Accelerating cavity

Accelerating cavities produce the electric field that accelerates the particles inside particle accelerators. Because the electric field oscillates at radio frequency, these cavities are also referred to as radio-frequency cavities.


A machine in which beams of charged particles are accelerated to high energies. Electric fields are used to accelerate the particles while magnets steer and focus them. Beams can be made to collide with a static target or with each other.

  • A collider is a special type of circular accelerator where beams travelling in opposite directions are accelerated and made to interact at designated collision points.
  • A linear accelerator (or linac) is often used as the first stage in an accelerator chain.
  • A synchrotron is an accelerator in which the magnetic field bending the orbits of the particles increases with the energy of the particles. This makes the particles move in a circular path.


The Antiproton Decelerator, the CERN research facility that produces the low-energy antiprotons.

ALICE (A Large Ion Collider Experiment)

One of the four large experiments that will study the collisions at the LHC.


Every kind of matter particle has a corresponding antiparticle. Charged antiparticles have the opposite electric charge to their matter counterparts. Although antiparticles are extremely rare in the Universe today, matter and antimatter are believed to have been created in equal amounts at the Big Bang.


The antiparticle of the proton.


One of the four large experiments that will study the collisions at the LHC.


All ordinary matter is made up of atoms, which are themselves composed of a nucleus and electrons. The protons and neutrons in the nucleus are made of quarks, the smallest known matter particles.


The particles in an accelerator are grouped together in a beam. Beams can contain billions of particles and can be divided into discrete portions called bunches. Each bunch is typically several centimetres long and just a few microns wide.

Big Bang

The name given to the explosive origin of the Universe.


The collective name given to the particles that have integer (whole number) spin and carry forces between particles of matter. (See also Particles.)


An instrument for measuring the amount of energy carried by a particle. In particular, the electromagnetic calorimeter measures the energy of electrons and photons, whereas the hadronic calorimeter determines the energy of hadrons, that is, particles such as protons, neutrons, pions and kaons.

CARE (Co-ordinated Accelerator Research in Europe)

An EU-supported activity to generate a structured and integrated area in accelerator research and development in Europe.

Cherenkov radiation

Light emitted by fast-moving charged particles traversing a dense transparent medium faster than the speed of light in that medium.

CLIC (Compact LInear Collider)

A site-independent feasibility study aiming at the development of a realistic technology at an affordable cost for an electron–positron linear collider for physics at multi-TeV energies.

CMS (Compact Muon Solenoid)

One of the four large experiments that will study the collisions at the LHC.

CNGS (CERN Neutrinos to Gran Sasso)

A project that aims at the first observation of the tau neutrino by sending a beam of muon neutrinos from CERN to the Laboratori Nazionali del Gran Sasso in Italy.


Special type of acclerator where counter-rotating beams are accelerated and interact at designated collision points. The collision energy is twice that of an individual beam, which allows higher energies to be reached than in fixed target accelerators.

Colour charge

Colour charge is a property of quarks and gluons analogous to electric charge. It is merely a term relating to the strong forces of interaction, and has nothing to do with the visual notion of colour. The three "colours" a quark may take are red, blue or green; those an anti-quark may take are anti-red, anti-blue or anti-green; while a gluon has a colour-anticolour combination.

Cosmic ray

A high-energy particle that strikes the Earth’s atmosphere from space, producing many secondary particles, also called cosmic rays.

CP violation

A subtle effect observed in the decays of certain particles that betrays Nature’s preference for matter over antimatter.

Cross section

A quantity proportional to the probability for a specified reaction (such as creation of a new particle) to occur, for example when the two proton beams collide as in the LHC. The name reflects the origin of the concept in classical mechanics (geometrical cross-sectional area of an object which could be hit by a beam), but in particle physics the probabilities are from quantum mechanics. At the LHC, cross sections are typically expressed in nanobarns (nb), picobarns (pb), and femtobarns (fb).

Cryogenic distribution line (QRL)

The system used to transport liquid helium around the LHC at very low temperatures. This is necessary to maintain the superconducting state of the magnets that guide the particle beam.


A refrigerator used to maintain extremely low temperatures.


The constrained minimal supersymmetric extension of the standard model, CMSSM, is a simplified and practical model of SUSY-breaking that is often used to illustrate and compare the reach of SUSY searches at colliders.

Dark matter

Only 4% of the matter in the Universe is visible. The rest is known as dark matter — 26%, and dark energy — 70%. Finding out what it consists of is a major challenge for modern science.


A device used to measure properties of particles. Some detectors measure the tracks left behind by particles, others measure energy. The term ‘detector’ is also used to describe the huge composite devices made up of many smaller detector elements. In the large detectors at the LHC each layer has a very specific task.


A magnet with two poles, like the north and south poles of a horseshoe magnet. Dipoles are used in particle accelerators to keep particles moving in a circular orbit. In the LHC there are 1232 dipoles, each 15 m long.

Enabling Grids for E-SciencE (EGEE) project

An EU-funded project led by CERN, now involving more than 90 institutions in over 30 countries worldwide, to provide a seamless Grid infrastructure that is available to scientists 24 hours a day.

Electronvolt (eV)

A unit of energy or mass used in particle physics. One eV is extremely small, and units of a million electronvolts, MeV, or thousand million electronvolts, GeV, are more common. The latest generation of particle accelerators reaches up to several million million electronvolts, TeV. One TeV is about the energy of motion of a flying mosquito.

Electromagnetic force

The electromagnetic force binds negative electrons to the positive nuclei in atoms, and underlies the interactions between atoms that give rise to molecules and to solids and liquids. Unlike gravity, it can produce both attractive and repulsive effects. Opposite electric charges (positive and negative) and opposite magnetic poles (north and south) attract, but charges or poles of the same type repel each other.

Electroweak interactions

The name given to the interactions (electromagnetic and weak) which were unified in the theory for which Glashow, Salam, and Weinberg received the Nobel Prize in 1979. The electroweak force carriers are the positively and negatively charged W bosons, and the electrically neutral photon (γ) and Z boson.


Detector placed at each end of a barrel-shaped detector to provide the most complete coverage in detecting particles.


If searches for a particle reveal statistically that it almost certainly doesn’t exist with certain characteristics (e.g. a particular mass), future searches exclude those characteristics. This narrows the search parameters within which the particle might be found. Establishing such exclusions is very important in the search for undiscovered particles.


There are four known fundamental forces in nature. Gravity is the most familiar to us, but it is the weakest. Electromagnetism is the force responsible for lightning and carrying electricity into our homes. The two other forces, weak and strong, are confined to the atomic nucleus. The strong force binds the nucleus together, whereas the weak force causes some nuclei to break up. The weak force is important in the energy-generating processes of stars, including the Sun. Physicists would like to find a theory that can explain all these forces. A big step forward was made in the 1960s when the electroweak theory uniting the electromagnetic and weak forces was proposed. This was later confirmed in a Nobel-prize-winning experiment at CERN.

Fundamental particle

One of the smallest known particles, from which all the other particles are made of.


Gluon is a special particle, called boson, that carries the strong force, one of the four fundamental forces, or interactions, between particles.


A subatomic particle that contains quarks, antiquarks, and gluons, and so experiences the strong force. (See also Particles.)

Higgs boson

A particle predicted by theory. It is linked with the mechanism by which physicists think particles acquire mass.


System that supplies particles to an accelerator. The injector complex for the LHC consists of several accelerators acting in succession.


An ion is an atom with one or more electrons removed (positive ion) or added (negative ion).


Slightly different versions of the same element, differing only in the number of neutrons in the atomic nucleus—the number of protons is the same.


A meson containing a strange quark (or antiquark). Neutral kaons come in two kinds, long-lived and short-lived. The long-lived ones occasionally decay into two pions, a CP-violating process. (See also Particles.)


A unit of temperature. One kelvin is equal to one degree Celsius. The Kelvin scale begins at absolute zero, –273.15°C, the coldest temperature possible.

LCG (LHC Computing Grid)

The mission of the LCG is to build and maintain a data-storage and analysis infrastructure for the entire high-energy physics community that will use the LHC.


The Large Electron–Positron Collider, which ran at CERN until 2000.


A class of elementary particle that includes the electron. Leptons are particles of matter that do not feel the strong force. (See also Particles.)


The Large Hadron Collider, CERN’s biggest accelerator.

LHCb (Large Hadron Collider beauty)

One of the four large experiments that will study the collisions at the LHC.


An abbreviation for linear accelerator.

Mass, invariant mass, effective mass

Particle physicists use the word "mass" to refer to the quantity (sometimes called "rest mass") which is proportional to the inertia of the particle when it is at rest. This is the "m" both in Newton's second law of motion, F=ma, and in Einstein's equation, E=mc2 (in which E must be interpreted as the energy of the particle at rest). When a particle decays and hence no longer exists, its mass before the decay can be calculated from the energies and momenta of the decay products. The inferred value of the mass is independent of the reference frame in which the energies and momenta are measured, so that that the mass called "invariant". The concept is frequently generalized, so that for any set of particles (e.g., two leptons emerging from a collision), one can apply the same formulas to obtain an "invariant mass" (also called the “effective mass”) of the set.

Mass spectrum

One of the important statistical tools in particle physics is to create a histogram of the invariant mass of a particle or a group of particles (thought to originate from the decay of something interesting) versus the frequency with which that particular mass was recorded. This plot is known as a mass spectrum, and is used to signify the presence of new particles and to establish their masses.

Model / Scientific model

A model is a representation of phenomena or processes that allows us to summarise and study them. They can be made up of a number of hypotheses working within a set of parameters, usually obtained from observation and/or experiment. Modelling, including the use of computer simulations, is an essential part of scientific activity as it gives us a framework within which to test our hypotheses - we can create a complex model, alter its parameters, test its reliability against the real world and even make predictions.


A particle similar to the electron, but some 200 times more massive. (See also Particles.)

Muon chamber

A device that identifies muons, and together with a magnetic system creates a muon spectrometer to measure momenta.


A neutral particle that hardly interacts at all. Neutrinos are very common and could hold the answers to many questions in physics. (See also Particles.)


A baryon with electric charge zero; it is a hadron with a basic structure of two down quarks and one up quark (held together by gluons).


The collective name for protons and neutrons.


There are two groups of elementary particles, quarks and leptons. The quarks are up and down, charm and strange, top and bottom. The leptons are electron and electron neutrino, muon and muon neutrino, tau and tau neutrino. There are four fundamental forces, or interactions, between particles, which are carried by special particles called bosons. Electromagnetism is carried by the photon, the weak force by the charged W and neutral Z bosons, the strong force by the gluon; gravity is probably carried by the graviton, which has not yet been discovered. Hadrons are particles that feel the strong force. They include mesons, which are composite particles made up of a quark–antiquark pair and baryons, which are particles containing three quarks. Pions and kaons are types of meson. Neutrons and protons (the constituents of ordinary matter) are baryons; neutrons contain one up and two down quarks; protons two up and one down quark.


The force carrier particle of electromagnetic interactions.

See Particles.


The least massive type of meson.


The antiparticle of the electron.


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.


The Proton Synchrotron, backbone of CERN’s accelerator complex.


A quantity (denoted by η) frequently used in colliding beam experiments to express angles with respect to the axis of the colliding beams (typically for rays emanating from the center of the detector). It has the value 0 for rays perpendicular to the beam, and large positive or negative values for rays at small angles to the beam. The sign is chosen by convention: in CMS positive η refers to the end of the detector closer to the nearby Jura mountains.


A magnet with four poles, used to focus particle beams rather as glass lenses focus light. There are 392 main quadrupoles in the LHC.

Quantum electrodynamics (QED)

The theory of the electromagnetic interaction.

Quantum chromodynamics (QCD)

The theory for the strong interaction, analogous to QED.


A class of elementary particle. Quarks are particles of matter that feel the strong force.

Quark–gluon plasma (QGP)

A new kind of plasma in which protons and neutrons are believed to break up into their constituent parts. QGP is believed to have existed just after the Big Bang.


A quench occurs in a superconducting magnet when the superconductor warms up and ceases to superconduct.


To maintain the conservation of baryon and lepton number, a discrete multiplicative symmetry of R-parity is often imposed in SUSY models. This term is defined by R = (-1)2S+3B+L, with spin S, baryon number B and lepton number L.

Ring Imaging CHerenkov (RICH) counter

A kind of particle detector that uses the light emitted by fast-moving particles as a means of identifying them.


The flash of light emitted by an electron in an excited atom falling back to its ground state.


A magnet with six poles, used to apply corrections to particle beams. At the LHC, eight- and ten-pole magnets will also be used for this purpose.


In particle physics, a detector system containing a magnetic field to measure momenta of particles.


Intrinsic angular momentum - a fundamental property of particles as described by quantum mechanics.


The Super Proton Synchrotron. An accelerator that provides beams for experiments at CERN, as well as preparing beams for the LHC.

Standard Model

A collection of theories that embodies all of our current understanding about the behaviour of fundamental particles.

Strong force

The strong force holds quarks together within protons, neutrons and other particles. It also prevents the protons in the nucleus from flying apart under the influence of the repulsive electrical force between them (because they all have positive charge). Unlike the more familiar effects of gravity and electromagnetism where the forces become weaker with distance, the strong force becomes stronger with distance.


A property of some materials, usually at very low temperatures, that allows them to carry electricity without resistance. If you start a current flowing in a superconductor, it will keep flowing forever—as long as you keep it cold enough.


A phase of matter characterized by the complete absence of resistance to flow.


A hypothesis that predicts the existence of heavy ‘superpartners’ to all known particles. It will be tested at the LHC.


A particle accererator in which a magnetic field bends the orbits of the particles, which increases their energy. The particles travel in a circular path.

Technical Design Report (TDR)

The blueprint for an LHC sub-detector system.

Technology transfer

The promotion and dissemination to third parties of technologies developed, for example at CERN, for socio-economic and cultural benefits.

Transfer line

Carries a beam of particles, e.g., protons, from one accelerator to another using magnets to guide the beam.

Transverse momentum (and energy)

The amount of a particle's momentum which is perpendicular to the beam direction. Transverse momentum carried by unobserved particles such as neutrinos and dark matter particles is called "missing" transverse momentum; it can be inferred from the observed transverse momenta. For technical reasons a closely related quantity called transverse energy is often calculated instead of transverse momentum.


An electronic system for spotting potentially interesting collisions in a particle detector and triggering the detector’s read-out system to record the data resulting from the collision.


A volume of space that is substantively empty of matter, so that gaseous pressure is much less than standard atmospheric pressure.

Weak force

The weak force acts on all matter particles and leads to, among other phenomena, the decay of neutrons (which underlies many natural occurrences of radioactivity) and allows the conversion of a proton into a neutron (responsible for hydrogen burning in the centre of stars). It can be either an attractive or a repulsive force.