton. A positively charged sub-atomic particle ‘discovered’ and so named by Ernest Rutherford in 1919. Rutherford actually identified that the nucleus of the hydrogen atom (which is a single proton) is a fundamental constituent of other atomic nuclei. The proton is a baryon consisting of two up- and one down-quarks with spin ½ and mass 938 MeV.
Quantum. A fundamental, indivisible unit of properties such as energy and angular momentum. In quantum theory, such properties are recognized not to be continuously variable but to be organized in discrete packets or bundles, called quanta. The use of the term is extended to include particles. Thus, the photon is the quantum particle of the electromagnetic field. This idea can be extended beyond the carriers of forces to include matter particles themselves. Thus, the electron is the quantum of the electron field, and so on. This is sometimes referred to as second quantization.
Quantum chromodynamics (QCD). The SU(3) quantum field theory of the strong colour force between quarks carried by a system of eight coloured gluons.
Quantum electrodynamics (QED). The U(1) quantum field theory of the electromagnetic force between electrically charged particles, carried by photons.
Quantum field. In classical field theory a ‘force field’ is ascribed a value at every point in space-time and can be scalar (magnitude but no direction) or vector (magnitude and direction). The ‘lines of force’ made visible by sprinkling iron filings on a piece of paper held above a bar magnetic provides a visual representation of such a field. In a quantum field theory, forces are conveyed by ripples in the field which form waves and – because waves can also be interpreted as particles – as quantum particles of the
field. This idea can be extended beyond the carriers of forces (bosons) to include matter particles (fermions). Thus, the electron is the quantum of the electron field, and so on.
Quantum number. The description of the physical state of a quantum system requires the specification of its properties in terms of total energy, linear and angular momentum, electric charge, etc. One consequence of the quantization of such properties is the appearance in this description of regular multiples of the associated quanta. For example, the angular momentum associated with the spin of an electron is fixed at the value ½ h/2π, where h is Planck’s constant. The recurring integral or half-integral numbers which multiply the sizes of the quanta are called quantum numbers. When placed in a magnetic field, the electron spin may be oriented along or against the field lines of force, giving rise to ‘spin-up’ and ‘spin-down’ orientations characterized by the quantum numbers +½ and –½. Other examples include the principal quantum number, n, which characterizes the energy levels of electrons in atoms, electric charge, quark colour charge, etc.
Quark. The elementary constituents of hadrons. All hadrons are composed of triplets of spin ½ quarks (baryons) or combinations of quarks and anti-quarks (mesons). The quarks form three generations, each with different flavours. The up- and down-quarks, with electric charges and and masses of 1.7–3.3 MeV and 4.1–5.8 MeV, respectively, form the first generation. Protons and neutrons are composed of up- and down-quarks. The second generation consists of the charm and strange-quarks, with electric charges and and masses of 1.27 GeV and 101 MeV, respectively. The third generation consists of bottom and top quarks, with electric charges and and masses of 4.19 GeV and 172 GeV, respectively. Quarks also carry colour charge, with each flavour of quark possessing red, green, or blue charges.
Renormalization. One consequence of introducing particles as the quanta of fields is that they may undergo self-interaction, i.e. they can interact with their own fields. This means that techniques, such as perturbation theory, used to solve the field equations tend to break down, as the self-interaction terms appear as infinite corrections. Renormalization was developed as a mathematical device used to eliminate these self-interaction terms, by redefining the parameters (such as mass and charge) of the field particles themselves.
SLAC. Acronym for Stanford Linear Accelerator Center, located in the Los Altos Hills near Stanford University in California.
Special relativity. Developed by Einstein in 1905, the special theory of relativity asserts that all motion is relative, and there is no unique or privileged frame of reference against which motion can be measured. All inertial frames of reference are equivalent – an observer stationary on earth should obtain the same results from the same set of physical measurements as an observer moving with uniform velocity in a spaceship. Out go classical notions of absolute space, time, absolute rest, and simultaneity. In formulating the theory, Einstein assumed that the speed of light in a vacuum represents an ultimate speed which cannot be exceeded. The theory is ‘special’ only in the sense that it does not account for accelerated motion; this is covered in Einstein’s general theory of relativity.
Spin. All elementary particles exhibit a type of angular momentum called spin. Although the spin of the electron was initially interpreted in terms of electron ‘self-rotation’ (the electron spinning on its own axis, like a spinning top), spin is a relativistic phenomenon and has no counterpart in classical physics. Particles are characterized by their spin quantum numbers. Particles with half-integral spin quantum numbers are called fermions. Particles with integral spin quantum numbers are called bosons. Matter particles are fermions. Force particles are bosons.
SSC. Acronym for Superconducting Supercollider, an American project to build the world’s largest particle accelerator at Waxahachie in Ellis County, Texas, capable of proton–proton collision energies of 40 TeV. The project was cancelled by Congress in October 1993.
Standard Model, of big bang cosmology. See lambda-CDM model.
Standard Model, of particle physics. The currently accepted theoretical model describing matter particles and the forces between them, with the exception of gravity. The Standard Model consists of a collection of quantum field theories with local SU(3) (colour force) and SU(2)×U(1) (weak nuclear force and electromagnetism) symmetries. The Model contains three generations of quarks and leptons, the photon, W, and Z particles, colour force gluons, and the Higgs boson.
Strangeness. Identified as a characteristic property of particles such as the neutral lambda, neutral and charged sigma and xi particles, and the kaons. Strangeness was used together with electric charge and isospin to classify particles according to the ‘Eightfold Way’ by Murray Gell-Mann and Yuval Ne’eman (see Figure 10, p. 69). This property was subsequently traced to the presence in these composite particles of the strange-quark (see Figure 12, p. 82).
Strange-quark. A second generation quark with charge , spin ½ (fermion), and a mass of 101 MeV. The property of ‘strangeness’ was identified as a characteristic of a series of relatively low-energy (low-mass) particles discovered in the 1940s and 1950s by Murray Gell-Mann and independently by Kazuhiko Nishijima and Tadao Nakano. This property was subsequently traced by Gell-Mann and George Zweig to the presence in these composite particles of the strange-quark (see Figure 12, p. 82).
Strong force. The strong nuclear force, or colour force, binds quarks and gluons together inside hadrons and is described by quantum chromodynamics. The force that binds protons and neutrons together inside atomic nuclei (also referred to as the strong nuclear force) is thought to be a ‘hang-over’ of the colour force binding quarks inside the nucleons. See colour force.
SU(2) symmetry group. The special unitary group of transformations of two complex variables. Identified by Chen Ning Yang and Robert Mills as the symmetry group on which a quantum field theory of the strong nuclear force should be based, SU(2) was subsequently identified with the weak force and, when combined with the U(1) field theory of electromagnetism, forms the SU(2)×U(1) field theory of the electro-weak force.
SU(3) symmetry group. The special unitary group of transformation of three complex variables. Used by Murray Gell-Mann and Yuval Ne’eman as a global symmetry on which the ‘Eightfold Way’ was constructed. Subsequently used by Gell-Mann, Harald Fritzsch, and Heinrich Leutwyler as a local symmetry on which to base a quantum field theory of the strong nuclear (colour) force between quarks and gluons.
Superconductivity. Discovered by Heike Kamerlingh Onnes in 1911. When cooled below a certain critical temperature, certain crystalline materials lose all electrical resistance and become superconductors. An electric current will flow indefinitely in a superconducting wire flowing with no energy input. Superconductivity is a quantum-mechanical phenomenon explained using the BCS mechanism, named for John Bardeen, Leon Cooper, and John Schrieffer.
Supersymmetry (SUSY). An alternative to the Standard Model of particle physics in which the asymmetry between matter particles (fermions) and force particles (bosons) is explained in terms of a broken supersymmetry. At high energies (for example, the kinds of energies that prevailed during the very early stages of the big bang) supersymmetry would be unbroken and there would be perfect symmetry between fermions and bosons. Aside from the asymmetry between fermions and bosons, the broken supersymmetry predicts a collection of massive super-partners with a spin different by ½. The supersymmetric partners of fermions are called sfermions. The partner of the electron is called the selectron; each quark is partnered by a corresponding squark. Likewise, for every boson there is a bosino. Supersymmetric partners of the photon, W and Z particles, and gluons are the photino, wino and zino, and gluinos. Supersymmetry resolves many of the problems with the Standard Model, but evidence for super-partners has not yet been found.
Symmetry-breaking. Spontaneous symmetry-breaking occurs whenever the lowest-energy state of a physical system has lower symmetry than higher-energy states. As the system loses energy and settles to its lowest-energy state, the symm
etry spontaneously reduces, or ‘breaks’. For example, a pencil perfectly balanced on its tip is symmetrical, but will topple over to give a more stable, lower-energy, less symmetrical state with the pencil lying along one specific direction.
Synchrotron. A type of particle accelerator in which the electric field used to accelerate the particles and the magnetic field used to circulate them in a ring are carefully synchronized with the particle beam.
Tera. A prefix denoting trillion. A tera electron volts (TeV) is a trillion electron volts, 1012 eV, or 1000 GeV.
Top quark. Also sometimes referred to as the ‘truth’ quark. A third-generation quark with charge , spin ½ (fermion), and a mass of 172 GeV. It was discovered at Fermilab in 1995.
Trillion. A thousand billion or a million million, 1012, or 1,000,000,000,000.
U(1) symmetry group. The unitary group of transformations of one complex variable. It is equivalent (the technical term is ‘isomorphic’) with the circle group, the multiplicative group of all complex numbers with absolute value of unity (in other words, the unit circle in the complex plane). It is also isomorphic with SO(2), a special orthogonal group which describes the symmetry transformation involved in rotating an object in two dimensions. In quantum electrodynamics, U(1) is identified with the phase symmetry of the electron wavefunction (see Figure 7, p. 34).
Uncertainty principle. Discovered by Werner Heisenberg in 1927. The uncertainty principle states that there is a fundamental limit to the precision with which it is possible to measure pairs of ‘conjugate’ observables, such as position and momentum, and energy and time. The principle can be traced to the fundamental duality of wave and particle behaviour in quantum objects.
Higgs:The invention and discovery of the 'God Particle' Page 20
