GLOSSARY
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 priori distinction 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.
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.
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.
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.
CLIC: The Compact Linear Collider (CLIC) 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).
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.
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.
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.
Down Quark (d): The second flavour of quark (in order of increasing mass), with electric charge -1/3.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Jet: a narrow cone of hadrons and other particles produced by the hadronization of a quark or gluon in a particle physics.
Kaon (K): A meson containing a strange quark and an anti-up (or anti-down) quark, or an anti-strange quark and an up (or down) quark.
Lepton: A fundamental fermion that does not participate in strong interactions. The electrically-charged leptons are the electron, the muon, the tau, and their antiparticles. Electrically-neutral leptons are called neutrinos.
LCG: LHC Computing Project (LCG) is to build and maintain a data storage and analysis infrastructure for the entire high energy physics community that will use the LHC.
Linacs: An abbreviation for linear accelerator, that is, an accelerator that has no bends in it. MassThe rest mass of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by the speed of light squared. When particle physicists use the word "mass" they always mean the "rest mass" of the object in question.
Magnetic monopoles: Magnetic monopoles are hypothetical particles with a single magnetic charge, either a north pole or a south pole. Some speculative theories suggest that, if they do exist, magnetic monopoles could cause protons to decay. These theories also say that such monopoles would be too heavy to be produced at the LHC. Nevertheless, if the magnetic monopoles were light enough to appear at the LHC, cosmic rays striking the Earth’s atmosphere would already be making them, and the Earth would very effectively stop and trap them. The continued existence of the Earth and other astronomical bodies therefore rules out dangerous proton-eating magnetic monopoles light enough to be produced at the LHC.
Matter creation: is the process inverse to particle annihilation. It is the conversion of massless particles into one or more massive particles.
Meson: A hadron made from an even number of quark constituents The basic structure of most mesons is one quark and one antiquark.
Muon: The second flavour of charged leptons (in order of increasing mass), with electric charge -1.
Muon Chamber: The outer layers of a particle detector capable of registering tracks of charged particles. Except for the chargeless neutrinos, only muons reach this layer from the collision point. Neutral: Having a net charge equal to zero. Unless otherwise specified, it usually refers to electric charge.
Neutrino: A lepton with no electric charge. Neutrinos participate only in weak and gravitational interactions and are therefore very difficult to detect. There are three known types of neutrinos, all of which are very light and could possibly have zero mass.
Neutron (n): A baryon with electric charge zero; it is a fermion with a basic structure of two down quarks and one up quark (held together by gluons).
Parity transformation (also called parity inversion): is the simultaneous flip in the sign of all spatial coordinates
Pauli Exclusion Principle: 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.
Photon: The carrier particle of electromagnetic interactions.
Pion: The least massive type of meson, pions can have electric charges of +1, -1, or 0.
Positron (e+): The antiparticle of the electron.
Proton (p): 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).
Quantum: The smallest discrete amount of any quantity.
Quantum Chromodynamics (QCD): The quantum theory of the strong interaction.
Quantum Electrodynamics (QED): The quantum theory of the electromagnetic interaction.
Quantum Mechanics: The laws of physics that apply on very small scales. The essential feature is that electric charge, momentum, and angular momentum,as well as charges, come in discrete amounts called quanta.
Quark (q): A fundamental fermion that has strong interactions. Quarks have electric charge of either +2/3 (up, charm, top) or -1/3 (down, strange, bottom) in units where the proton charge is 1.
Quark-gluon plasma (QGP): A "soup" of Nature's most basic particles, loosed from their usual confined state within hadrons. Thought to have existed a few millionths of a second after the Big Bang, before matter cooled and organized into hadrons and atoms. Also thought to exist at the centers of neutron stars.
Quench: A quench is a resistive transition, i.e. when a magnet changes from the state of superconductivity to the state of resistivity; it occurs when either the critical temperature or the critical current or the critical field is by-passed. It can also occur in case of beam loss.
Relativistic: Describes anything travelling at nearly the speed of light, and obeying the special laws of behavior that apply at such speeds.
Residual Interaction: Interaction between objects that do not carry a charge but do contain constituents that have that charge. Although some chemical substances involve electrically-charged ions, much of chemistry is due to residual electromagnetic interactions between electrically-neutral atoms. The residual strong interaction between protons and neutrons, due to the strong charges of their quark constituents, is responsible for the binding of the nucleus.
Rest Mass: The rest mass of a particle is the mass defined by the energy of the isolated (free) particle at rest, divided by the speed of light squared. When particle physicists use the word "mass", they always mean the "rest mass" of the object in question.
Spin: Intrinsic angular momentum.
Stable: Does not decay. A particle is stable if there exist no processes in which a particle disappears and in its place different particles appear.
Standard Model: Physicists' name for the theory of fundamental particles and their interactions. It is widely tested and is accepted as correct by particle physicists.
Strange Quark (s): The third flavour of quark (in order of increasing mass), with electric charge -1/3.
String theory: is an incomplete mathematical approach to theoretical physics, whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point particles that form the basis for the standard model of particle physics. By replacing the point-like particles with strings, an apparently consistent quantum theory of gravity emerges, which has not been achievable under quantum field theory.
Strong interaction: The interaction responsible for binding quarks, antiquarks, and gluons to make hadrons. Residual strong interactions provide the nuclear binding force.
Subatomic Particle: Any particle that is small compared to the size of the atom.
Supersymmetry (SUSY): is a proposed property of the universe. It is one of the best motivated extensions of the Standard Model, so the study of SUSY is a primary goal of the LHC. Supersymmetry requires every type of particle to have an associated supersymmetric particle, called its superpartner, wich is a heavy replica of the particle.
Strangelets: Strangelet is the term given to a hypothetical microscopic lump of ‘strange matter’ containing almost equal numbers of particles called up, down and strange quarks. According to most theoretical work, strangelets should change to ordinary matter within a thousand-millionth of a second. But could strangelets coalesce with ordinary matter and change it to strange matter? This question was first raised before the start up of the Relativistic Heavy Ion Collider, RHIC, in 2000 in the United States. A study at the time showed that there was no cause for concern, and RHIC has now run for eight years, searching for strangelets without detecting any. At times, the LHC will run with beams of heavy nuclei, just as RHIC does. The LHC’s beams will have more energy than RHIC, but this makes it even less likely that strangelets could form. Strangelet production at the LHC is therefore less likely than at RHIC, and experience there has already validated the arguments that strangelets cannot be produced.
Synchrotron: A type of circular accelerator in which the particles travel in synchronized bunches at fixed radius.
T-symmetry: Is the symmetry of physical laws under a time reversal transformation. Although in restricted contexts one may find this symmetry, the universe itself does not show symmetry under time reversal. This is due to the uncertainty principle (at quantum scales) and thermodynamic entropy (at larger scales).
Tau: The third flavour of charged lepton (in order of increasing mass), with electric charge -1.
TeV: 1 trillion electron Volts. (1012 eV)
Top Quark (t): The sixth flavour of quark (in order of increasing mass), with electric charge 2/3. Its mass is much greater than any other quark or lepton.
Track: The record of the path of a particle traversing a detector.
Tracking: The reconstruction of a "track" left in a detector by the passage of a particle through the detector.
Trigger system: it carries out the selection process in several stages. In LHC detectors, a decision to keep data from an event is made less than two microseconds after the event ocurred. Of 40 millions bunch crossings per seconds, less than 100000 pass the first level of decision.
Uncertainty Principle: The quantum principle, first formulated by Heisenberg, that states that is is not possible to know exactly both the position x and the momentum p of an object at the same time. The same is true with energy and time (see virtual particle).
Up Quark (u): The least massive flavour of quark, with electric charge 2/3.
Vacuum bubbles: There have been speculations that the Universe is not in its most stable configuration, and that perturbations caused by the LHC could tip it into a more stable state, called a vacuum bubble, in which we could not exist. If the LHC could do this, then so could cosmic-ray collisions. Since such vacuum bubbles have not been produced anywhere in the visible Universe, they will not be made by the LHC.
Vertex Detector: A detector placed very close to the collision point in a colliding beam experiment so that tracks coming from the decay of a short-lived particle produced in the collision can be accurately reconstructed and seen to emerge from a `vertex' point that is different from the collision point.
Virtual Particle: A particle that exists only for an extremely brief instant in an intermediary process. Then the Heisenberg Uncertainty Principle allows an apparent violation of the conservation of energy. However, if one sees only the initial decaying particle and the final decay products, one observes that the energy is conserved.
W Boson: A carrier particle of the weak interactions. It is involved in all electric-charge-changing weak processes.
Weak Interaction: The interaction responsible for all processes in which flavour changes, hence for the instability of heavy quarks and leptons, and particles that contain them. Weak interactions that do not change flavour (or charge) have also been observed.
Z Boson: A carrier particle of the weak interactions. It is involved in all weak processes that do not change flavour.
