antimatter (redirected from Antitritium)

antimatter: see antiparticle.

---

antimatter

Substance composed of elementary particles having the mass and electric charge of ordinary matter (such as electrons and protons) but for which the charge and related magnetic properties are opposite in sign. The existence of antimatter was posited by the electron theory of P.A.M. Dirac. In 1932 the positron (antielectron) was detected in cosmic rays, followed by the antiproton and the antineutron detected through the use of particle accelerators. Positrons, antiprotons, and antineutrons, collectively called antiparticles, are the antiparticles of electrons, protons, and neutrons, respectively. When matter and antimatter are in close proximity, annihilation occurs within a fraction of a second, releasing large amounts of energy.

---

antimatter

a form of matter composed of antiparticles, such as antihydrogen, consisting of antiprotons and positrons

---

antimatter [′an·tē‚mad·ər]

(physics)

Material consisting of atoms which are composed of positrons, antiprotons, and antineutrons.

---

Antimatter

Matter which is made up of antiparticles. At the most fundamental level every type of elementary particle has its anticounterpart, 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 Electron, Elementary particle, Positron, Proton, Quantum mechanics, Relativity

The mass, intrinsic angular momentum (spin), and lifetime (in the case of unstable particles) of antiparticles and their particles are equal, while their electromagnetic properties, that is, charge and magnetic moment, are equal in magnitude but opposite in sign. Some neutrally charged particles such as the photon and &pgr;⁰ meson are their own antiparticles. Certain other abstract properties such as baryon number (protons and neutrons are baryons and have baryon number) and lepton number (electrons and muons are leptons and have lepton number) are reversed in sign between particles and antiparticles. See Angular momentum, Baryon, Lepton

The quantum-mechanical operation of turning particles into their corresponding antiparticles is termed charge conjugation (C), that of reversing the handedness of particles is parity conjugation (P), and that of reversing the direction of time is time reversal (T). A fundamental theorem, the CPT theorem, states that correct theories of particle physics must be invariant under the simultaneous operation of C, P, and T. Simply put, the description of physics in a universe of antiparticles with opposite handedness where time runs backward must be the same as the description of the universe. One consequence of the CPT theorem is that the above-mentioned properties of antiparticles (mass, intrinsic angular momentum, lifetime, and the magnitudes of charge and magnetic moment) must be identical to those properties of the corresponding particles. This has been experimentally verified to a high precision in many instances. See CPT theorem, Parity (quantum mechanics)

When a particle and its antiparticle are brought together, they can annihilate into electromagnetic energy or other particles and their antiparticles in such a way that all memory of the nature of the initial particle and antiparticle is lost. Only the total energy and total angular momentum remain. In the reverse process, antiparticles can be produced in particle collisions with matter if the colliding particles possess sufficient energy to create the required mass. For example, a photon with sufficient energy which interacts with a nucleus can produce an electron-positron pair. See Electron-positron pair production

Since mesons do not possess baryon or lepton number, only charge, energy, and angular momentum need be conserved in their production. Thus, a process such as a collision of a proton with a proton can produce a single neutral pi meson. Other quantum numbers, such as strangeness and charm, must be conserved if production of mesons possessing these quantum numbers is to proceed through strong or electromagnetic interactions. In these cases a particle with the negative values of the particular quantum number must also be produced. Such a process is termed associated production. See Charm, Quantum numbers

Isolated neutral particles, notably K⁰ and B⁰ mesons, can spontaneously transform into their antiparticles via the weak interaction. These quantum-mechanical phenomena are termed K– or mixing, respectively. Mixing can lead to particle-antiparticle oscillations wherein a K⁰ can become its antiparticle, a⁰, and later oscillate back to a K⁰. It was through this phenomenon that observation of CP violation first occurred. That observation, coupled to the CPT theorem, implies that physics is not exactly symmetric under time reversal, for example, that the probability of a K⁰ becoming a ⁰ is not exactly the same as that in the reverse process.

Experimental observations, both ground- and balloon-based, indicate that the number of cosmic ray antiprotons is less than 1/10,000 that of protons. This number is consistent with the antibaryon production that would be expected from collisions of cosmic protons with the Earth's atmosphere, and is consistent with the lack of appreciable antimatter in the Milky Way Galaxy. Attempts to find antimatter beyond the Milky Way involve searches for gamma radiation resulting from matter-antimatter annihilation in the intergalactic gas that exists between galactic clusters. The null results of these searches suggests that at least the local cluster of galaxies consists mostly of matter. If matter dominates everywhere in the universe, a question arises as to how this came to be. In the standard model of cosmology, the big bang model, the initial condition of the universe was that the baryon number was zero; that is, there was no preference of matter over antimatter. The current theory of how the matter-antimatter asymmetry evolved requires three ingredients: interactions in which baryon number is violated, time reversal (or CP) violation, and a lack of thermodynamic equilibrium. The last requirement was satisfied during the first few microseconds after the big bang. Time reversal violation has been observed in the laboratory in K⁰ decays, albeit perhaps not of sufficient size to explain the observed baryon-antibaryon asymmetry. But the first ingredient, baryon number violation, has not yet been observed in spite of sensitive searches. Thus, the origin of the dominance of matter over antimatter remains an outstanding mystery of particle and cosmological physics. See Thermodynamic processes