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Particle Physics Glossary

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

accelerator: 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
ever seen, exposing the quarks inside a proton. At the same time, high-energy accelerators can produce collisions that recreate the conditions of the early universe ? though in a much smaller volume. Creating tiny fireballs of high density and high temperature, physicists produce the particles that were abundant in the early universe, a trillionth of a second after the Big Bang. More. http://www2.slac.stanford.edu/vvc/accelerator.html

AGS (Alternating Gradient Synchrotron): An accelerator based at Brookhaven National Laboratory (BNL). For more information, see: http://www.bnl.gov/bnlweb/facilities/AGS.html

AMANDA (Antartic Muon And Neutrino Detector Array): A Cherenkov detector, embedding in the ice located at the south pole, designed to look at very high energy neutrinos. For more information see http://amanda.berkeley.edu/amanda/amanda.html.

antimatter: 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. Understanding how this asymmetry was produced is a major goal in particle physics and astrophysics.

antiquark: The anti-matter partner of the quark.

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

antiproton: The anti-matter partner of the proton.

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: http://press.web.cern.ch/Atlas/Welcome.html.

 B
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.

B-Meson: 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.

BaBar: 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: http://www.slac.stanford.edu/BFROOT/.

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

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

BELLE: 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: http://bsunsrv1.kek.jp/.

BEPC: Beijing Electron Positron Collider

BNL (Brookhaven National Laboratory): A national laboratory located in Long Island, New York. This multidisciplinary laboratory, operated by the Department of Energy, is the site of the AGS and RHIC (relativistic heavy-ion collider) accelerators. For more information, see: http://www.bnl.gov/world/.

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: http://www-boone.fnal.gov/.

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.

BTeV: A proposed experiment to take place at the Fermilab Tevatron. It would study B-Meson decay in detail. For more information see: http://www-btev.fnal.gov/btev.html.

 C
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: http://www-cdf.fnal.gov/.

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 particles.

CERN (European Organization for Nuclear Research): A particle physics laboratory in Geneva, Switzerland that is supported by 20 member states. It is the site of the Large Hadron Collider (LHC), which is currently under construction. Over the last fifteen years, the experiments at CERN have provided particle physicists crucial information about electroweak unification. For more information see: http://welcome.cern.ch/welcome/gateway.html.

CESR (The Cornell Electron-positron Storage Ring): A machine, located at Cornell University, funded by the National Science Foundation, that collides a beam of electrons and a beam of positrons of equal energy. The total Energy is adjustable from 9-12 GeV. For more information see: http://www.lns.cornell.edu/public/CESR/.

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 Cherenkov detectors.

CLEO: This experiment is based at the CESR accelerator. It is most noted for its study of the B-meson system. For more information see: http://www.lns.cornell.edu/public/CLEO/.

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: http://cern.ch/CLIC-Study.

CKM (Cabbibo-Kobyashi-Maskawa) Mixing Matrix: The matrix of mixing angles that describes mixing among quarks.

CKM (Charged Kaons at the Main Injector): A proposed fixed target experiment to take place at Fermilab. Its goal is to measure the decay of charged K-mesons to pions and neutrinos to study the parameters of the CKM mixing matrix. Further information can be found at : http://www.fnal.gov/projects/ckm/Welcome.html.

CMS: (Compact Muon Solenoid): This large, multi-purpose detector is currently under construction for use at the Large Hadron Collider (LHC) in Geneva, Switzerland. For more information see: http://cmsinfo.cern.ch/Welcome.html/.

color: 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.

cross-section: 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 measured in pb (picobarns), which are equal to 10-12 barns.

 D
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 constant". It is distinct from dark matter, which gravitates in the same way as ordinary matter, but is not luminous.

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.

decay: 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.

DESY: A laboratory located in Hamburg, Germany. It is the site of the HERA accelerator, which hosts the ZEUS and H1 experiments. For more information see: http://www.desy.de/html/home/fastnavigator.html.

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.

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: http://www-d0.fnal.gov/.

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-neutrinos.

 E
electron: 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.

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

Fermilab: A laboratory, operated by the Department of Energy, located in Batavia, Illinois. It is the site of the machine that currently operates at the highest energy in the world, the Tevatron. For more information, see: http://www.fnal.gov.

fermion: 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: 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: up(u), down(d), strange(s), charm(c), bottom(b), and top(b).

FNAL (Fermi National Accelerator Laboratory): See Fermilab.

 G
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: electromagnetism (photons), the weak force (W and Z-bosons), and the strong force (gluons).

gaugino: 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 theory is currently the best hope for a theory of quantum gravity.

GeV (Giga-electron Volts): 109 electron volts

GLAST (Gamma Ray Large Area Space Telescope): A satellite-based experiment that searches for gamma rays. For more information, see: http://www-glast.stanford.edu/.

gluons: The spin-one particle that carries the strong force.

gluino: The hypothetical superpartner of the gluon. The gluino has spin ?.

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: http://www.lngs.infn.it/.

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.

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

 H
H1: An experiment located at the DESY laboratory. H1 uses the electron-proton collider called HERA. For more information, see: http://www-h1.desy.de.

HEPAP (High-Energy Physics Advisory Panel): The committee that advises the Department of Energy and National Science Foundation on the particle physics program.

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.

hadron: 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.

HERA: An accelerator, located at the DESY laboratory, that collides 920 GeV protons and 30 GeV electrons. For further information, see http://desyntwww.desy.de/hera/ .

HERA-B: 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://www-hera-b.desy.de/

HERMES: An experiment located at the DESY laboratory. HERMES uses the polarized electron beam of HERA. For more information, see: http://www-hermes.desy.de/.

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.

HiRes (The High Resolution Fly'sEye): A cosmic ray detector based in Utah. This detector is especially sensitive to the highest energy cosmic rays. For more information see: http://www.cosmic-ray.org/.

Homestake: A former gold-mine located in South Dakota that is the proposed site of a National Underground Sciences Laboratory. This mine also contained the experiment that first detected solar neutrinos.

 I
IceCube: A proposed extension of the AMANDA experiment. It will be 10 times larger, at a kilometer on each side. For more information, see http://www.ssec.wisc.edu/a3ri/icecube/.

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.

 J
Jlab: Jefferson Laboratory. For more information, see: http://www.jlab.org

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: http://neutrino.kek.jp/jhfnu/.

JINR: The Joint Institute for Nuclear Research in Dubna, Russia. For more information, see: http://www.jinr.ru/

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

 K
K2K: This experiment aims a neutrino beam from the KEK accelerator to the SuperKamiokande detector. It is searching for neutrino oscillations. For more information see: http://neutrino.kek.jp/.

KamLAND: 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: http://www.awa.tohoku.ac.jp/html/KamLAND/index.html.

kaons: See K-Meson.

K-Meson: 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.

KEK: A high energy physics laboratory located in near Tokyo, Japan. It houses the BELLE experiment, as well as the accelerator used in the K2K experiment. For further information, see http://www.kek.jp/intra.html.

K0PI0: 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: http://pubweb.bnl.gov/users/e926/www/index.html.

 L
LBNL (Lawrence Berkeley National Laboratory): A Department of Energy facility located in Berkeley, California, with a broad-based program of basic and applied research. For more information see: http://www.lbl.gov.

LEP (Large Electron Positron Collider): This collider, operated from 1989 until 2001 at the CERN laboratory in Geneva, Switzerland, was used to make detailed studies of the weak force. The 27 km tunnel that housed this accelerator will now house the LHC.

lepton: 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: electron, muon, and tau.

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.

LLNL (Lawrence Livermore National Laboratory): A Department of Energy facility located in Livermore, California, with a broad-based program of basic and applied research. For more information see: http://www.llnl.gov.

LSST (Large-aperture Synoptic Survey Telescope): A proposed telescope, whose possible applications range from the study of dark matter to searching for asteroids that could collide with the Earth. For more information, see: http://www.lssto.org/lssto/index.htm

luminosity: 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.

 M
MECO (Muon to Electron COnversion Experiment): A proposed experiment to look for the conversion of muons to electrons in the presence of nuclei. This would be a signal of lepton flavor violation. Further information is available at: http://meco.ps.uci.edu/index.html.

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

MeV (Mega-electron Volts): 106 electron volts

MiniBooNE (Mini- Booster Neutrino Experiment): This experiment, based at Fermilab, represents the first stage of the BooNE experiment. It is searching for oscillations between muon and electron neutrinos. If an encouraging signal is seen at MiniBooNE, construction of BooNE will proceed.

MINOS (Main Injector Neutrino Oscillation Search): An experiment, that uses an accelerator at Fermilab to send a beam of neutrinos to the Soudan Mine to search for the oscillation of muon neutrinos. For more information see: http://www-numi.fnal.gov/.

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.

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

 N
neutralino: 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: an uncharged, weakly interacting lepton, most commonly produced in nuclear reactions such as those in the sun. There are three known flavors of neutrino, corresponding to the three flavors of leptons. Recent experimental results indicate that all neutrinos have tiny masses.

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.

neutron: 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: http://www-project.slac.stanford.edu/lc/nlc.html.

NSAC: 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.

NuMI (Neutrinos at the Main Injector): A project at Fermilab to construct a beam of neutrinos for use in the MINOS experiment.

 P
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.

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

parton: 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.

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

PHENIX: An experiment based at Brookhaven National Laboratory (BNL). For more information, see: http://www.phenix.bnl.gov/.

photon: The gauge boson that mediates the force of electromagnetism. The photon is the quantum of light.

Pierre Auger Observatory: A detector, currently under construction in Argentina, that will be used to study the highest energy cosmic rays. For more information, see: http://www.auger.org/.

pions: Pions are the lightest mesons. They are composed of up quarks, down quarks, and their anti-quark counterparts. Pions of charge +1, -1, and 0 are denoted pi +, pi -, and pi 0, respectively.

positron: the anti-matter partner of the electron. It is identical in all respects to an electron, but it has positive charge.

p-p collisions: Collisions between two protons.

proton: One of the constituents of atoms. Along with the neutron, the proton can be found in the nuclei (centers) or atoms. Protons have a positive electric charge, and are composed of two up quarks and a down quark.

 Q
QCD (Quantum Chromodynamics): The theory of the strong interaction. It describes the exchange of gluons between quarks.

quantum dimensions: As yet undiscovered dimensions of space-time that convert force particles to matter particles and vice versa. Such dimensions are predicted in theories with supersymmetry or superstrings. The pair of particles that are converted into each other are called superpartners.

quantum gravity: At very small distances, the principles of quantum mechanics are necessary to accurately describe physical phenomena. Developing a theory that incorporates both the principles of quantum mechanics and gravity, a theory of "quantum gravity", has proven to be extremely difficult. String theory is the first real hope of providing such a theory.

quantum mechanics: In microscopic systems, particles such as electrons and protons behave like waves. Quantum mechanics replaces the more familiar Newtonian mechanics to describe such phenomena.

quantum numbers: The name given to the labels that describe various characteristics of elementary particles, atoms and molecules. Examples include the charge and spin of a particle.

quark: One of the fundamental constituents of matter. These particles posses spin ?. They come in six flavors: up (u), down (d), strange (s), charm (c), bottom (b), and top (t). Only the up and down quarks are commonly found outside of accelerators. Protons and neutrons are composed of up and down quarks.

 R
RHIC (Relativistic Heavy Ion Collider): An experiment based at Brookhaven National Laboratory (BNL). For more information, see: http://www.bnl.gov/rhic/

Run II: The term describing the current run of data collection at the Fermilab Tevatron. The goal is to collect roughly ten times more data than in the first run, when the top quark was discovered.

Rutherford Appleton Laboratory: Located in Oxfordshire in the United Kingdom, this laboratory has a broad-based research program that includes investigations in nuclear and particle physics. For more information, see: http://www.rl.ac.uk/.

 S
SAGENAP: (Scientific Assessment Group for Experiments in Non-Accelerator Physics) A committee that reports to the Department of Energy and National Science Foundation.

sfermion: the superpartner of a fermion

SLAC (Stanford Linear Accelerator Center): Located in Menlo Park, California, this Department of Energy laboratory is the site of the BaBar experiment. For more information, see: http://www.slac.stanford.edu/.

SLC (Stanford Linear Collider): This accelerator, located on the SLAC site, was completed in 1989 was used to study the Z-boson in detail.

SNAP (Supernova/Acceleration Probe): A proposed satellite-based experiment for finding and studying supernovae. The characteristics of such supernovae have proved useful in studying dark energy. For more information, see: http://snap.lbl.gov/.

SNO (Sudbury Neutrino Observatory): This experiment, located 2 km beneath the surface in an active nickel mine in Sudbury, Ontario, is currently searching for neutrino oscillations in an attempt to disentangle the Solar Neutrino Problem. For more information, see: http://www.sno.phy.queensu.ca/.

Solar Neutrino Problem: The number of neutrinos observed to be coming from the sun is much less than the number predicted based on our understanding of the sun's inner workings. This discrepancy is known as the solar neutrino problem, and is one of the reasons to believe that there are neutrino oscillations.

space-time: See Special Relativity.

Special Relativity: Einstein discovered that time and space are interconnected. Height, width, length, and time make up the dimensions of space-time. The famous equation E=mc2 is a consequence of this theory.

spin: A number that labels the intrinsic angular momentum of a particle, essentially how much the particle rotates around its axis. This number can only take on discrete values. Particles with different spins will interact in different ways. Electrons have spin ?, while photons have spin one.

squark: The hypothetical spin-zero superpartner of the quark.

string theory: A theory that seeks to incorporate a quantum theory of gravity into the Standard Model. In this theory, the fundamental constituents of matter are not particles, but strings. The particles that are observed are manifestations of the vibrations of fundamental strings.

Strong force: One of four known fundamental forces (the others are the weak force, electromagnetism and gravity). The strong force is felt only by quarks and gluons, and is responsible for binding quarks together to make hadrons. For example, two up quarks and a down quark are bound together to make a proton. The strong interaction is also responsible for holding protons and neutrons together in atomic nuclei.

SuperKamiokande: This experiment, located in the Kamioka mine in Japan, is the successor to the smaller Kamiokande experiment. SuperKamiokande provided compelling evidence for neutrino oscillations in neutrinos created by cosmic rays bombarding the Earth's atmosphere. For more information, see: http://www-sk.icrr.u-tokyo.ac.jp/doc/sk/.

supernova: When a star exhausts its nuclear fuel, it under goes a catastrophic collapse. The resulting explosion is known as a supernova. It often is brighter than an entire galaxy.

superparticle: Supersymmetry predicts the existence of superpartners of the Standard Model particles. These new types of particles, the superparticles, would represent a new quantum dimension.

superpartner: see superparticle.

supersymmetry: A hypothetical symmetry relating particles of different spins. Under this symmetry, matter particles (spin one-half fermions) are related to force particles (spin-zero or spin-one bosons).

superstrings: When supersymmetry is imposed on string theory, it becomes, superstring theory. The fundamental constituents of this theory are known as superstrings.

symmetry: Physicists use symmetries to restrict possible theories of fundamental particles. As an analogy, consider the human face. If you were able to see only the right side of a person's face, you would still be able to guess what the other side of the face looks like, because of the symmetry that our bodies possess. The other side of the face cannot look like just anything, you know what it looks like-because of symmetry. Similarly, symmetries observed in nature limit what theories look like.

synchrotron radiation: When a charged particle is accelerated, it emits light known as synchrotron radiation. This radiation can be used to probe the structure of materials and biological molecules.

 T
teraflops: A trillion floating point operations per seconds.

TESLA (TeV Energy Superconducting Linear Accelerator): A proposal for a next-generation linear collider made by an international collaboration based at DESY. For further information, see: http://tesla.desy.de/.

TeV (Tera-electron Volts): 1012 electron volts

Tevatron: A 2 TeV proton on anti-proton collider that operates at Fermilab in Batavia, Illinois. The top quark was discovered using this accelerator.

trilinear coupling: The strength of an interaction involving three particles. In the case where all three particles are identical, it is known as the trilinear self-coupling.

 U
Uncertainty Principle: In the world of quantum mechanics, there is an intrinsic uncertainty in studying the position and the momentum of a particle at the same time. This means studying physics at small distances, where an accurate determination of the position is needed, requires high momentum and hence high energy.

UNO (Underground Nucleon decay and Neutrino Observatory): A proposed Cherenkov detector experiment to search for proton decay, and neutrino oscillations.

 V
VERITAS (Very Energetic Radiation Imaging Telescope Array System): A proposed experiment, based in the Arizona desert, that will study gamma rays of slightly higher energy than the GLAST experiment. For more information, see http://veritas.sao.arizona.edu/.

veto detector: A veto detector is used for eliminating possible false signals in an experiment. When a false signal has a notable feature, a veto detector can be built to look for that feature. In this way, when the veto detector registers a signal, the data associated with that signal can be flagged, and removed from the subsequent analysis.

 W
weak force: This force is carried by heavy particles known as the W-boson and the Z-boson. The most common manifestation of this force is beta decay, in which a neutron in a nucleus is transformed into a proton, by emitting an electron and a neutrino.

weak neutral current: A very weak interaction that is independent of the electric charge of a particle. Particles can exchange energy through this mechanism, but other characteristics of the particles remain unchanged. This force is mediated by the Z-boson.

WIMP: (Weakly Interacting Massive Particle): These as yet undiscovered particles are a leading candidate for dark matter.

WIPP (Waste Isolation Pilot Program): This underground cavern, located in Carlsbad, New Mexico, is a storage site for nuclear waste. It is also a proposed location for a National Underground Sciences Laboratory.

 Z

Z-boson: A gauge boson with no electric charge. It mediates the weak neutral current. It was studied in spectacular detailed by experiments at LEP and the SLC.

ZEUS: An experiment located at the DESY laboratory. ZEUS uses the electron-proton collider called HERA. For more information, see: http://www-zeus.desy.de/.