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What is Antimatter? - Page 5

The operation of changing particles with anti-particles is called Charge conjugation (C). Particles and anti-particles have the exact same mass and equal, but opposite charges and magnetic moments; if they are unstable, they have the same lifetime. This period is called the Charge Conjugation-Parity-Time (CPT) invariance, which establishes the fact that if you interchange particles for anti-particles (C), look in a three dimensional mirror (P) and reverse time (T), you cannot tell the difference between the them. The most stringent tests of CPT to date are measurements of the ratio of the magnetic moments of the electron and positron to two parts in a trillion (R. Van Dyck, Jr. and P. B. Schwinberg, University of Washington,1987) and measurements of charge per mass of the proton and antiproton--found to be 0.999,999,999,91 to 90 parts per trillion (G. Gabrielse, Harvard, 1998).

Antimatter came about as a solution to the fact that the equation describing a free particle in motion (the relativistic relation between energy, momentum and mass) has not only positive energy solutions, but negative ones as well! If this were true, nothing would stop a particle from falling down to infinite negative energy states, emitting an infinite amount of energy in the process--something which does not happen. In 1928, Paul Dirac postulated the existence of positively charged electrons. The result was an equation describing both matter and antimatter in terms of quantum fields. This work was a truly historic triumph, because it was experimentally confirmed and it inaugurated a new way of thinking about particles and fields.

In 1932, Carl Anderson discovered the positron while measuring cosmic rays in a Wilson chamber experiment. In 1955 at the Berkeley Bevatron, Emilio Segre, Owen Chamberlain, Clyde Wiegand and Thomas Ypsilantis discovered the antiproton. And in 1995 at CERN, scientists synthesized anti-hydrogen atoms for the first time.
When a particle and its anti-particle collide, they annihilate into energy, which is carried by "force messenger" particles that can subsequently decay into other particles. For example, when a proton and anti-proton annihilate at high energies, a top-anti-top quark pair can be created!


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