A machine used to accelerate particles to high speeds, and thus high energy compared to their rest-mass energy.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
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 particle of positive electric charge called the positron. Most boson types also have antiparticles except for those that have zero value for all charges, such as a photon or a composite boson made from a quark and its corresponding antiquark. In these cases there is no distinction between the particle and the antiparticle; they are the same object.
[ Top ] [ Close Glossary ]
The antiparticle of a quark.
[ Top ] [ Close Glossary ]
The physics of astronomical objects such as stars and galaxies.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The particle stream injected into an accelerator. These particles travel in clusters.
[ Top ] [ Close Glossary ]
The theory that the expanding universe began as an infinitely dense and hot medium. The initial instant is called the Big Bang.
[ Top ] [ Close Glossary ]
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 even numbers of fermion constituents (quarks) are bosons.
[ Top ] [ Close Glossary ]
The fifth flavor of quark (in order of increasing mass), with electric charge of -1/3.
[ Top ] [ Close Glossary ]
CERN (European Laboratory for Particle Physics) is the major European international accelerator laboratory located near Geneva, Switzerland.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The observation that electric charge is conserved in any process of transformation of one group of particles into another.
[ Top ] [ Close Glossary ]
Charm (c) is the fourth quark (in order of increasing mass), with an electric charge +2/3.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The quantum number that determines participation in strong interactions. Quarks and gluons carry nonzero color charges.
[ Top ] [ Close Glossary ]
The property of the strong interaction that quarks or gluons are never found separately but only inside color-neutral composite objects.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The study of the history of the universe
[ Top ] [ Close Glossary ]
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 that there is also dark matter between the galaxies, but this will be much harder to verify.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The down quark (d) is the second flavor of quark (in order of increasing mass), with an electric charge -1/3.
[ Top ] [ Close Glossary ]
The quantum number that determines participation in electromagnetic interactions.
[ Top ] [ Close Glossary ]
The interaction due to electric charge; this includes magnetic interactions.
[ Top ] [ Close Glossary ]
The least massive electrically-charged particle, hence absolutely stable. It is the most common lepton, with electric charge -1.
[ Top ] [ Close Glossary ]
In the Standard Model, electromagnetic and weak interactions are related (unified); physicists use the term electroweak to encompass both of them.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
Fermi National Accelerator Laboratory in Batavia, Illinois (near Chicago). Named for particle physics pioneer Enrico Fermi.
[ Top ] [ Close Glossary ]
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 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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
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 lepton has a different mass.
[ Top ] [ Close Glossary ]
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 interaction that is responsible for fundamental particle masses.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The carrier particle of the strong interactions.
[ Top ] [ Close Glossary ]
The interaction of particles due to their mass/energy.
[ Top ] [ Close Glossary ]
The carrier particle of the gravitational interactions; not yet directly observed.
[ Top ] [ Close Glossary ]
A particle made of strongly-interacting constituents (quarks and/or gluons). These include the meson and baryons. Such particles participate in residual strong interactions.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
A fundamental fermion that does not participate in
strong interactions.
The electrically-charged leptons are the electron (e-),
the muon (
),
the tau (
),
and their antiparticles.
Electrically-neutral leptons
are called neutrinos (
).
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
An abbreviation for linear accelerator, that is, an accelerator that has no bends in it.
[ Top ] [ Close Glossary ]
See rest mass.
[ Top ] [ Close Glossary ]
A hadron made from an even number of quark-antiquark constituents. The basic structure of most meson is one quark and one antiquark.
[ Top ] [ Close Glossary ]
)The second flavor of charged leptons (in order of increasing mass), with electric charge -1.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
Having a net charge equal to zero. Unless otherwise specified, it usually refers to electric charge.
[ Top ] [ Close Glossary ]
)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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
A collection of neutrons and protons that forms the core of an atom (plural: nuclei).
[ Top ] [ Close Glossary ]
A subatomic object with a definite mass and charge.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
)The force carrier particle of electromagnetic interactions.
[ Top ] [ Close Glossary ]
)The least massive type of meson, pions can have electric charges of +1, -1, or 0.
[ Top ] [ Close Glossary ]
The antiparticle of the electron.
[ Top ] [ Close Glossary ]
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, which is why the number of protons is what distinguishes the different chemical elements.
[ Top ] [ Close Glossary ]
The smallest discrete amount of any quantity (plural: quanta).
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
An 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.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The Stanford Linear Accelerator Center in Stanford, California.
[ Top ] [ Close Glossary ]
A quantum particle property of intrinsic angular momentum.
[ Top ] [ Close Glossary ]
Does not decay. A particle is stable if there exist no processes in which a particle disappears and in its place different particles appear.
[ Top ] [ Close Glossary ]
Physicists have developed a theory of fundamental particles and interactions called the Standard Model. This site describes various aspects of this model.
[ Top ] [ Close Glossary ]
The third flavor of quark (in order of increasing mass), with electric charge -1/3.
[ Top ] [ Close Glossary ]
The interaction responsible for binding quarks, antiquarks, and gluons to make hadrons. Residual strong interactions provide the nuclear binding force.
[ Top ] [ Close Glossary ]
Any particle that is small compared to the size of the atom.
[ Top ] [ Close Glossary ]
A type of circular accelerator in which the particles travel in synchronized bunches at fixed radius.
[ Top ] [ Close Glossary ]
)The third flavor of charged lepton (in order of increasing mass), with electric charge -1.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
The record of the path of a particle traversing a detector.
[ Top ] [ Close Glossary ]
The reconstruction of a "track" left in a detector by the passage of a particle through the detector.
[ Top ] [ Close Glossary ]
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).
[ Top ] [ Close Glossary ]
The least massive flavor of quark, with electric charge 2/3.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
A carrier particle of the weak interactions. It is involved in all electric-charge-changing weak processes.
[ Top ] [ Close Glossary ]
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.
[ Top ] [ Close Glossary ]
A carrier particle of the weak interactions. It is involved in all weak processes that do not change flavor
[ Top ] [ Close Glossary ]