Glossary

Particle Adventure glossary

80 definitions · page 1 of 2.

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…
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…
Antiparticle: For most particle types (and every fermion type) there is another particle 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…
Antiquark: The antiparticle of a quark.
Astrophysics: The physics of astronomical objects such as stars and galaxies.
B-Factory: The B-Factory is an accelerator at SLAC designed to maximize the production of B mesons. The properties of the B meson are then studied with special detectors.
Baryon: A hadron made from three quarks. The proton (up-up-down) and the neutron (up-down-down) are both baryons. They may also contain additional quark-antiquark pairs.
Beam: The particle stream injected into an accelerator. These particles travel in clusters.
Big Bang Theory: 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) and composite particles with…
Bottom Quark: The fifth flavor of quark (in order of increasing mass), with electric charge of -1/3.
CERN: CERN (European Laboratory for Particle Physics) is 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: Charm (c) is the fourth quark (in order of increasing mass), with an electric charge +2/3.
Collider: A collider is 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 is always the same before and after a particle reaction, it is said to be conserved. Such quantities include electric charge, energy, and momentum.
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 that we can observe. The nature of 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…
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: The down quark (d) is the second flavor of quark (in order of increasing mass), with an electric charge -1/3. [ Top ] [ Close Glossary ]
Electric Charge: The quantum number that determines participation in electromagnetic interactions.
Electromagnetic Interaction: The interaction due to electric charge; this includes magnetic interactions.
Electron: 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. All particles are either fermions or bosons. Fermions obey a rule called the Pauli Exclusion Principle, which states that no two fermions…
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…
Fundamental Interaction: In the Standard Model the fundamental interactions are the strong, electromagnetic, weak, and gravitational interactions. Four interaction types are all that are needed to explain all observed physical phenomena. The theory proposes at least one more…
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. There are three generations of matter.
Gluon: 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 meson 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). The four fundamental interactions are gravitational, electromagnetic, strong, and weak.
Kaon: 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 (u), the tau (t), and their antiparticles. Electrically-neutral leptons are called neutrinos (v).
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…
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-antiquark constituents. The basic structure of most meson is one quark and one antiquark.
Muon (u): 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 (v): 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 have very little 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…
Nucleus: A subatomic object with a definite mass and charge.
Pauli Exclusion Principle: The principle that no two particles in the same quantum state may exist in the same place at the same time. Particles that obey this principle are called fermions; particles that do not are called bosons.
Photon (y): The force carrier particle of electromagnetic interaction
Pion (x): 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…
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.