In contrast to ordinary matter we talk about antimatter, that is the particular matter made of antiparticles, in which instead of each single fundamental particle is replaced by the particle that is obtained from it by charge conjugation.
In other words, antimatter is the matter consisting of antiparticles, corresponding in mass to the particles of ordinary matter, but having some quantum numbers, such as electric charge, of opposite sign. The laws that govern the combinations of antiparticles to form antielements (or antiatoms) and antimolecules are symmetrical to those that govern matter.
The concept of anti-matter was introduced for the first time, purely theoretically, by P. A. M. Dirac, in 1930, who predicted the existence of positive electrons, then actually found in cosmic radiation in 1932. Later were observed antiparticles of all fundamental particles, and were also produced in laboratory nuclei of light atoms of antimatter.
Atoms formed from hydrogen antimatter, i.e., hydrogen antiatoms consisting of antiprotons and positive electrons, were first obtained in the laboratory, but in small numbers, at CERN in Geneva, in 1995-1996 and then at Fermilab in Chicago, in 1998. A large amount of antimatter in this same form has been subsequently produced, always at CERN, in 2002. In these last experiments, started in 2000, have been produced about 50.000 hydrogen antiatoms at velocities so low to be considered at rest.
Properly stored, these antimatter atoms are also used to compare the similarities and differences between matter and antimatter. Everything seems to show that these atoms behave similarly to ordinary matter atoms, however, if matter and antimatter come in contact, they both annihilate giving rise to the formation of mesons and other particles that, in turn, in a few microseconds, turn into gamma rays γ and neutrinos, or the particles involved turn into other pairs of particles-antiparticles; in any case such that the sum of the total energy, before and after the event, remains constant, according to the mass-energy conservation principle. Under certain conditions particles and antiparticles can originate for very short times unstable particles, such as mesons, or an exotic atom, such as positronium.
Although it is believed that originally matter and antimatter were equivalent, in the current universe is detectable antimatter in small quantity, of which a part produced by experiments, in a very short time annihilated by matter. The reason that led to the prevalence of matter is under active study.
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