The antimatter counterpart of the simplest chemical element, hydrogen. An atom of hydrogen consists of two particles of matter: a single, positively charged proton as the atomic nucleus, bound to a single, negatively charged electron. An atom of antihydrogen consists of two particles of antimatter which, by definition, possess equivalent properties except for electric charge; these antimatter particles are a negatively charged antiproton as the atomic nucleus, bound to a single, positively charged positron (see figure). See also: Atom; Atomic nucleus; Chemical element; Electric charge; Electron; Positron

Matter that is composed of antiparticles, which have the same masses, spins, and decay lifetimes (if unstable) as particles of ordinary matter, but opposite electric charges, magnetic moments, lepton numbers, baryon numbers, strangeness, and charm. At the most fundamental level every type of elementary particle has its anti-counterpart, its antiparticle. The existence of antiparticles was implied by the relativistic wave equation derived in 1928 by P. A. M. Dirac in his successful attempt to reconcile quantum mechanics and special relativity. The antiparticle of the electron (the positron) was first observed in cosmic rays by C. D. Anderson in 1932, while that of the proton (the antiproton) was produced in the laboratory and observed by E. Segré, O. Chamberlain, and their colleagues in 1955. See also: Electron; Elementary particle; Positron; Proton; Quantum mechanics; Relativity

Strongly interacting composite particle that accounts for almost all of the visible mass of the universe. The proton and neutron that make up the atomic nucleus are examples of baryons. Strongly interacting particles (hadrons) are either baryons, which are fermions with half-integral spin, or mesons, which are bosons with integer spin. See also: Fundamental interactions; Hadron; Meson; Neutron; Proton; Quantum statistics

Light emitted by a high-speed charged particle when the particle passes through a transparent, nonconducting, solid material at a speed greater than the speed of light in the material. The emission of Cerenkov radiation—also spelled Cherenkov—is analogous to the emission of a shock wave by a projectile moving faster than sound, because in both cases the velocity of the object passing through the medium exceeds the velocity of the resulting wave disturbance in the medium. A common example of Cerenkov radiation is the blue glow observed in the water of a nuclear reactor, close to the active fuel elements, when electron particles travel through the water faster than the light that the electrons emit (Fig. 1); most Cerenkov light, however, is emitted in the ultraviolet and not the visible range. See also: Electron; Light; Motion; Sound; Speed; Ultraviolet radiation; Velocity; Wave motion

Charged, energetic particles, mostly hydrogen and helium nuclei, that travel at nearly the speed of light through space and bombard Earth from all directions. Cosmic rays are generated in astrophysical environments, ranging from the Sun locally to the cores of galaxies many billions of light-years away. When cosmic rays reach Earth, they shatter molecules in the atmosphere, triggering so-called air showers of secondary particles that can propagate all the way to the ground if the original cosmic ray is sufficiently energetic (Fig. 1). See also: Atmosphere; Atomic nucleus; Galaxy; Helium; Hydrogen; Light-year; Molecule; Sun

The particle physics principle that physical laws should remain unchanged when replacing matter with antimatter in point reflection of space, and the observations that this symmetry is violated in some rare processes involving the weak nuclear force. CP symmetry is the technical name for an almost, but not quite, exact symmetry of the laws of physics (Fig. 1). It is the symmetry between the laws of physics for matter and those for antimatter. CP violation refers to the very small effects showing that this symmetry is not exact. The symmetry appears to be exact for three out of the four fundamental interactions, or forces of nature, namely the strong, electromagnetic, and gravitational interactions. Only in a few weak interaction processes is any violation of this symmetry found. See also: Antimatter; Electromagnetism; Fundamental interactions; Gravity; Matter; Strong nuclear interactions; Weak nuclear interactions

A fundamental ingredient in quantum field theories, which dictates that all interactions in nature, all the force laws, are unchanged (invariant) on being subjected to the combined operations of particle-antiparticle interchange (so-called charge conjugation, C), reflection of the coordinate system through the origin (parity, P), and reversal of time, T. The operations may be performed in any order; TCP, TPC, and so forth, are entirely equivalent. If an interaction is not invariant under any one of the operations, its effect must be compensated by the other two, either singly or combined, in order to satisfy the requirements of the theorem. See also: Nonrelativistic quantum theory; Quantum field theory

Highly energetic collisions of elementary particles, namely, leptons and nucleons, which probe the nucleons' internal structure; or collisions between two heavy ions in which the two nuclei interact strongly while their nuclear surfaces overlap.

A member of a class of subatomic particles called baryons, which exists in four electric charge states and has a total spin of J = 3/2. In the underlying quark model, the delta resonance (Δ) consists of three quarks whose intrinsic spins of ½ are lined up in the same direction. The Δ is closely related to the more familiar nucleon constituents of atomic nuclei, the neutrons (n) and protons (p). See also: Nucleon; Quarks

The negatively charged constituent of ordinary matter, responsible for the chemical properties of matter, and the carrier of electricity. The electron is the lightest known particle that possesses an electric charge. Its rest mass m_e is 9.1 × 10⁻³¹ kg (2.0 × 10⁻³⁰ lb), about 1/1836 of the mass of the proton or neutron, which are, respectively, the positively charged and neutral constituents of ordinary matter. The rest mass of an electron can also be expressed as 0.511 MeV/c², where c is the speed of light. Electrons were discovered in 1895 by J. J. Thomson in the form of cathode rays. The electron was the first elementary particle to be identified. See also: Atomic structure and spectra; Cathode rays; Electric charge; Electronvolt; Elementary particle; Nuclear structure