Glossary - The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics - Robert Oerter

The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics - Robert Oerter (2006)


Alpha particle A helium atom with its electrons stripped off. An alpha particle consists of two protons and two neutrons.

Antimatter Particles with all of the properties of normal matter reversed. An antimatter particle can combine with its normal counterpart: The two annihilate each other and their energy is converted into photons.

Asymptotic freedom The strange property of QCD that tells us the strong force gets weaker when color-carrying particles are closer together.

Baryon Originally a term for any heavy particle, it is now used for any particle made up of three quarks (or three antiquarks), such as the proton and the neutron.

Beta decay A form of radioactivity in which an electron (formerly called a beta ray) is released. The simplest beta decay process is when a neutron decays into a proton, an electron, and an antineutrino. Beta decay is caused by the weak force.

Boson Any particle with a whole-number spin value—for example, the Higgs particle (spin 0) or the photon (spin 1).

Cathode In a cathode-ray tube (CRT), such as a TV tube, the cathode is the source of the electrons that eventually strike the screen to make the picture.

CERN (European Organization for Nuclear Research) The world’s largest particle accelerator facility in Geneva, Switzerland, where the W and Z° particles were discovered.

Charm One of the six quark flavors. Particles containing at least one charm or anticharm quark are said to be “charmed.”

Classical Descriptive of the physics that predates quantum theory, or for which there is not yet a quantum description (as for gravity).

Cloud chamber An early form of particle detector in which particles are seen by their trails through some form of vapor.

Color The property of quarks that makes them subject to the strong force. The three colors (usually called red, blue, and green) are related by SU(3) symmetry. In nature, only color-neutral combinations such as (red + blue + green) or (red + antired) seem to exist.

Confinement (of quarks) The curious fact that quarks are never found floating about by themselves; they are always confined inside protons, neutrons, or other particles. Quark confinement was given a theoretical foundation with the discovery of asymptotic freedom by David Gross, Frank Wilzcek, and David Politzer, who shared the 2004 Nobel Prize in Physics for the discovery. Confinement still lacks a complete theoretical explanation, though.

Conserved quantity Take any of the basic measurable physical quantities (like mass, charge, spin, position, and velocity) and combine them mathematically according to some fixed formula. If the resulting combination does not change when the particles interact, it is a conserved quantity. Examples are the total energy and total momentum of a system.

Constructive / destructive interference Combine waves from two (or more) different sources, and you will find that the waves reinforce each other at some locations, causing larger ups and downs (constructive inference), and cancel each other at other locations, causing reduced oscillation or none at all (destructive interference).

Coupling constant A number that expresses the strength of a particular interaction.

CP symmetry / CP violation The C (charge conjugation) operation exchanges all the particles of a theory with their antiparticles. The P (parity) operation reflects everything into its mirror image. If a theory remains unchanged when these two operations are performed simultaneously, it has CP symmetry. The observed asymmetry between matter and antimatter can only be explained if CP symmetry is violated.

Dark energy The generic term used for whatever is driving the accelerating expansion of the universe.

Dark matter Astronomical observations of galaxies and galaxy clusters reveal that they contain much more mass than can be ascribed to the visible matter (stars and dust) they contain. Dark matter is the term for whatever accounts for the remaining mass. This missing mass cannot be normal matter or antimatter.

Decay Atoms or particles that spontaneously transform into other particles are said to decay. The decays are random, but they can be characterized by the lifetime, the average time it takes for a collection of the particles to decay.

Dirac equation The equation, formulated by P. A. M. Dirac, that gives a relativistic description of electron and positron behavior.

Eightfold Way A classification of particles by SU(3) symmetry. The Eightfold Way was superseded by the quark model.

Electric charge A property of some particles that causes them to attract or repel one another. All known particles are either uncharged (neutral) or have charge equal to a whole number multiple of one-third of the proton’s charge. In this book, electric charges are always given as a multiple of the proton charge.

Electric field The means by which a charged particle influences other charged particles. Classically, the electric field is represented by an arrow at every point in space. In relativistic quantum field theory, the electric field is the result of a cloud of uncountable virtual photons.

Electromagnetic waves Mutually reinforcing changes in the electric and magnetic fields that carry energy from one place to another. Radio waves, microwaves, X rays, and visible light are all electromagnetic waves.

Electroweak force In the Standard Model, the electromagnetic force is unified with the weak force into a single force, known as the electroweak force.

Electron Along with the proton and neutron, one of the particles that make up ordinary atoms. The electron’s electric charge is equal but opposite to the proton’s, and the electron’s mass is about 1/2000 of the proton’s mass.

Electron-volt The amount of energy gained by an electron moving through one volt of electric potential. Also used as a unit of mass through E = mc2.

Elementary particle A particle that cannot be separated into constituent particles.

Energy A conserved quantity that measures the capability of an object to influence other objects. Energy comes in two forms: kinetic energy that an object has due to its motion, and potential energy that an object has due to its interactions with other objects.

Family The elementary fermions of the Standard Model can be organized into three families. The first (or lightest) family consists of the electron, the electron neutrino, the up quark, and the down quark. The second family consists of the muon, mu neutrino, charm quark, and strange quark, all of which have charge and spin identical to the corresponding first-family particles. The third family consists of the tau, tau neutrino, top quark, and bottom quark, again with the same pattern of charges and spins.

Fermilab (Fermi National Accelerator Laboratory) One of the largest particle accelerator facilities in the world, located in Batavia, Illinois. Fermilab experiments discovered the bottom and top quarks and the tau neutrino.

Fermion Any particle with spin 1/2, 1 1/2, 2 1/2, and so on. Fermions obey the Pauli exclusion principle: Two identical fermions can never occupy the same quantum state.

Fine structure The details of the line spectrum of an element.

Fine structure constant The coupling constant that expresses the strength of the electromagnetic force. Denoted by α, it is related to the electron charge e, Planck’s constant ℏ, and the speed of light c by


Flavor The name for the type of quark: up, down, charm, strange, top, and bottom. Each quark flavor has a different mass. The term does not, of course, imply that quarks can be distinguished by tasting them.

Frame of reference A system that can be used to locate the positions of objects and the time of events.

Fundamental units It is much easier to talk about particle properties in terms of fundamental units rather than laboratory units. In this book, electric charge is given in multiples (or fractions, in the case of quarks) of the proton’s charge, and spin is measured in units of Planck’s constant, ℏ. For instance, using fundamental units we can say, “the electron is a particle with charge -1 and spin 1/2,” rather than, “the electron is a particle with charge -1.609x10-19coulombs and spin 5.28x10-35 Joule-seconds.”

Gamma ray A high-energy photon, generated in some radioactive decays and in particle-antiparticle annihilations, as well as in other processes.

General relativity Albert Einstein’s theory of gravity as a consequence of curved spacetime.

Gluon The massless intermediate particle that carries the color force.

GUTs (grand unified theories) Theories in which the Standard Model’s SU(3) x SU(2) x U(1) symmetry is replaced by a larger symmetry. These theories reproduce all of the predictions of the Standard Model, but they predict additional particles and interactions (such as proton decay) that do not occur in the Standard Model. GUTs do not unify gravity with the other forces.

Hadron A generic term for any particle built of quarks.

Harmonic oscillator Any vibrating system in which the vibration frequency does not depend on the amplitude. Almost any vibration can be treated as approximately harmonic for small oscillations.

Higgs particle The symmetry-breaking particle of the Standard Model. It has not yet been detected in experiments, but there is hope that it will be found in the next generation of CERN experiments.

Hole In Dirac’s electron theory, a hole is a positive charge that is left behind when an electron is lifted out of the negative-energy “sea.” In modern terms, we simply call it a positron.

Intermediate particle A particle whose exchange is responsible for one of the forces of the Standard Model: strong, weak, or electromagnetic. Intermediate particles all have spin 1. They are also known as Yang-Mills bosons or gauge bosons.

Invariance If a theory predicts that the result of any experiment is independent of some property (for instance, the location or orientation of the experiment), we say the theory has an invariance.

Isospin A particle property invented to explain certain types of nuclear interactions. In the Standard Model, these interactions are explained in terms of quarks.

Kaon (K+, K-, K0, and1300) A meson formed of one up or down quark and one antistrange quark, or else of one strange quark and one antiup or antidown quark. Kaons were the first strange particles discovered.

Lagrangian The mathematical expression that summarizes all of the properties and interactions of the particles in a relativistic quantum field theory.

Larnbda-zero particle (A°) A baryon formed of an up quark, a down quark, and a strange quark.

Least action principle The global formulation of the laws of physics, which states that a particle will follow the path along which the sum of its Lagrangian values is the smallest.

Lepton Any of the lightweight particles: electron, muon, tau, or their neutrinos. The leptons are all fermions and have spin 1/2.

Lifetime (of a particle) See Decay.

Line spectrum The pattern of bright lines that forms when the light emitted by excited atoms or molecules is passed through a prism.

Magnetic field The field created by a moving electric charge. Elementary particles such as the electron also have an intrinsic magnetic field that cannot be ascribed to the motion of charge since the electron, as far as we know, has no structure.

Magnetic moment A measure of the magnetic strength of a particle (or of a magnet). It can be measured by applying a magnetic field and measuring the rate at which the particle’s spin axis rotates.

Mass Roughly speaking, the mass of an object is simply its weight. More precisely, the mass is the object’s inertia—that is to say, its resistance to a force.

Mass-energy Thanks to Einstein’s E = mc2, we know that matter can be converted into energy and vice versa. It therefore makes sense to lump mass and energy together in a single term.

Maxwell’s equations The equations that describe the behavior of classical electric and magnetic fields.

Meson Originally, a term for the middleweight particles: heavier than the electron but lighter than the proton. In modern usage, any particle formed from one quark and one antiquark.

Mexican hat potential The particular form of the Higgs field’s self-interaction that allows spontaneous symmetry breaking to occur. Drawn on a graph, it looks like a sombrero.

Muon (µ+, µ-) A lepton with the same electric charge and spin as the electron, but a mass 200 times larger.

Neutrino (νe’ νµ, ντ) A lepton with no electric charge and a mass much smaller than the electron’s mass. Neutrinos come in three flavors: electron neutrino, muon neutrino, and tau neutrino. They only interact via the weak force.

Neutron A neutral, spin -1/2 particle that is one of the three main constituents of ordinary matter. The neutron is slightly heavier than the proton. It consists of one up quark and two down quarks.

Noether’s theorem States that for any continuous symmetry of a theory there is a corresponding conserved quantity.

Nucleon In isospin theory the neutron and the proton are considered two different states of a single particle, the nucleon.

Nucleus The heavy central core of an atom, consisting of protons and neutrons and bound together by the strong force.

Omega-minus-) A baryon formed of three strange quarks.

Pair production The process in which a particle and its antiparticle are produced from pure energy.

Particle accelerator A device for accelerating particles, usually electrons or protons, to extremely high energies in order to perform collision experiments. Accelerator designs include cyclotrons, synchrotrons, and linear accelerators. Today’s most powerful accelerators are many miles long.

Parity The operation of flipping an object into its mirror image. For instance, if you raise your right hand, your mirror image raises its left hand. If the parity operation leaves a situation or theory unchanged, it is said to be invariant under parity.

Parton The name used for a particle that is a constituent of a proton or neutron. Parton is used instead of quark when the full theory of QCD is not assumed.

Pauli exclusion principle The principle that two identical fermions cannot occupy the same quantum state.

Perturbation expansion Solving a problem by starting with only the largest influences, then adding successive corrections. In relativistic quantum field theory, the procedure of adding successively more complicated Feynman diagrams is a perturbation expansion.

Photoelectric effect The flow of electric current produced when certain metals are illuminated with light of sufficiently short wavelength. Einstein’s theory of the photoelectric effect provided some of the first evidence for photons.

Photon A quantum or particle of light. The photon has spin 1 and has no rest mass. The photon is its own antiparticle.

Pion +, π-, π0) A meson that carries the force that binds protons and neutrons in the nucleus.

Planck energy (Planck length, Planck time) The energy obtained by combining the three fundamental constants of nature: the speed of light, Planck’s constant, and the gravitational constant. Particle collisions that occur at this energy would presumably need to be described by a theory that unifies gravity and quantum mechanics. The same three constants can be combined to get the Planck length, which is about 10-35 meter, and the Planck time, which is about 10-43 second.

Planck’s constant (h) The fundamental constant of quantum mechanics. It is also the fundamental unit of spin.

Positron (e+) The antiparticle of the electron. It has the same mass and spin as the electron but the opposite electric charge.

Proton A spin 1/2 particle with charge +1 that is one of the three main constituents of ordinary matter. It consists of two up quarks and one down quark.

QED (quantum electrodynamics) The relativistic quantum field theory of electrons, positrons, and photons.

Quantum field The mathematical function that describes the motion of a particle or particles in quantum mechanics and relativistic quantum field theory. The square of the field value gives the probability of detecting a particle at a particular location. In quantum mechanics the quantum field is also known as the wavefunction.

Quantum mechanics The theory developed to explain the bizarre behavior of very small objects. Quantum mechanics introduced a new fundamental constant, Planck’s constant, and relinquished the certainty of classical mechanics for probability.

Quark An elementary spin 1/2 particle of the Standard Model. Quarks come in six flavors: up, down, charm, strange, top, and bottom. Each flavor comes in three colors: red, blue, and green. Only up quarks and down quarks are present in everyday objects, in the protons and neutrons of atomic nuclei.

Relativistic quantum field theory The extension of the early version of quantum mechanics to make it compatible with special relativity. Except for string theories, all modern elementary particle theories are relativistic quantum field theories.

Relativity / relativistic In this book, these terms always refer to special relativity, Einstein’s theory of the behavior of objects moving at high velocity. The basic assumption of the theory is that the speed of light is a constant of nature, independent of the motion of the light source.

Renormalization The procedure for eliminating the infinities that arise in calculations in relativistic quantum field theories. Not all such theories can be renormalized; renormalizability is usually considered a requirement for a viable theory.

Rest mass / rest energy According to special relativity, an object with mass has energy even when it is at rest, in the amount E = mc2. This energy can be released by particle-antiparticle annihilation, for instance. A photon always moves at the speed of light; it has no rest mass.

Scaling A property observed in some high energy particle collisions that gave experimental support to the quark model.

Schrödinger equation The basic equation of quantum mechanics. The solution to this equation is called the quantum field.

Special relativity See Relativity.

Spin Along with mass and electric charge, a fundamental property of elementary particles. It helps to think of a particle as a small ball that spins constantly without ever slowing down. Only the direction of the spin axis can be changed. However, this picture is necessarily incorrect. The spin of a particle cannot be explained in terms of ordinary rotation, it must simply be accepted as a defining property of the particle.

Spontaneous symmetry breaking A sudden change from a symmetrical situation to an asymmetrical one, as when a pencil balanced on its point spontaneously falls.

Strange particles Particles containing at least one strange or antistrange quark.

String theory A class of speculative theories in which elementary particles are not pointlike objects but rather extremely small one-dimensional objects called strings.

Strong force The force that binds protons and neutrons in the atomic nucleus and is responsible for certain types of particle decays. Nowadays used also for the SU(3) color force that binds quarks together.

SU(3) symmetry (SU(3) color force) The symmetry that relates red, blue, and green quarks. The existence of the symmetry explains the color force between quarks, because any theory with such symmetry will necessarily have gluons that mediate the force. The SU(3) symmetry of the Eightfold Way, however, is a different type of symmetry, unrelated to the quark colors.

Superposition state When a particle has a measurable probability of being in two (or more) different quantum states.

Supersymmetry A symmetry that requires a fermion corresponding to every boson, and vice versa. If supersymmetry is unbroken, the fermion and its corresponding boson must have the same mass. If supersymmetry truly describes our universe, it must be a broken symmetry.

Symmetry An operation that leaves an object or theory unchanged, for example, rotating a sphere.

Tau (τ+, τ-) The tau, like the muon, is a particle with the same charge and spin as the electron, but with a much larger mass.

Totalitarian theorem “Anything that is not forbidden is compulsory.” An expression of the fact that, in relativistic quantum field theory, any particle interaction that is not ruled out by a symmetry or conservation law will happen at least some of the time.

Vacuum The state with no particles—in other words, empty space.

Virtual particle An undetectable bit of a relativistic quantum field. Virtual particles can exhibit bizarre behavior such as traveling faster than light, but since they cannot be detected directly, they do not violate special relativity. Though the particles are undetectable, their effects are important; they are the source of the forces between real particles.

W particle (W+, W-) Particles predicted to exist by the Standard Model’s spontaneous symmetry breaking, and detected for the first time in particle collisions at CERN in 1983. The W particles (and the Z0) mediate the weak force.

Weak force (weak nuclear force) The force responsible for beta-decay and certain other forms of radioactivity. In the Standard Model, it is simply one aspect of the unified electroweak interaction.

WIMP (weakly interacting massive particle) The generic term for particles that may make up some, or most, of the dark matter in the universe. Although (by definition) they interact weakly with each other and with normal matter, they need not interact via the weak force.

Yang-Mills theory A theory with a continuous symmetry (called a gauge symmetry) such as the SU(3) symmetry of the color force. Also known as a gauge theory.

Zeeman effect The splitting of the lines of the hydrogen atom’s spectrum when the atom is subjected to a magnetic field.

Z° particle Along with the W particle, an intermediate particle of the unified electroweak theory. The Z0 was detected for the first time in particle collisions at CERN in 1983.