A - J

Taking a closer look at LHC

Accelerating cavity: Accelerating cavities produce the electric field that accelerates the particles inside particles accelerators. Because de electric field oscillates at radio frecuency, thes cavities are also referred to as radio-frecuency (RF) cavities.

Accelerator: A machine used to accelerate particles to high speeds (and thus high energy compared to their rest-mass energy).

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. The produced particles may be any combination allowed by conservation of energy and momentum and of all the charge types.

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 prioridistinction 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. AstrophysicsThe physics of astronomical objects such as stars and galaxies.

B-factory: An accelerator designed to maximize the production of B mesons. The properties of the B mesons are then studied with special detectors.

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

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 .

Barn (b):  A unit used to measure cross-section (1 b = 10-28 m2). The inverse of this unit is used to measure integrated luminosity.  

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.

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

Big Bang Theory: The theory of an expanding universe that begins as an infinitely dense and hot medium. The initial instant is called the Big Bang.

Black Holes (microscopic): According to some theoretical models, microscopic black holes could be produced in collisions at the LHC. Most people think it is very unlikely, but it would be fascinating if it were possible, and definitely harmless. Microscopic-black holes would very quickly decay and be detected by experiments (the tinier the black hole, the faster it evaporates). The energy available in the decay of a microscopic-black hole produced at LHC is exactly the same as the energy for any other collision event at LHC. When one asks if microscopic-black holes could be dangerous, it is important to remember that cosmic rays are continuously bombarding Earth's atmosphere with far more energy than protons will have at the LHC, so cosmic rays would produce everything LHC can produce. They have done so throughout the 4.5 billion years of the Earth's existence, and the Earth is still here! The LHC just lets us see these processes in the lab (though at a much lower energy than some cosmic rays).

Boson: A particle that has integer intrinsic angular momentum (spin) measured in units of h-bar (spin =0, 1, 2, ...). All particles are eitherfermions 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 flavour of quark (in order of increasing mass), with electric charge of -1/3.

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.

C-symmetry: Means the symmetry of physical laws under a charge-conjugation transformation. Electromagnetism, gravity and the strong interaction all obey C-symmetry, but weak interactions violate C-symmetry maximally.

Calorimeter: An experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles' energy is deposited in the calorimeter, collected, and measured.

Center-of-mass energy: It is that energy available for producing new particles. In fixed target mode the center-of-mass only increases as the square root of the energy of the projectile. In collider mode, it increases proportionally to the energy of the colliding projectiles.

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 2 charges has 2 interactions, etc.

Charge Conservation: The observation that electric charge is conserved in any process of transformation of one group of particles into another.

Charm Quark (c): The fourth quark (in order of increasing mass), with electric charge +2/3. ColliderAn accelerator in which two beams traveling in opposite directions are steered together to provide high-energy collisions between the particles in one beam and those in the other.

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.

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

CLIC (The Compact Linear Collider): is an electron-positron Linear Collider in the post-LHC era for Physics up to the multi-TeV center of mass colliding beam energy range (nominal 3 TeV).

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

Colour Charge: The quantum number that determines participation in h2 interactions. Quarks and gluons carry nonzero colour charges.

Combination of Charge and Parity Reversal: It was believed for some time that C-symmetry could be combined with the parity-inversion transformation to preserve a combined CP-symmetry. However, violations of even this symmetry have now been identified in the weak interactions (particularly in the kaons and B mesons).

Confinement: The property of the strong interaction that quarks or gluons are never found separately but only inside colour-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.

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 .

CP violation: is a violation of the postulated CP symmetry of the laws of physics. It plays an important role in theories of cosmology that attempt to explain the dominance of matter over antimatter in the present Universe.

CPT symmetry: Is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity and time simultaneously. The implication of CPT symmetry is that a mirror-image of our universe — with all objects having momenta and positions reflected by an imaginary plane (corresponding to a parity inversion), with all matter replaced by antimatter (corresponding to a charge inversion)— would evolve exactly like our universe. CPT symmetry is recognized to be a fundamental property of physical laws.

Crab cavities are a form of electromagnetic cavity used in particle accelerators to provide a transverse deflection to particle bunches. They can be used to provide rotation to a charged particle bunch by applying a time varying magnetic field. This rotation of the bunch can be used as a diagnostic tool to measure the length of a bunch (the longitudinal dimension is projected into the transverse plane, and imaged) or as a means of increasing the luminosity at an interaction point of a collider if the colliding beams cross each other at an angle (then called crab crossing). They can also be used in order to minimise beam-beam effects, which are important for circular colliders.

Cross-Section: is 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 barn, and also femtobarn(10-15 b) and attobarn (10-18 b).

Cryogenic distribution line (QRL): The system used to transport liquid helium aroud the LHC at very low temperature. This is necessary to mantein the superconducting state of the magnets that guide the particle beam.

Cryostat: It is the thermally insulating device that houses the the cold part of the accelerator.

Dark energy: is a hypothetical form of energy 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: Matter that is in space but is not visible to us because it emits no radiation by which to observe it. The motion of stars around the centers of their galaxies implies that about 90% of the matter in a typical galaxy is dark. Physicists speculate that there is also dark matter between the galaxies but this is harder to verify.

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

Decay channel: Decay channels are the possible transformations a particle can undergo as it decays. When a particle decays, it does not break into smaller bits; its energy does. Even fundamental particles can decay. Many particles in the Standard Model exist for only a limited time before decaying.

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

Down Quark (d): The second flavour of quark (in order of increasing mass), with electric charge -1/3.

Duoplasmatron: The source of all protons at CERN. It ionizes hydrogen gas from wich the protons are extracted by an electric field.

Electric Charge: The quantum number that determines participation in electromagnetic interactions.

Electromagnetic Interaction: The interaction due to electric charge; this includes magnetic interactions.

Electroweak Interaction: In the Standard Model, electromagnetic and weak interactions are related (unified); physicists use the term electroweak to encompass both of them.

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.

Electron volt (eV): 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.

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.

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

Fermion: Any particle that has odd-half-integer (1/2, 3/2, ...) intrinsic angular momentum (spin), measured in units of h-bar. 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 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.

Feynman diagrams: Each of the three basic interactions can be described using a symbol called a Feynman vertex. To the particle physicist, each Feynman vertex represents a component of some sophisticated mathematics that is used to calculate various aspects of particle interactions. But we can use the vertices in a non-mathematical way to illustrate how quarks and leptons interact with each other. There are three basic vertices, each one associated with each of the fundamental interactions. There is an electromagnetic interaction vertex, a weak interaction vertex and a strong interaction vertex.

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.

Flavour: The name used for the different quarks types (up, down, strange, charm, bottom, top) and for the different lepton types (electron, muon, tau). For each charged lepton flavour there is a corresponding neutrino flavour. In other words, flavour is the quantum number that distinguishes the different quark/lepton types. Each flavour 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 electromagnetic, weak, strong 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+ and W- bosons, and the Z bosons are fundamental. All other objects are made from these.

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 (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 of particles.

General Relativity: Einstein generalized his theory of Special Relativity to include gravity, and called it a General Theory of Relativity. 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.

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.

GeV (Giga-electron Volts): 109 electron-volts

Gluino: The hypothetical 

Gluon (g): The carrier particle of the strong interactions. Gravitational InteractionThe interaction of particles due to their mass/energy.

Graviton: The carrier particle of the gravitational interactions; not yet directly observed.

GRID: Is a service for sharing computer power and data storage capacity over the Internet. The Grid goes well beyond simple communication between computers, and aims ultimately to turn the global network of computers into one vast computational resource.

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.

Heavy flavour physics: It is the study of properties of quarks with large mass and their decay products, with an emphasis on the physics of the b and c quarks.

Hermetic Detector: A particle detector sensitive to particles emitted at all angles from the interaction point.

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 viceversa.

Higgs boson: Hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. It is the only Standard Model particle not yet observed, but would help explain how otherwise massless elementary particles still manage to construct mass in matter.

Higgs field: proposed field that full the universe. Disturbances in this field as particles move through it cause objects to have 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.

ILC (International Linear Collider): proposed electron-positron collider, it will complement the Large Hadron Collider, together unlocking some of the deepest mysteries in the universe. With LHC discoveries pointing the way, the ILC—a true precision machine—will provide the missing pieces of the puzzle.

Injector: The injector refers to the system that supplies particles to an accelerator. The injector complex for the LHC consists of several accelarators acting in sucession.

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.

Interaction length: It refers to the average distance a hadron will travel before interacting with a given material. A large number of interaction lengths means that hadrons will be effectively stopped or filtered out by that material.

Jet: a narrow cone of hadrons and other particles produced by the hadronization of a quark or gluon in a particle physics.


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Xabier Cid Vidal, PhD in experimental Particle Physics for Santiago University (USC). Research Fellow in experimental Particle Physics at CERN from January 2013 to Decembre 2015. He was until 2022 linked to the Department of Particle Physics of the USC as a "Juan de La Cierva", "Ramon y Cajal" fellow (Spanish Postdoctoral Senior Grants), and Associate Professor. Since 2023 is Senior Lecturer in that Department.(ORCID).

Ramon Cid Manzano, until his retirement in 2020 was secondary school Physics Teacher at IES de SAR (Santiago - Spain), and part-time Lecturer (Profesor Asociado) in Faculty of Education at the University of Santiago (Spain). He has a Degree in Physics and a Degree in Chemistry, and he is PhD for Santiago University (USC) (ORCID).



CERN Directory

CERN Experimental Program

Theoretical physics (TH)

CERN Experimental Physics Department

CERN Scientific Committees

CERN Structure

CERN and the Environment



Detector CMS

Detector ATLAS

Detector ALICE

Detector LHCb

Detector TOTEM

Detector LHCf

Detector MoEDAL

Detector FASER

Detector SND@LHC



 For the bibliography used when writing this Section please go to the References Section

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