K - Z

Taking a closer look at LHC

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.

Klystron: It is a linear-beam vacuum tube that functions as a high-gain radio-frecuency amplifier. The klystroms supply the radio frecuency that acceleates the LHC proton beam.

Lambda Point: The temperature (2.17 K) at which liquid helium makes the transition to the superfluid state.

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.

LEP (Large Electron-Positron collider): Collider that ran at CERN until 2000.

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.

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.

Luminosity: is a measurement of the number of collisions  that can be produced in a detectorper unit area and per unit time. 

Luminosity-integrated:  is the integral of the delivered luminosity over time . It is a measurement of the collected data size, and it is an important value to characterize the performance of an accelerator.

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.

MeV (Mega-electron Volts): 106 electron volts.

Meson: A hadron made from an even number of quark constituents The basic structure of most mesons is one quark and one antiquark.

Minimum-bias processes: Most of the interactions in a hadron collider occur between low-energy quarks and gluons of the proton and are lacking the energy to produce high-mass final state. The trigger and data acquisition system of the detectors is optimized to filter out these minimum-bias process by selecting final states with large transverse momentum.

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.

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.

Neutralino: A hypothetical electrically neutral supersymmetric  particle. The  superpartners of photon, Z_boson and Higgs Boson actually get blended together. These particles are called as neutralinos.

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.

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 (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.

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

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.

Pile-up: When the bunches intersect at the interaction points, hundreds of billions of protons cross each other, resulting in many (almost) simultaneous collisions (vertices). This phenomenon is called "pile-up".

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).

Pseudorapidity: A spatial coordinate describing the deviation of a particle respect to the axis of the beam pipe. It is related to the polar angle of the trajectory.  It is defined as  ɳ=–ln[tan(θ/2)]; where θ is the angle between the particle momentum P and the beam axis. In hadron collider physics, the pseudorapidity is preferred over the polar angle because, loosely speaking, particle production is constant as a function of pseudorapidity.

Quadrupole: A magnet with four poles, used to focus particle beams rather as glass lenses focus light. There are 393 main quadrupoles in the LHC.

Quantum: The smallest discrete amount of any quantity.

Quantum Chromodynamics (QCD): The quantum 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 Electrodynamics (QED): The quantum theory of the electromagnetic interaction.

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: 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.

Scintillation: The Flash of light emitted by an electron in an excited atom falling back to its groud state.

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

Sigma: it is a unit that describes how much a set of experimental data deviates from what’s expected. When scientists search for new physics, they compare what they observe to what theories predict. If an experiment sees something that doesn't match with theory, it could be evidence of something new—or it could be merely a result of random fluctuations in the data. Scientists use the statistical measure sigma to express the probability of a statistical fluke as large as the observed mismatch between theory and experiment. Three sigmas correspond to a 1-in-740 chance of a statistical quirk, while four sigmas equal a 1-in-32,000 chance and five sigmas a 1-in-3.5 million chance. In particle physics, a three-sigma result usually means that the experimental finding is a promising hint. Four sigmas are considered a sign of a likely discovery. And a definitive discovery generally requires at least a five-sigma result. The more unexpected or important the discovery, or the more narrow the scope of the search, the greater the number of sigmas physicists require to convince themselves it's real.

Spectrometer: In particule physics, a detector system containing a magnetic field to measure momenta of particles.

Spin: Intrinsic angular momentum.

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

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.

Superconductivity: 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 for ever -as long as you keep it cold enough.

Superfluidity: A phase of matter characterized by the complete absence of resistence to flow.

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

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

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), equivalent to 1.6·10-7 J.

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.

Transfer line: Carries a beam of particles from one accelerator to another using magnets to guide the beam.

Transverse momentum (PT): In a proton collider, collisions come from interactions of proton constituents, i.e., quarks and gluons, each carrying only a fraction of the proton total energy. Most of the interesting events arise when the energy of the constituents is high; experimentally this results in the emission of particle with large momentum perpendicular to the colliding beam (pT). High momentum to the direction parallel to the beam is not necessarily an indication of a high-energy collision. 

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.

WLCG (Worldwide LHC Computing Grid: Infrastructure to maintain data-storage and analysis for the entire High Energy Physics community.

Z Boson: A carrier particle of the weak interactions. It is involved in all weak processes that do not change flavour.


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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

© Xabier Cid Vidal & Ramon Cid - rcid@lhc-closer.es  | SANTIAGO (SPAIN) |