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 such as a Z boson. The produced particles may be any combination allowed by conservation of energy and momentum and of all the charge types and other rules.

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. An antiquark is denoted by putting a bar over the corresponding quark (, , , etc.).

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.

baryon-antibaryon asymmetry: The observation that the universe contains many baryons but few antibaryons; a fact that needs explanation.

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

boson: A particle that has integer intrinsic angular momentum (spin) measured in units of  (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 -1/3.

calorimeter: A device that can measure the energy deposited in it. (originally devices to measure heat energy deposited, using change of temperature; particle physicists use the word for any energy measuring device.)

CERN: The major European international accelerator laboratory located near Geneva, Switzerland, the European Laboratory for Particle Physics.

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 particle into another.

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

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

colliding-beam experiments: Experiments done at colliders.

color charge: The quantum number that determines participation in strong interactions, quarks and gluons carry non-zero color charges.

color neutral: An object with no net color charge. For composites made of color charged particles the rules of neutralization are complex. Three quarks (baryon) or a quark plus an antiquark (meson) can both form color-neutral combinations.

confinement: The property of the strong interactions 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.

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

DESY: Deutsches Elektronen-Sychrotron at Hamburg/Germany. The laboratory performs basic research in high-energy and particle physics as well as in the production and application of synchrotron
radiation.

detector: Any device used to sense the passage of a particle. Also a collection of such devices designed so that each serves a particular purpose in allowing physicists to reconstruct particle events.

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 effects which have to do with moving electric charges.

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.

eV (electron-volt): The energy gained by an electron which accelerates through a potential difference of one volt.

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.

exclusion principle: See  fermion.

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 . 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 place and 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 quark 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^± bosons, and 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 strong interactions.

grand unified theory: Any of a class of theories which contain the Standard Model but go beyond it to predict further types of interactions mediated by particles with masses of order 10¹⁵ GeV/c². At energies large compared to this mass (times c²) the strong, electromagnetic and weak interactions are seen as different aspects of one unified interaction.

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.

Higgs boson: The carrier particle or quantum excitation of the additional force needed to introduce particle masses in the Standard Model. Not yet observed.

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.

jet: Depending on their energy, the quarks and gluons emerging from a collision will materialize into 5-30 particles (mostly mesons and baryons). At high momentum, these particles will appear in clusters called ``jets,'' that is, in groups of particles moving in roughly the same direction, centered about the original quark or gluon.

kaon (K): A meson containing a strange quark and an anti-up (or an 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 (), 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 2005, it will be the most powerful particle accelerator in the world. It is hoped that it will unlock many of the remaining secrets of particle physics.

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

luminosity: The number of particles per square-centimeter per second generated in the beams of high energy particle experiments. The higher the luminosity, the greater the number of events produced for study.

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.

MeV: One million electron-volts, where one electron-volt is the energy gained by an electron which accelerates through a potential difference of one volt.

microwave: An electromagnetic wave with wavelength in the micron range.

muon (): The second flavor of charged lepton (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 specified otherwise, it usually refers to electric charge.

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

nucleon: A proton or a neutron; that is, one of the particles that makes up a 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.

photon (): The carrier particle of electromagnetic interactions.

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

plasma: A gas of charged particles.

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 (-1). 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 energy, momentum, and angular momentum as well as charges come in discrete amounts called quanta.

quantum theory: The theory which describes laws of physics that apply on very small scales. The essential feature is that energy, 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 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 c². When particle physicists use the word ``mass,'' they always mean the ``rest mass'' (m) of the object in question. The total energy of a free particle is given by  where p is the momentum of the particle. Note that for p=0 this simplifies to Einstein's famous E=mc². For a general particle with mass and momentum, it can also be written as E =  m c² where  Some textbooks on special relativity identify  m as the ``mass'' of a moving  particle; this definition is not used in particle physics. The quantity E includes both the mass energy and the kinetic energy.

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

spin: Intrinsic angular momentum of a particle, given in units of , the quantum unit of angular momentum, where = h/2 = 6.58 x 10^-34 Js.

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

Standard Model: Physicists' name for the theory of fundamental particles and their interactions. 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 lepton: The third flavor of charged lepton (in order of increasing mass), with electric charge -1.

top quark: 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 it is not possible to know exactly both the position x and the momentum p of an object at the same time.  x  p . It can be written as  E  t  where  E means the uncertainty in energy and  t the uncertainty in lifetime of a state (see  virtual particle).

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

vertex detector: A detector placed very close to the collision point in a colliding beam experiment so that tracks coming from the decay of a short-lived particle produced in the collision can be accurately reconstructed and seen to emerge from a `vertex' point that is different from the collision point.

virtual particle: A particle that exists only for an extremely brief instant as an intermediary in a process. The intermediate or virtual particle stages of a process cannot be directly observed. If they were observed, we might think that conservation of energy was violated. However, the Heisenberg Uncertainty Principle (which can be written as  E ^.  t > /2) allows an apparent violation of the conservation of energy. If one sees only the initial decaying particle (such as a meson with the c quark) and the final decay products (such as s + _e + e⁺), one observes that energy is conserved. The ``virtual'' particle (such as the W^±) exists for such a short time that it can never be observed.

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 weak interactions. It is involved in all weak processes that do not change flavor.