11.3
Antimatter and antiparticles:
In particle physics and quantum chemistry, antimatter is composed of antiparticles
in the same way that normal matter is composed of particles. For example an
antielectron (a positron, an electron with a positive charge) and an antiproton
(a proton with a negative charge) could form an antihydrogen atom in the same
way that an electron and a proton form a normal matter hydrogen atom. Mixing
of matter and antimatter would lead to the annihilation of both as mixing
of antiparticles and particles does, thus giving rise to high-energy photons
(gamma rays) or other particle-antiparticle pairs. The particles resulting
from matter-antimatter annihilation are endowed with energy equal to the difference
between the rest mass of the products of the annihilation and the rest mass
of the original matter-antimatter pair, which is often quite large. Antimatter
is not found naturally on Earth, except very briefly and in vanishingly small
quantities (as the result of radioactive decay or cosmic rays). This is because
antimatter which came to exist on Earth outside the confines of a suitable
physics laboratory would almost instantly meet the ordinary matter that Earth
is made of, and be annihilated. Antiparticles and some stable antimatter (such
as antihydrogen) can be made in tiny amounts, but not in enough quantity to
do more than test a few of its theoretical properties.
- Antineutrinos: Antiparticles of neutrinos are neutral particles produced
in nuclear beta decay where a neutron turns into a proton. They have a spin
of 1/2, and they are part of the lepton family of particles. The antineutrinos
observed so far all have right-handed helicity, while the neutrinos are
left-handed. Antineutrinos interact with other matter only through the gravitational
and weak forces, making them very difficult to detect experimentally. Antineutrinos
have very small mass.
- Electron-positron annihilation: This occurs when an electron and a positron
(the electron's anti-particle) collide. The result of the collision is the
creation of gamma ray photons or, less often, other particles. The process
must satisfy a number of conservation laws, including:
- Conservation of charge. The net charge before and after is zero.
- Conservation of linear momentum and total energy. This forbids the creation
of a single gamma ray.
- Conservation of angular momentum.
As with any two charged objects, electrons and positrons may also interact
with each other without annihilating, in general by elastic scattering.