LEP Physics Glossary

Accelerator

A device (i.e., machine) used to produce high-energy high-speed beams of charged particles, such as electrons, protons, or heavy ions, for research in high-energy and nuclear physics, synchrotron radiation research, medical therapies, and some industrial applications. The LEP collidor is an electron-positron accelerator.

Annihilation

A process in which a particle meets its corresponding antiparticle and both disappear. Their energy and momentum appears in some other form, producing other particles together with their antiparticles and providing their motion.

Antimatter

A material made from antiparticles. The particles that are common in our universe are defined as matter and their antiparticles as antimatter. In the particle theory there is almost no a priori distinction between matter and antimatter. Their interactions are almost identical. The asymmetry of the universe between these two classes of particles is a deep puzzle which is yet to be fully understood.

Antiparticles

In particle physics every particle with any type of charge or fermion label has a corresponding antiparticle type. Any particle and its antiparticle have identical mass and spin but opposite charges. For example the antiparticle of an electron is a positron. It has exactly the same mass as an electron but positive charge. Some particles are their own antiparticles, the antiparticle of a photon is a photon for instance. Conserved quantities such as baryon number and lepton number are further types of "charges" that are reversed for particle and antiparticle. Thus an electron and an electron neutrino both have electron number +1 while their antiparticles the positron and the anti-electron-neutrino have electron number -1.

Beam

A unidirectional or approximately unidirectional flow of electromagnetic radiation or particles.

Bottom Quark or B Quark

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

Carrier Particle

A fundamental boson associated with quantum excitations of the force field corresponding to some interaction. Gluons are carrier particles for strong interactions (color force fields), photons are carrier particles of electromagnetic interactions, and the W and Z bosons are carrier particles for weak interactions.

Calorimeter

In particle physics, any device that can measure the energy deposited in it by particles (originally a device that measured heat energy deposited, thus a calorie-meter).

CERN

The major European International Accelerator Laboratory located near Geneva, Switzerland. (originally called Centre European pour Rechearche Nucleaire). The WWW was created at CERN.

Charge

A quantity carried by a particle that determines its participation in an interaction process. A particle with electric charge has electrical interactions; one with strong charge (or color charge) has strong interactions, etc.

Charm

One "flavor" of quarks. Also known as the C quark.

Decay

Any  process in which a particle disappears and in its place two or more different particles appear.

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 or D Quark

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

Electric Field

A force field which defines what acceleration an electric charge placed at rest at any point in space will feel. Electric charges cause electric fields around them, which then apply a force to any other electric charge placed in the field.  The electric field E has both a magnitude and a direction at each point in space, and the magnitude and direction of the resulting force on a charge q at that point is given by F= qE. When you get a shock from a door handle after scuffing your feet on a carpet you feel the effect of an electric field accelerating electrons.

eV (electronvolt)

The basic unit of energy used in high energy physics. It is the energy gained by one electron when it moves through a potential difference of one volt. By definition an eV is equivalent to 1.6 x 10^-19 joules. This is a very small amount of energy and the more commonly used multiples are MeV (million eV), GeV (billion eV or giga-electronvolt) and TeV (trillion eV).

Electromagnetic Interaction

The interaction due to electric charge; this includes magnetic effects that have to do with moving electric charges.

Electron

The least massive electrically charged particle, therefore absolutely stable. It is the most common lepton with charge -1. An electron is one of the fundamental particles in nature. Fundamental means that, as far as we know, an electron cannot be broken down into smaller particles. Electrons are responsible for many of the phenomena that we observe in everyday life. Mutual repulsion between electrons in the atoms of the floor and those within your shoes keeps you from sinking and disappearing into the floor!!! Electrons carry electrical current and successful manipulation of electrons allows electronic devices, such as the one you are using, to function.

Electron Accelerator

Electrons carry electrical charge and successful manipulation of electrons allows electronic devices to function. The picture and text on the video terminal in front of you is caused by electrons being accelerated and focused onto the inside of the screen , where a phosphor absorbs the electrons and light is produced. A television screen is a simple, low-energy example of an electron accelerator.

Electron Beam

The stream of electrons generated by the electron gun and accelerated by the accelerator guide.

Elementary Particles

The name given to protons, neutrons and electrons before it was discovered that protons and electrons had substructure (quarks). Today we use the term "fundamental" for the six types of quarks and the six leptons and their antiparticles, which  have no known substructure. Gluons, photons and W and Z bosons are also fundamental particles. All other particles are composite, that is  made from combinations of fundamental particles.

Event

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

Fermion

General name for a particle that is a matter constituent, characterized by spin in odd half integer quantum units (1/2,3/2,5/2...). Named for Italian physicist Enrico Fermi.  Quarks, leptons and baryons are all fermions.

Flavor

The name used for the different quark types and the different lepton types. The six flavors of quarks are up, down, strange, charm, bottom, top, in increasing order of mass. The flavors of charged leptons are electron, muon and tau, again in increasing order of mass. For each charged lepton flavor there is a corresponding neutrino flavor.

Fundamental Interaction

The known fundamental interactions are the strong, electromagnetic, weak and gravitational interactions. These interactions explain all observed physical processes but do not explain particle masses. Any force between two objects is due to one or another of these interactions. All known particle decays can be understood in terms of these strong, electromagnetic or weak interactions.

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 particles

Gamma Rays

Gamma rays are electromagnetic waves or photons emitted from the nucleus (center) of an atom. See also: photon.

GeV (Giga Electron Volt)

Unit of energy equal to that acquired by a particle with one electronic charge in passing through a potential difference of one billion volts.

Gluon

The carrier particle of the strong interaction.

Hadron

Any  particle made of quarks and gluons, i.e. a meson or a baryon. All such particles have no strong charge (i.e are strong charge neutral objects) but  participate in residual strong interactions due to the strong charges of their constituents.

High-Energy Physics

A branch of science that tries to understand the interactions of the fundamental particles, such as electrons, photons, neutrons and protons (and many others than can be created). These particles are the basic building blocks of everyday matter, making up the human body as well as the entire universe. This type of physics is called high-energy because very powerful machines, are created to make these particles go very fast so that they can probe deeply into other particles and try to understand what they are made of.

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).

Jet

The name physicists give to a cluster of particles emerging from a collision or decay event all traveling in roughly the same direction and carrying a significant fraction of the energy in the event. The particles in the jet are chiefly hadrons.

Lepton

A fundamental matter particle that does not participate in strong interactions. The charged leptons are the electron, the muon, the tau and their antiparticles. Neutral leptons are called neutrinos.

Linear Accelerator

A type of particle accelerator in which charged particles are accelerated in a straight line, either by a steady electrical field or by means of radiofrequency electric fields. In the latter variety, the passage of the particle is synchronized with the phase of the accelerating field. The SLAC Linear Accelerator (linac) is a two-mile long accelerator, consisting of a cylindrical, disc-loaded, copper waveguide placed on concrete girders in a tunnel about 25 feet underground.

Meson

A hadron with the basic structure of one quark and one antiquark.

MeV (Mega Electron Volt)

Energy equal to that acquired by a particle with one electronic charge in passing through a potential difference of one million volts.

Muon

The second 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.  The detector is designed so that the only charged particles that can get out to this layer are muons.

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 have very low or possibly even zero mass.

Neutron

A baryon with electric charge zero. Its basic structure is two down quarks and one up quark.

Nucleus

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

Pair Production and Annihilation

Whenever sufficient energy is available to provide the mass-energy, a particle and its matching antiparticle can be produced (pair production). When a particle collides with its matching antiparticle they may annihilate -- which means they both disappear and their energy appears as some other particles -- with balanced number of particles and antiparticles for each type.  All conservation laws are obeyed in these processes.

Particle

In "particle physics", a subatomic object with definite mass and charge.

Photon

The carrier particle of the electromagnetic interaction. Depending on its frequency (and therefore its energy) photons can have different names such as visible light, X rays and gamma rays. We describe light in several ways. When we talk about "photons" we generally think of uncharged particles with out mass that carry energy (but be careful, there are other particles like this!). Photons of light are known by other names too, such as gamma rays and x-rays. Low-energy forms are called ultraviolet rays, infrared rays, even radio waves! A photon is one of the fundamental particles in nature and it plays an important role involving electron interactions. Photons are the most familiar particles in everyday existence. The light we see, the radiant heat we feel, microwaves we cook with, are make use of photons of different energies. An x-ray is simply a name given to the most energetic of these particles.

Planck's Constant

A fundamental physical constant, the elementary quantum of action, It is the ratio of the energy of a photon to its frequency and is equal to 6.62620 x 10^-34 joule-second. Symbolized by h.

Polarization

A polarized particle beam is a beam of particles whose spins are aligned in a particular direction. The polarization of the beam is the  fraction of the particles with the desired alignment.

Positron

Antiparticle of the electron.

Proton

A baryon with electric charge +1. Protons contain a basic structure of two up quarks and one down quark. The nucleus of a hydrogen atom is a proton. A nucleus with atomic number Z contains Z protons; therefore the number of protons is what distinguishes the different chemical elements.

Quantum

The smallest discrete amount of any substance (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 charge come in discrete amounts called quanta.

Quantum Number

A number that labels a state, it denotes  the number of quanta of a particular type that the state contains. Electric charge given as an integer multiple of  the electron's charge is an example of a quantum number.

Quark

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

Shower (also called Electromagnetic Cascade Shower)

Electrons can create photons by interacting with a medium. In a similar way, photons can create electrons and their antiparticles, positrons, by interacting with a medium. So, imagine a very high-energy electron, impinging on some material. The electron can set photons into motion and these photons can, in turn, set electrons and positrons into motion, and this process can continue to repeat. One high-energy electron can set thousands of particles into motion. Albert Einstein's famous relation governing the equivalence of matter and energy (E = mc²) governs this process -- namely, matter (electrons and positrons) can be creased from pure energy and vice versa. The particle creation process only stops when the energy runs out.

Spin

The name given to the angular momentum carried by a particle. For composite particles the spin is made up from the combination of the spins of the constituents plus the angular momentum of their motion around one-another. For fundamental particles spin is an intrinsic and inherently quantum property, it cannot be understood in terms of motions internal to the object.

Stable

Does not decay.

Standard Model

Physicists' name for the current theory of fundamental particles and their interactions.

Strange Quark

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

Strong Force

The fundamental strong force is the force between quarks and gluons that makes them combine to form the observed hadrons, such as protons and neutrons. It also causes forces between hadrons, such as the strong nuclear force that makes protons and neutrons bind together to form nuclei.

Strong Interaction

The interaction responsible for binding quarks and gluons to make hadrons. Residual strong interactions provide the nuclear binding force. In nuclear physics the term strong interaction is also used for this residual effect. (As a parallel, the force between electrically charged particles is an electromagnetic interaction, the force between neutral atoms that leads to the formation of molecules is a residual electromagnetic effect.) 

Subatomic Particle

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

Tau

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

TeV (Tera Electron Volt)

Unit of energy equal to that acquired by a particle with one electronic charge in passing through a potential difference of one trillion volts

Top Quark

The sixth flavor of quark (in order of increasing mass) with electric charge +2/3.

Track

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

Tracking Chamber

A section of a particle detector capable of detecting the passage of electrically charged particles.

Up Quark

The first flavor of quark (in order of increasing mass), with electric charge +2/3.

W Boson

A carrier particle of the weak interaction.

Z Boson

Also known as a Z Particle. A carrier particle of weak interactions. It is involved in weak processes that do not change flavor.

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