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Interactions - Particle physics glossary
Category: Sciences > Particle Physics
Date & country: 15/01/2011, UK
Words: 143

ab (atobarn)
10-18 barns. A unit used to measure cross-section. The inverse of this unit is used to measure integrated luminosity.

Accelerators are ring-shaped or linear devices that accelerate charged particles. More powerful than any other microscope, high-energy accelerators allow physicists to study matter at the smallest scale human beings have

All particles of ordinary matter (electrons, protons, neutrons) have anti-matter partners that appear identical in all respects (e.g. mass, spin) except that they have the opposite electric charge. We believe that in the Big Bang equal quantities of matter and antimatter were created. The fact that the universe now contains matter and not anti-matter is known as the matter-anti-matter asymmetry. U...

The generic term for an anti-matter partner of a particle.

The anti-matter partner of the proton.

The anti-matter partner of the quark.

ATLAS (A Toroidal LHC Apparatus)
This multi-purpose experiment is currently under construction for use as a detector at the Large Hadron Collider (LHC) in Geneva Switzerland. For more information, see

B Physics
The study of particles containing the bottom (b) quark. The b quark is the second heaviest quark, and is found only at particle accelerators. B-Mesons are ideal objects to study the tiny differences between matter and anti-matter.

A meson containing a bottom (b) quark, and one lighter anti-quark. The b quark is the second heaviest quark, and is found only at particle accelerators. Only the top quark is heavier.

This experiment, which focuses on B-mesons, uses the PEP II storage ring at the Stanford Linear Accelerator Center (SLAC). One of its primary goals is to investigate why the universe appears to be made entirely from matter, and not from anti-matter. There is a competing experiment, BELLE, located at the KEK laboratory. For more information on the BaBar experiment see

A unit of cross-section, a barn is equal to 10-28 m2.

A hadron composed of three quarks. Examples include the protons and neutrons found in ordinary nuclei.

This experiment, which focuses on B-mesons, is located at the KEK laboratory in Japan. One of its primary goals is to investigate why the universe appears to be made entirely from matter, and not from anti-matter. There is a competing experiment, BaBar, located at the Stanford Linear Accelerator Center. For more information on the BELLE experiment, see

Beijing Electron Positron Collider

BooNE (Booster Neutrino Experiment)
A proposed experiment to be based at Fermilab. It is the expanded version of the Mini-BooNE experiment, which is currently under construction. Further information is available at

branching ratio/fraction
When a particle decays, it often can decay in several ways. The likelihood of it decaying to a particular mode is known as its branching ratio for that decay mode.

A proposed experiment to take place at the Fermilab Tevatron. It would study B-Meson decay in detail. For more information see

CDF (Collider Detector at Fermilab)
This is one of the two large multi-purpose experiments located at the Fermilab Tevatron. Along with D0, it discovered the top quark in 1995. For more information see

center-of-mass energy
In particle collisions, this is the energy that can go into making new particles. For a collider experiment where two beams of equal energy collide head-on, this is simply the sum of the energy of the two beams. In fixed target experiments, in which a beam of particles strikes a stationary target, the center-of-mass energy is significantly less than the sum of the energies of the two colliding par...

charmed particle
Any particle that contains the exotic charmed (c) quark is known as a charmed particle.

Cherenkov detector
Light travels more slowly in materials, such as water, than it does in a vacuum. No particle can travel faster than the speed of light in a vacuum. Particles that travel faster than the speed of light in a given material emit a cone of light, in the same way that objects exceeding the speed of sound emit a sonic boom. Detectors that use this light to detect subatomic particles are known as Cherenk...

This experiment is based at the CESR accelerator. It is most noted for its study of the B-meson system. For more information see

CLIC (Compact Linear Collider)
A proposed linear collider under study by an international group based at CERN. The proposal is for a collider that could reach energies of 5 TeV. For more information, see

(Compact Muon Solenoid)

A property of quarks and gluons. Gluons can exchange color between quarks and other gluons. This process is the origin of the strong force. It has nothing to do with the color perceived by the human eye.

cosmic rays
Any of the particles from outer space that are continuously colliding with the Earth's atmosphere. They are mostly protons, with some nuclei, electrons, and photons. Their interactions with the atmosphere produce a variety of particles, including pions, muons, and neutrinos.

coupling constant
The parameter that describes the strength of a given force.

CP violation
CP is a symmetry that relates particles to anti-particles. CP violation occurs when there is a difference in the way that particles and anti-particles interact. It is believed to be necessary for the excess of matter over antimatter we observe in the universe. This phenomenon is currently being studied in detail at BaBar and BELLE.

a measure of the likelihood of a given process occurring at an accelerator. The idea is that two objects with a larger cross-sectional area are more likely to hit one another. So, larger cross-sections mean that a process is more likely to occur. Cross-sections are measured in barns, 10-28 m2. A barn is an extremely large cross-section in particle physics. Many interesting cross-sections are measu...

D0 (D-Zero)
One of the two large multi-purpose experiments located at the Tevatron at Fermilab. Along with CDF, it discovered the top quark in 1995. For more information, see

dark energy
A poorly understood (and non-luminous) substance that exerts a pressure that tends to accelerate the expansion of the universe. This dark energy counteracts gravity's natural tendency to slow the expansion of the universe. It is particularly important to understand this substance, as it appears to make up the majority of our universe. Dark energy is also often referred to as the "cosmological co...

dark matter
Astronomical measurements indicate that luminous matter, such as our sun, makes up only a small percentage of the total matter in the universe. The missing mass that makes up the remainder is known as dark matter.

Exotic particles produced at accelerators are often very short-lived, and can transform into lighter, less exotic products, such as electrons and photons. This process of transformation is known as decay.

DOE (Department of Energy)
One of the principal federal agencies supporting research in the physical sciences in the United States. Through its Office of Science it provides approximately 90% of the support for High Energy and Nuclear Physics.

double beta decay
The process in which a neutron in a nucleus transforms into a proton by emitting an electron and an anti-neutrino is known as beta decay. When two neutrons undergo this transformation simultaneously, the process is called double beta-decay. Of particular interest is the hypothetical process of neutrinoless double beta decay, in which the neutrons transform by emitting two electrons, but no anti-ne...

A fundamental constituent of matter. Along with protons and neutrons, electrons are the building blocks of atoms. They have negative electric charge.

electroweak symmetry breaking
Although electromagnetism and the weak force have the same strength at high energies, electromagnetism is much stronger than the weak force in our everyday experience. The process by which a single unified electroweak force becomes two separate forces is electroweak symmetry breaking. See Higgs Boson.

electroweak unification
This theory describes two of the four fundamental forces, elctromagnetism and the weak interaction (responsible for nuclear decays) as a single force at high energy.

eV (electron volt)
A unit of energy equal to the amount kinetic energy an electron gains after being accelerated through an electric potential of 1 Volt. It can also be used as a unit of mass by applying Einstein's relation E=mc2.

fb (femtobarn)
10-15 barns. A unit used to measure cross-section. The inverse of this unit is used to measure integrated luminosity.

The term for a particle with half-integer spin. Examples include the quarks and leptons of the Standard Model.

fixed target experiment
An experiment in which a single energetic beam strikes a stationary target such as a block of metal. This is to be contrasted with collider experiments, in which two energetic beams collide head on.

Flavor has two different meanings. For leptons, it is the label used to differentiate the three generations. That is to say, electrons, muons, and taus are said to have different flavor. For quarks, it is a label that discriminates between all quarks of different masses. There are six known flavors of quarks

galactic halo
the region of dark matter that surrounds the visible portion of a galaxy

gamma rays
photons of high energy. The most energetic forms of light are known as gamma rays.

gauge bosons
The particles that carrry the forces of the Standard Model

The generic term describing the hypothetical superpartner of any of the gauge bosons of the Standard Model.

General Relativity
Einstein generalized his theory of special relativity to include gravity, and called it a general theory of relativity. It showed that apples fall to the ground because the Earth's mass curves the adjacent space-time, forcing apples to move in a special way-towards the surface of the Earth. It has proved, however, extremely difficult to unify General Relativity with quantum mechanics. String theo...

GeV (Giga-electron Volts)
109 electron volts

The hypothetical superpartner of the gluon. The gluino has spin �.

The spin-one particle that carries the strong force.

Gran Sasso
An underground laboratory located near Rome, Italy. It is the site of dark matter detection experiments, a double beta decay experiment, and neutrino detection experiments. For more information, see

gravitational lensing
According to Einstein's theory of general relativity, the presence of matter can warp space-time. This warping of space can affect the path which light-rays follow, much in the same way that a lens does. This is known as gravitational lensing.

The as-yet unobserved spin-two particle that carries gravity.

An experiment located at the DESY laboratory. H1 uses the electron-proton collider called HERA. For more information, see

Any particle made out of quarks and/or anti-quarks. Protons, neutrons, and anti-protons are examples of hadrons.

hadron collider
A machine that accelerates hadrons to high-energies in two beams, and then collides the beams head-on.

HEPAP subpanel
HEPAP appoints a subpanel every few years to have intensive and detailed discussions on specific problems. This subpanel was appointed in March 2001 to discuss the future of the particle physics program in the US as specified in the charge in Appendix B.

An accelerator, located at the DESY laboratory, that collides 920 GeV protons and 30 GeV electrons. For further information, see http

An experiment located at the DESY laboratory. HERA-B used the 920 GeV proton beam of HERA (data acquisition until 2003). For more information see http

An experiment located at the DESY laboratory. HERMES uses the polarized electron beam of HERA. For more information, see

hidden dimensions
Hypothetical additional dimensions of space-time, either a classical dimension in which particles can move, or a quantum dimension that converts a force particle into a matter particle and vice versa.

Higgs boson (H)
An as yet undiscovered particle that appears when sufficient energy is imparted to the Higgs Field.

Higgs Field
A hypothetical medium that permeates space and time, and is thought to impart masses to all of the particles of the Standard Model. Without the Higgs Field, all particles in the Standard Model would have no mass.

high-energy physics
Because particle physics often requires high-energy particles to probe short distances, particle physics and high-energy physics are often used interchangeably. See accelerator.

A proposed extension of the AMANDA experiment. It will be 10 times larger, at a kilometer on each side. For more information, see http

integrated luminosity
This is a measure of the total data collected at an accelerator. It is the intensity of the accelerator, summed over some specified time period. A barn-1 (inverse barn) of data will give one event for a process that has a cross-section of one barn. A picobarn-1 (a much larger amount of data) will give one event for a much rarer process that has a cross-section of one picobarn.

JHF( Japan Hadron Facility)
An accelerator currently under construction for nuclear physics experiments that will produce a very intense 50 GeV proton beam. It can also be made to generate an intense beam of neutrinos. A proposal exists to use such a beam with the SuperKamiokande detector. For more information, see

The Joint Institute for Nuclear Research in Dubna, Russia. For more information, see

Jefferson Laboratory. For more information, see

JLC (Japanese Linear Collider)
A development effort for a next-generation linear collilder based at KEK. For more information, see

This is the name for a meson that contains the exotic strange (s) quark and either an up or down quark. These particles are also known as Kaons.

An experiment that searches for a rare decay of uncharged K-mesons into neutral pions. This decay provides valuable information about the CKM Mixing Matrix and CP violation. Further information can be found at

This experiment aims a neutrino beam from the KEK accelerator to the SuperKamiokande detector. It is searching for neutrino oscillations. For more information see

An experiment based at the Kamioka mine in Japan that is designed to look at anti-neutrinos produced by nearby nuclear power plants. For more information, see

See K-Meson.

Along with quarks, leptons make up all known matter. Unlike quarks, leptons do not participate in strong interactions. Neutrinos and electrons are two common examples. There are three flavors of leptons

lepton flavor violation
In the Standard Model, leptons do not change flavor. For example, a muon would never turn into an electron. If this were observed, it would be a signal for new physics. Evidence for Neutrino oscillations already indicate that lepton flavor violation may occur. See MECO.

LHC (Large Hadron Collider)
A 14 TeV proton-proton collider under construction at the CERN laboratory in Geneva, Switzerland.

linear collider
An accelerator that accelerates two beams of charged particles in a straight line, and then collides them head on.

This figure of merit for an accelerator quantifies the intensity of the beams. It is directly related to the number of events produced at the machine.

A hadron composed of a quark and an anti-quark.

MeV (Mega-electron Volts)
106 electron volts

mixing angles
A particle of a given flavor sometimes transforms into a similar particle of a different flavor. The parameters that quantify how likely this is to occur are known as mixing angles.

MRE (Major Research Equipment)
The MRE account was created by the National Science Foundation for the funding and construction of large, cutting-edge research facilities.

A fundamental particle, identical to the electron, but approximately 200 times heavier. It is commonly found in cosmic rays.

A hypothetical electrically neutral supersymmetric particle. The superpartners of the photon, Z-boson, and Higgs Boson actually get blended together. These particles are called as neutralinos.

neutrino oscillations
If neutrinos have mass, it is possible for them to convert from one flavor to another, and back again. This process is known as neutrino oscillation. If neutrinos oscillate from a flavor that is detectable into a flavor that is more difficult to detect, it may appear that the neutrinos have disappeared. This is a possible solution to the solar neutrino problem.

One of the constituents of atoms. Along with the proton, the neutron is found in the nuclei (centers) of atoms. Neutrons have no electric charge, and are composed of two down quarks and an up quark.

NLC (Next Linear Collider)
A proposal for a next-generation linear collider made by an international collaboration based at SLAC. For more information, see

Nuclear Science Advisory Committee that reports to the DOE (Department of Energy) and the National Science Foundation.

NSF (National Science Foundation)
One of the principal federal agencies supporting research in the physical sciences in the United States. Through its Division of Physics it provides approximately 30% of the support of the university High Energy Physics program.

p-p collisions
Collisions between two protons.

PAC (Physics Advisory Committee)
Each accelerator laboratory has a PAC to conduct rigorous reviews on proposed experiments and advise the laboratory director on the laboratory's program.

If a process is equally probable when the coordinates of space are reflected (as in a mirror), it is said to have parity symmetry.

The constituents of protons and neutrons are quarks, gluons and anti-quarks. They are collectively known as partons.

parton distribution functions
These functions parameterize the composition (quarks, anti-quarks and gluons) of various hadrons. They describe the probability of finding a given constituent with a given momentum.

pb (picobarn)
10-12 barns. A unit used to measure cross-section. The inverse of this unit is used to measure integrated luminosity.

An electron-positron collider, sited at SLAC, where the BaBar experiment is located.

An experiment based at Brookhaven National Laboratory (BNL). For more information, see