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In 1930, Paul Dirac developed the first description of the electron that was consistent with quantum mechanics and special relativity. His theory included a remarkable prediction, he said that an anti-particle of the electron should exist. This "anti electron" should have the same mass as the electron but an opposite electric charge and magnetic moment. Although opposite in charge might make you think proton, note that this anti electron would have the same mass as a normal electron making it much smaller than a proton.
In 1932, Carl Anderson was examining tracks produced by cosmic rays in a cloud chamber. One particle made a track like an electron would but the curvature of its path in the magnetic field showed that it was positively charged. He named this particle a positron. The particle Anderson detected was none other than the anti electron predicted by Dirac two years ago.
In the 1950s physicists at the Lawrence Radiation Laboratory used the Bevatron accelerator to produce the anti proton, or a particle with the same mass and spin as the proton but with negative charge and opposite magnetic moment. In order to create the anti proton, protons were accelerated to very high energy and then smashed into a target containing other protons. Occasionally the energy brought in to the collision would product a proton-anti proton pair in addition to the two original protons. This result gave credibility to the idea that for every particle there is a corresponding antiparticle.
What is antimatter?
A particle and and its antimatter particle annihilate when they meet, both particles disappear and their kinetic + rest-mass energy is converted into other particles following E=mc2. For example, when an electron and a positron annihilate at rest, two gamma rays are produced. These gamma rays go off in opposite directions to conserve the energy and momentum of the interaction. The annihilation of positrons and electrons is the basis of Positron Emission Tomography or the more commonly known PET scans. When a proton and antiproton annihilate at rest, other particles are usually produced but the total kinetic + rest mass energies of these products will add up to twice the rest mass energy of the proton showing conservation.
Antimatter is also produced in some radioactive decays. The beta decay of 14C is a good example. As earlier explained in the Radioactivity section, a carbon neutron decays into a proton with the additional products of an electron and an antineutrino. This process is considered beta-minus decay because our emitted beta particle is an electron or β⁻.
Parallel to beta-minus decay, we have beta-plus decay. When 19Ne decays, a proton in the nucleus decays to a neutron with the additional products of a positron, e+, and a neutrino. Beta-plus decay emits a positron, β⁺.
The grey numbers below each decay show lepton number. Leptons are point-like particles that interact with the electromagnetic, weak, and gravitational force, but not the strong force. Strong force affects particles like quarks and is responsible for holding nuclei together. During each reaction, lepton number must be conserved so when a lepton is produced an anti lepton is created alongside it to balance out the lepton number.
Where is antimatter?
From a distance matter and anti matter would look identical. Although the name makes it seem like antimatter and matter are always found together, there appears to be very little antimatter in our universe. This conclusion is partly based on the low observed abundance of antimatter in cosmic rays or particles that constantly rain down on us from outer space. All of the antimatter present in cosmic rays can be accounted for by radioactive decays or reactions like those described above.
We can learn about the relative prominence of matter and antimatter by studying emissions from the edges of galaxies. If some galaxies were matter dominated while others were antimatter dominated, then we would except large annihilation signatures where these galaxies meet. However, we do not see annihilation photons from these edge regions so we believe essentially all of the objects we see in our universe are made of matter not antimatter.
Antimatter in the Lab
Elementary particle physicists create massive particles by accelerating lower mass particles close to the speed of light and then smashing them together. The mass and energy of the collided particles is converted into the mass of the created particles allowing particles of heavier mass to be created.
One method includes taking accelerated positrons and electrons and smashing them into each other. Out of this energy, very massive particles such as quarks, tau-particles, and the Z0 can be created. Studies of such electron-positron annihilators are carried out at SLAC and at the LEP facility at CERN.
Credit: Fermilab
A similar technique is used at the Fermi National Accelerator Laboratory except it involves colliding protons with anti-protons. Collisions of this kind were used to produce the sixth type of quark known as the top. This massive particle has a rest mass energy of approximately 160,000 MeV, nearly the same mass of the nucleus of a gold atom.
Read more about the Fermi lab top quark discovery on their site.
Atoms of anti-hydrogen consist of a positron orbiting an anti proton. These atoms are believed to have been created in 1995 at the CERN laboratory. Physicists are now searching for very small differences between the properties of matter atoms and antimatter atoms. This will help confirm or confound our understanding of the symmetry between matter and anti matter.
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