kinetic energy the energy a particle has during its motion; defined as the work needed to accelerate a particle with mass from rest to its stated velocity.
K meson (K* meson) an elementary particle in the class of pseudoscalar mesons (K spin 0) or pseudoscalar vector mesons (K* spin 1) distinguished by the strangeness quantum number; it is a bound state of a strange quark (or antiquark) and an up or down quark (or antiquark)
Lagrangian the function that describes the dynamics of a physical system; it is named after French mathematician Joseph Louis Lagrange, who reformulated classical mechanics
large electron positron (LEP) collider a multinational, circular collider at CERN, built in a tunnel straddling the border of Switzerland and France; operating from 1989 to 2000, it was the largest accelerator of leptons ever built, reaching an energy of 209 GeV by 2000
large hadron collider (LHC) the world’s largest high-energy accelerator, built at CERN from 1998 to 2008 using the former LEP circular tunnel; built by a collaboration of more than 10,000 scientists and engineers from about 100 countries, it has a circumference of 27 km (17 mi) beneath the French–Swiss border near Geneva
lepton an elementary particle with spin ½ that does not undergo strong interactions and satisfies the Pauli exclusion principle—in other words, no two leptons can occupy the same quantum state; the electron, muon, and tau are charged leptons whereas the neutrinos are electrically neutral; the electron governs the chemical properties of atoms; there are six types or flavors of leptons, forming three generations or families, matching the three generations of quarks
local (locality) the concept that a particle’s position and momentum can be localized at a point in space; standard quantum field theory is based on the axiom of locality, in which two particles separated by a distance in spacetime cannot interact; this is in accordance with special relativity, in which no particle can exceed the speed of light; for example, the electromagnetic force between charged particles is mediated by photons with a finite speed, which makes the force local, in contrast to Newton’s formulation of gravity, in which the force of attraction between massive bodies acted instantaneously (non-locally)
Lorentz invariance invariance of physical laws under Lorentz transformations from one inertial frame to another
Lorentz transformations mathematical transformations from one inertial frame moving with uniform velocity to another inertial frame such that the laws of physics remain the same; named after Hendrik Lorentz, who developed them in 1904, these transformations form the basic mathematical equations underlying special relativity
luminosity in astronomy, the total amount of energy emitted by an astronomical object per unit time; in particle accelerators, the number of collisions per unit area of the target cross-sections over time; the number of collisions can be calculated by multiplying the integration over time of the luminosity by the total cross-section for the collisions; at the LHC, this count is measured as the inverse femtobarns for the time period
mass scale the mass or energy scale in particle physics at which the strengths of particle forces take on characteristic properties; for example, the Planck energy (mass) of 1.22 × 1019 GeV (Planck mass, 2.18 × 10−8 kg) is the energy when the quantum effects of gravity are expected to become strong
matrix in mathematics, a rectangular array of symbols, such as numbers, arranged in rows and columns.
meson a short-lived boson composed of a quark and an antiquark, believed to bind protons and neutrons together in the atomic nucleus
Michelson–Morley experiment an experiment conducted in 1887 by Albert Michelson and Edward Morley that proved that the ether did not exist; beams of light traveling in the same direction, and in perpendicular directions, in the supposed ether showed no difference in speed or arrival time at their destination
Minkowski light cone the path that light, emanating from a single event E, would take through four-dimensional spacetime (three space dimensions and one time dimension); confined to a two-dimensional plane, the light spreads out in a circle from E; if we graph the growing circle with the vertical axis representing time and the horizontal axis representing space, the spreading light forms two cones, one of which is the past light cone and the other the future light cone; the sides of the cones represent light traveling at speed c, and massive particles cannot pass through them for, in special relativity, no particle can travel faster than the speed of light
MOG a relativistic modified theory of gravitation that generalizes Einstein’s general relativity and can fit astronomical and cosmological data without exotic dark matter; MOG stands for “modified gravity”
multiverse a hypothetical set of finite or infinite universes including the universe we inhabit; currently it is postulated as a paradigm that allows for fine-tuning of the calculation of parameters and constants in particle physics, and is closely related to the anthropic principle
muon from the Greek letter mu (μ), an unstable elementary particle with spin-½ similar to the electron; it belongs to the family of leptons, together with the neutrinos and the tau
neutralino a hypothetical particle predicted to exist by supersymmetry that is a popular dark-matter particle candidate; there are four neutralinos that are electrically neutral fermions, the lightest of which is stable
neutrino an elementary particle with zero electric charge and a tiny mass that has not yet been measured accurately; very difficult to detect, it is created in radioactive decays and is able to pass through matter almost without disturbing it; there are three flavors of neutrinos: νe, νμ, and ντ
neutron an electrically neutral particle found in the atomic nucleus, and having about the same mass as the proton
Noether’s theorem a mathematical theorem stating that any symmetry of the action of a physical system (the integral over the Lagrangian density function) has an associated conservation law; for example, the rotational invariance of the action leads to the conservation of angular momentum; the theorem was published in 1918 by the German mathematician Emmy Noether
nonlocality the direct action-at-a-distance between two particles that are separated in space with no mediating mechanism; Newton’s formulation of gravity was nonlocal, as the force of attraction between massive bodies acted instantaneously; quantum mechanics, too, acts nonlocally, through the quantum entanglement of two photons or electrons, which Einstein called “spooky action at a distance”; nonlocal quantum field theory can be formulated consistently as a finite theory and may violate causality at small distances
parity a symmetry property of particles under spatial inversion
perturbation theory a mathematical method for finding an approximate solution to an equation that cannot be solved exactly, by expanding the solution in a series in which each successive term is smaller than the preceding one
phase of waves in sinusoidal shapes of waves, the initial angle of a sinusoidal function at its origin is termed the phase difference, which is the fraction of a wave cycle that has elapsed relative to the origin
photon the quantum particle that carries the energy of electromagnetic waves; the spin of the photon is 1 times Planck’s constant h.
pi meson (pion) the lightest, unstable spin-0 pseudoscalar (negative parity) meson; the positively charged pion is composed of an up quark and an antidown quark
Planck length a constant unit of length equal to 1.616199(97) × 10−35 m first described by Max Planck; it is defined using the three fundamental constants: the gravitational constant G, the speed of light c in a vacuum, and Planck’s constant h
Planck mass or energy the Planck constant formed from G, c, and h, expressed as a unit of mass or energy
Planck’s constant (h) a fundamental constant that plays a crucial role in quantum mechanics, determining the size of quantum packages of energy such as the photon; it is named after Max Planck, a founder of quantum mechanics; the symbol ħ, which equals h divided by 2π, is used in quantum mechanical calculations
positron the antip
article of the electron with positive electrical charge and spin ½; it is a stable particle with the same mass as the electron
potential energy in gravitation, the stored energy of a particle when it is held at an elevated position; other force fields such as electromagnetism also have potential energy
proton a particle that carries a positive electric charge and is the nucleus of a hydrogen atom; it is a spin-½ hadron composed of two up quarks and one down quark.
pseudoscalar the property of a particle that changes sign under a spatial parity inversion; this is in contrast to the scalar property of a particle that does not change sign
quantum chromodynamics (QCD) a theory of the strong-interaction color force, it describes the interactions between quarks and gluons that make up hadrons; QCD is described by a nonabelian SU(3) Yang–Mills gauge theory, which consists of colored gluon fields and confined quarks
quantum electrodynamics (QED) the relativistic quantum field theory of the electromagnetic field, describing how charged particles interact with photons at the quantum level; QED makes extremely accurate predictions, such as the anomalous magnetic moment of the electron and the Lamb shift of the energy levels of hydrogen
quantum field theory the modern relativistic version of quantum mechanics used to describe the physics of elementary particles; it can also be used in nonrelativistic field-like systems in condensed-matter physics
quantum mechanics the theory of the interaction between quanta (radiation) and matter; the effects of quantum mechanics become observable at the submicroscopic distance scales of atomic and particle physics, but macroscopic quantum effects can also be seen in the phenomenon of quantum entanglement
quantum spin the intrinsic quantum angular momentum of an elementary particle; this is in contrast to the classical orbital angular momentum of a body rotating about a point in space.
quark the fundamental constituent of all particles that interact through the strong nuclear force; quarks have spin-½, are fractionally charged, and come in several varieties or “flavors” called up, down, charm, strange, top, and bottom; the lightest quark is the up quark; because of their color charge, they are confined within particles such as protons and neutrons, so they cannot be detected as free particles; quarks, like leptons, form three generations
quark–gluon plasma a phase of quantum chromodynamics that exists at an extremely high temperature and density, such as at the beginning of the universe
renormalization in quantum field theory, a technique used to deal with infinities arising in calculated quantities; it was first developed in quantum electrodynamics to treat infinities occurring in the calculation of the charge and mass of an electron
rotational invariance the property of a physical system when it behaves the same regardless of how it is oriented in space; if the action (the integral over time of the Lagrangian) of a system is invariant under rotations, then by Noether’s theorem, the angular momentum is conserved
scalar field a physical term that associates a value without direction to every point in space, such as temperature, density, and pressure; it is in contrast to a vector field, which has a direction in space; in Newtonian physics or in electrostatics, the potential energy is a scalar field and its gradient is the vector force field; in quantum field theory, a scalar field describes a boson particle with spin zero; see vector field
S-matrix connecting the initial and final states of a scattering process, it is defined technically as the unitary matrix, relating asymptotic particle states in the Hilbert space of physical states
special relativity Einstein’s initial theory of relativity, published in 1905, in which he explored the “special” case of transforming the laws of physics from one uniformly moving (inertial) frame of reference to another; he called it “special” because it did not include gravity, or accelerated frames of reference; the equations of special relativity revealed that the speed of light is a constant, that objects appear contracted in the direction of motion when moving at close to the speed of light, and that E = mc2, or energy is equal to mass times the speed of light squared
spin see quantum spin
spontaneous symmetry breaking the breaking of a symmetric state into an asymmetric state; for example, the equations of motion or the Lagrangian of a physical system obey rotational symmetry, but the lowest energy or ground-state solutions of the equations of motion do not have that symmetry; spontaneous symmetry breaking plays a pivotal role in the electroweak theory in the standard model
standard deviation represented by the symbol sigma (σ), in statistics and probability theory it tells how much variation or dispersion exists from the mean or average value of an experimental result
standard model of particle physics a model of the electromagnetic, weak, and strong interactions based on the dynamics of the known quarks, leptons, and gauge bosons, as well as the Higgs boson; developed by a collaborative effort by many particle theorists and experimentalists throughout the mid to late 20th century, it has been very successful in explaining a wide variety of experiments
string theory a theory based on the idea that the smallest units of matter are not point particles, but vibrating strings; a popular research pursuit in physics for several decades, string theory has some attractive mathematical features, but has yet to make a testable prediction
strong force one of the four fundamental forces, the strong interaction (or nuclear force) is about 100 times stronger than the electromagnetic force and orders of magnitude stronger than the weak and gravitational forces; at the nuclear level, it binds protons and neutrons together to form the nucleus of an atom; at a smaller scale, it is the force mediated by colored gluons that binds quarks together to form hadrons such as protons and neutrons
superconductivity the physical phenomenon of zero electrical resistance and the expulsion of magnetic fields in certain materials cooled below a critical temperature; a quantum mechanical phenomenon discovered by Dutch physicist Heike Kamerlingh Onnes in 1911, its theoretical explanation was proposed in 1957 by Bardeen, Cooper, and Schrieffer
supergravity a field theory that combines gravity (general relativity) and supersymmetry; it contains a spin-2 field with a quantum particle, the graviton, which super-symmetry demands has a superpartner with spin called the gravitino
superstring theory an attempt to unify the four fundamental forces of nature in a version of string theory that incorporates fermions as well as bosons in a supersymmetric framework; in it, particles are modeled as tiny vibrating strings
supersymmetry a theory developed during the 1970s that, proponents claim, describes the most fundamental spacetime symmetry of particle physics: for every boson particle there is a supersymmetric fermion partner, and for every fermion there exists a supersymmetric boson partner; to date, no supersymmetric particle partner has been detected
symmetry breaking in nature, the breaking of a symmetry of a physical system; there are two kinds of symmetry breaking: explicit, in which the physical laws of a system are not invariant under a symmetry generated by a set of transformations, and spontaneous symmetry breaking, in which the physical laws are invariant under a given set of transformations, but the vacuum state or lowest energy state (ground state) is not invariant
symmetry group mathematically, the group of all isometries (the congruence of two geometrical figures) under which an object is invariant
synchrotron a type of particle accelerator developed from the cyclotron, in which the guiding magnetic field, which bends the particles into a closed path, is time dependent, and is synchronized to a particle beam with increasing kinetic energy; it is often the initial accelerator in high-energy colliders
TeV a measure of energy equal to one trillion electron volts or 1.60217657 × 10−7 joules (J), called a teraelectron volt
Tevatron accelerator a circular particle accelerator at the Fermi National Accelerator Laboratory (Fermi Lab) in Batavia, Illinois, that was the second-largest high-energy accelerator in the world a
fter the large hadron collider at CERN; a synchrotron that accelerated protons and antiprotons in a 6.86-km (4.26-mi) ring to beam energies of up to about 1 TeV (which explains its name), it was completed in 1983 and closed down in September 2011
unitarity a restriction in particle physics on the permitted evolution of a quantum system that guarantees that the sum of all probabilities of a physical process (such as a scattering process) always equals unity, or one; the S-matrix describing a scattering of particles must be a unitary operator
vacuum in quantum mechanics, the lowest energy state, which corresponds to the vacuum state of particle physics; in modern quantum field theory, it is the state of perfect balance of the creation and annihilation of particles and antiparticles
vacuum expectation value in quantum field theory, the average quantum value of a field operator in a vacuum
variable speed of light (VSL) cosmology an alternative to inflation theory in which the speed of light was much faster at the beginning of the universe than it is today; like inflation, this theory solves the horizon and flatness problems in the very early universe in the standard Big Bang model
vector field a field with direction in space, such as the force field of gravity or the electric and magnetic force fields in Maxwell’s field equations; see scalar field
void cosmology an alternative to the standard LambdaCDM cosmology, in which we inhabit a large cosmic void that is surrounded by matter in the form of galaxies; in a simple version of the model, the void is a spherically symmetric “bubble” described by the exact Lemaître–Tolman–Bondi cosmological solution of Einstein’s field equations; agreeing well with available cosmological data, the model does not hypothesize the existence of dark energy or an accelerated expansion of the universe
Cracking the Particle Code of the Universe Page 28
