Glossary

See also the page on pronunciation.

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accelerator: A machine used to accelerate particles to high speeds (and thus high energy compared to their rest-mass energy).

annihilation: A process in which a particle meets its corresponding antiparticle and both disappear. The energy appears in some other form, perhaps as a different particle and its antiparticle (and their energy), perhaps as many mesons, perhaps as a single neutral boson. The produced particles may be any combination allowed by conservation of energy and momentum and of all the charge types.

antimatter: Material made from antifermions. We define the fermions that are common in our universe as matter and their antiparticles as antimatter. In the particle theory there is no a priori distinction between matter and antimatter. The asymmetry of the universe between these two classes of particles is a deep puzzle for which we are not yet completely sure of an explanation.

antiparticle: For every fermion type there is another fermion type that has exactly the same mass but the opposite value of all other charges (quantum numbers). This is called the antiparticle. For example, the antiparticle of an electron is a particle of positive electric charge called the positron. Bosons also have antiparticles except for those that have zero value for all charges, for example a photon or a composite boson made from a quark and its corresponding antiquark. In this case there is no distinction between the particle and the antiparticle, they are the same object.

antiquark: The antiparticle of a quark.

astrophysics: The physics of astronomical objects such as stars and galaxies.

B-factory: An accelerator designed to maximize the production of B mesons. The properties of the B mesons are then studied with special detectors.

baryon: A hadron made from three quarks. The proton (uud) and the neutron (udd) are both baryons. They may also contain additional quark-antiquark pairs.

beam: The particle stream produced by an accelerator usually clustered in bunches.

big bang theory: The theory of an expanding universe that begins as an infinitely dense and hot medium. The initial instant is called the Big Bang.

boson: A particle that has integer intrinsic angular momentum (spin) measured in units of h-bar (spin =0, 1, 2, ...). All particles are either fermions or bosons. The particles associated with all the fundamental interactions (forces) are bosons. Composite particles with even numbers of fermion constituents (quarks) are also bosons.

bottom quark (b): The fifth flavor of quark (in order of increasing mass), with electric charge of -1/3.

CERN: The major European international accelerator laboratory located near Geneva, Switzerland.

charge: A quantum number carried by a particle. Determines whether the particle can participate in an interaction process. A particle with electric charge has electrical interactions; one with strong charge has strong interactions, etc.

charge conservation: The observation that electric charge is conserved in any process of transformation of one group of particles into another.

charm quark (c): The fourth quark (in order of increasing mass), with electric charge +2/3.

collider: An accelerator in which two beams traveling in opposite directions are steered together to provide high-energy collisions between the particles in one beam and those in the other.

color charge: The quantum number that determines participation in strong interactions. Quarks and gluons carry nonzero color charges.

confinement: The property of the strong interaction that quarks or gluons are never found separately but only inside color-neutral composite objects.

conservation: When a quantity (e.g. electric charge, energy, or momentum) is conserved, it is the same after a reaction between particles as it was before.

cosmology: The study of the history of the universe.

dark matter: Matter that is in space but is not visible to us because it emits no radiation by which to observe it. The motion of stars around the centers of their galaxies implies that about 90% of the matter in a typical galaxy is dark. Physicists speculate that there is also dark matter between the galaxies but this is harder to verify.

decay: A process in which a particle disappears and in its place different particles appear. The sum of the masses of the produced particles is always less than the mass of the original particle.

down quark (d): The second flavor of quark (in order of increasing mass), with electric charge -1/3.

electric charge: The quantum number that determines participation in electromagnetic interactions .

electromagnetic interaction: The interaction due to electric charge; this includes magnetic interactions.

electron (e): The least massive electrically-charged particle, hence absolutely stable. It is the most common lepton, with electric charge -1.

electroweak interaction: In the Standard Model, electromagnetic and weak interactions are related (unified); physicists use the term electroweak to encompass both of them.

event: What occurs when two particles collide or a single particle decays. Particle theories predict the probabilities of various possible events occurring when many similar collisions or decays are studied. They cannot predict the outcome for any single event.

Fermilab: Fermi National Accelerator Laboratory in Batavia, Illinois (near Chicago). Named for particle physics pioneer Enrico Fermi.

fermion: Any particle that has odd-half-integer (1/2, 3/2, ...) intrinsic angular momentum (spin), measured in units of h-bar. As a consequence of this peculiar angular momentum, fermions obey a rule called the Pauli Exclusion Principle, which states that no two fermions can exist in the same state at the same time. Many of the properties of ordinary matter arise because of this rule. Electrons, protons, and neutrons are all fermions, as are all the fundamental matter particles, both quarks and leptons.

fixed-target experiment: An experiment in which the beam of particles from an accelerator is directed at a stationary (or nearly stationary) target. The target may be a solid, a tank containing liquid or gas, or a gas jet.

flavor: The name used for the different quarks types (up, down, strange, charm, bottom, top) and for the different lepton types (electron, muon, tau). For each charged lepton flavor there is a corresponding neutrino flavor. In other words, flavor is the quantum number that distinguishes the different quark/lepton types. Each flavor of quark and charged lepton has a different mass. For neutrinos we do not yet know if they have a mass or what the masses are.

fundamental interaction: In the Standard Model the fundamental interactions are the strong, electromagnetic, weak, and gravitational interactions. There is at least one more fundamental interaction in the theory that is responsible for fundamental particle masses. Five interaction types are all that are needed to explain all observed physical phenomena.

fundamental particle: A particle with no internal substructure. In the Standard Model the quarks, leptons, photons, gluons, W+ and W- bosons, and the Z bosons are fundamental. All other objects are made from these.

generation: A set of one of each charge type of quark and lepton, grouped by mass. The first generation contains the up and down quarks, the electron and the electron neutrino.

gluon (g): The carrier particle of the strong interactions.

gravitational interaction: The interaction of particles due to their mass/energy.

graviton: The carrier particle of the gravitational interactions; not yet directly observed.

hadron: A particle made of strongly-interacting constituents (quarks and/or gluons ). These include the mesons and baryons. Such particles participate in residual strong interactions.

interaction: A process in which a particle decays or it responds to a force due to the presence of another particle (as in a collision). Also used to mean the underlying property of the theory that causes such effects.

kaon (K): A meson containing a strange quark and an anti-up (or anti-down) quark, or an anti-strange quark and an up (or down) quark.

lepton: A fundamental fermion that does not participate in strong interactions. The electrically-charged leptons are the electron (e), the muon (µ), the tau (), and their antiparticles. Electrically-neutral leptons are called neutrinos ().

LHC: The Large Hadron Collider at the CERN laboratory in Geneva, Switzerland. LHC will collide protons into protons at a center-of-mass energy of about 14 TeV. When completed in the year 2004, it will be the most powerful particle accelerator in the world. It is hoped that it will unlock many of the secrets of particle physics.

linacs: An abbreviation for linear accelerator, that is, an accelerator that has no bends in it.

mass: see rest mass.

meson: A hadron made from an even number of quark constituents The basic structure of most mesons is one quark and one antiquark.

muon (µ): The second flavor of charged leptons (in order of increasing mass), with electric charge -1.

muon chamber: The outer layers of a particle detector capable of registering tracks of charged particles. Except for the chargeless neutrinos, only muons reach this layer from the collision point.

neutral: Having a net charge equal to zero. Unless otherwise specified, it usually refers to electric charge.

neutrino (): A lepton with no electric charge. Neutrinos participate only in weak and gravitational interactions and are therefore very difficult to detect. There are three known types of neutrinos, all of which are very light and could possibly have zero mass.

neutron (n): A baryon with electric charge zero; it is a fermion with a basic structure of two down quarks and one up quark (held together by gluons). The neutral component of an atomic nucleus is made from neutrons. Different isotopes of the same element are distinguished by having different numbers of neutrons in their nucleus.

nucleus: A collection of neutrons and protons that forms the core of an atom (plural: nuclei).

particle: A subatomic object with a definite mass and charge.

Pauli exclusion principle: see fermions

photon: The carrier particle of electromagnetic interactions.

pion (): The least massive type of meson, pions can have electric charges of +1, -1, or 0.

positron (e+): The antiparticle of the electron.

proton (p): The most common hadron, a baryon with electric charge +1 equal and opposite to that of the electron. Protons have a basic structure of two up quarks and one down quark (bound together by gluons). The nucleus of a hydrogen atom is a proton. A nucleus with electric charge Z contains Z protons; therefore the number of protons is what distinguishes the different chemical elements.

quantum: The smallest discrete amount of any quantity (plural: quanta).

quantum mechanics: The laws of physics that apply on very small scales. The essential feature is that electric charge, momentum, and angular momentum, as well as charges, come in discrete amounts called quanta.

quark (q): A fundamental fermion that has strong interactions. Quarks have electric charge of either +2/3 (up, charm, top) or -1/3 (down, strange, bottom) in units where the proton charge is 1.

residual interaction: Interaction between objects that do not carry a charge but do contain constituents that have that charge. Although some chemical substances involve electrically-charged ions, much of chemistry is due to residual electromagnetic interactions between electrically-neutral atoms. The residual strong interaction between protons and neutrons, due to the strong charges of their quark constituents, is responsible for the binding of the nucleus.

rest mass: The rest mass (m) of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by the speed of light squared. When particle physicists use the word "mass, " they always mean the "rest mass" (m) of the object in question.

SLAC: The Stanford Linear Accelerator Center in Stanford, California.

spin: Intrinsic angular momentum, where h-bar=h/2 = 6.58x10^(-34) Js.

stable: Does not decay. A particle is stable if there exist no processes in which a particle disappears and in its place different particles appear.

Standard Model: Physicists' name for the theory of fundamental particles and their interactions, described in these pages. It is widely tested and is accepted as correct by particle physicists.

strange quark (s): The third flavor of quark (in order of increasing mass), with electric charge -1/3.

strong interaction: The interaction responsible for binding quarks, antiquarks, and gluons to make hadrons. Residual strong interactions provide the nuclear binding force.

subatomic particle: Any particle that is small compared to the size of the atom.

synchrotron: A type of circular accelerator in which the particles travel in synchronized bunches at fixed radius.

tau (): The third flavor of charged lepton (in order of increasing mass), with electric charge -1.

top quark (t): The sixth flavor of quark (in order of increasing mass), with electric charge 2/3. Its mass is much greater than any other quark or lepton.

track: The record of the path of a particle traversing a detector.

tracking: The reconstruction of a "track" left in a detector by the passage of a particle through the detector.

uncertainty principle: The quantum principle, first formulated by Heisenberg, that states that is is not possible to know exactly both the position x and the momentum p of an object at the same time. The same is true with energy and time (see virtual particle).

up quark (u): The least massive flavor of quark, with electric charge 2/3.

virtual particle: A particle that exists only for an extremely brief instant in an intermediary process. Then the Heisenberg Uncertainty Principle allows an apparent violation of the conservation of energy. However, if one sees only the initial decaying particle and the final decay products, one observes that the energy is conserved.

W+, W- boson: A carrier particle of the weak interactions. It is involved in all electric-charge-changing weak processes.

weak interaction: The interaction responsible for all processes in which flavor changes, hence for the instability of heavy quarks and leptons, and particles that contain them. Weak interactions that do not change flavor (or charge) have also been observed.

Z boson: A carrier particle of the weak interactions. It is involved in all weak processes that do not change flavor.