# antimatter

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Related to antimatter: black hole, CERN, dark matter

## antimatter:

see antiparticleantiparticle,
elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment.
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## 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

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

## antimatter

Matter composed entirely of antiparticles. Ordinary matter and antimatter would annihilate on contact. Although individual antiparticles are produced in cosmic-ray showers and in high-energy particle accelerators, the search for antimatter in the Universe has so far proved unsuccessful. It is thought therefore that the Universe is not now symmetric between matter and antimatter, although initially equal amounts were created. An excess of matter over antimatter may have resulted from processes occurring very early in the evolution of the Universe while it was out of equilibrium.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Antimatter

material which is constructed of antipar-ticles. The nuclei of the atoms of matter consist of protons and neutrons, and the shells of atoms are formed of electrons. In antimatter the nuclei consist of antiprotons and antineutrons, with positrons occupying the place of electrons in their shells.

According to a contemporary theory, the nuclear forces which determine the stability of atomic nuclei are identical for particles and antiparticles. The same can be said of electromagnetic and exchange forces, owing to which stable configurations of electrons in atoms and molecules take place; because the charges of all antiparticles are opposite to the charges of the corresponding particles, the negatively charged nuclei of antiatoms attract positrons in exactly the same way as nuclei attract electrons in atoms. Therefore the whole hierarchy of the construction of matter from particles must also be feasible for antimatter, which consists of antiparticles. In 1965 it was first demonstrated experimentally that aggregates of the same type as those made up of particles can be constructed from antiparticles. A group of American physicists under the direction of L. Lederman obtained on the accelerator and recorded the first anti-nucleus, an antideuteron (a cohesive aggregate of an antiproton and an antineutron). In 1969, in experiments on a Serpukhov proton accelerator of 70 giga electron volts, Soviet physicists (under Iu. D. Prokoshkin) recorded nuclei of antihelium-3, consisting of two antiprotons and one anti-neutron.

Inasmuch as the laws of physics are identical for particles and antiparticles, the question arises as to whether there is an equal amount of matter and antimatter in the whole universe. In the part of the universe observable to us, no significant accumulation of antimatter has been discovered. In particular, there are no antiprotons or antinuclei in cosmic rays. However, the question of the abundance of antimatter in the universe, important to astrophysics and cosmology, remains open.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

## antimatter

(physics)
Material consisting of atoms which are composed of positrons, antiprotons, and antineutrons.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## antimatter

a form of matter composed of antiparticles, such as antihydrogen, consisting of antiprotons and positrons
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
"That's why there is so little naturally occurring antimatter in the Universe around us," says Stone, a Fellow of the American Physical Society, which has awarded him this year's W.K.H.
Even a tiny bit of difference between the two would have helped scientists explain the imbalance between matter and antimatter, but they both behaved identically to an incredibly high-degree of precision, (https://phys.org/news/2018-04-physicists-mysterious-antimatter.html) Phys.org reported.
When the lambda-b baryon decays, particles produced by the decay speed away at different angles and momenta for matter and antimatter versions of the baryon, the scientists report online September 16 at arXiv.org.
While antimatter is rare, a huge amount of energy is released when particles collide with antiparticles, which many see as a new form of energy.
With the restart of CERN's accelerator chain getting underway, the laboratory's antimatter research programme is set to resume soon.
Scientists think the universe contained equal parts of matter and antimatter after the Big Bang, believed to have started everything 13.7 billion years ago.
As far as we know, the Big Bang created equal amounts of matter and antimatter, but today's universe contains matter only.
This is the first time antimatter has been captured in a "normal" state rather than the short-lived.
Although antimatter particles have been known about and created in labs for some time, they had never before been held in captivity, so to speak.
Holding the antimatter in a vacuum for this fraction of a second allowed the physicists to study the atoms, CERN said in an article in the British journal Nature.
In Isaiah 45:7 we read, "He Who forms the light and creates the darkness." In that sense, until light and darkness were separated, they were mixed together in that hot soup of quarks and gluons, matter and antimatter, and finally, photons and electrons.
Quinn and Yossi Nir; THE MYSTERY OF THE MISSING ANTIMATTER; Princeton University Press \$0.00 ISBN: 9780691133096

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