This entry contributed by Leonardo Motta

An antimatter counterpart of an elementary particle. Antiparticles are denoted my placing a bar above the symbol for a given particle. For example, the proton is denoted p, so the antiproton is denoted . An exception to this rule is that the antiparticle of the electron is called the positron and denoted .

When Paul Dirac extended quantum mechanics to include special relativity, he derived a formula known as the Dirac equation. He noticed in 1928 that this equation predicted that an electron should have a positively charged counterpart. This particle, the positron, was soon discovered in the cosmic radiation by Carl Anderson in 1932. It gradually become clear that every particle has a corresponding antiparticle with the same mass and spin but, for charged particles, with a charge (and other quantum numbers) of the opposite sign.

When a particle meets its antiparticle, they can annihilate each other and disappear, their combined rest energies becoming available to appear in other forms. For an electron annihilating with its antiparticle, this energy appears as two gamma ray photons,

with energy MeV.

Today, scientists can produce antiparticles in particle accelerators. The most mysterious aspect of antimatter is why our universe is composed predominantly of matter and not antimatter. This question involves complicated questions in cosmology theory, and is not entirely settled even today.

The following table gives some common particles and antiparticles.

The following table compares the properties of particles and antiparticles.

property	particle	antiparticle
baryon number	B
charge	Q
half-life
isospin	I	I
isospin z-component
lepton number	L
mass	m	m
parity	P
spin	s	s
strangeness	S

Antibaryon, Antihydrogen, Antimatter, Antimeson, Antineutrino, Antiproton, Particle, Positron