Antimatter first arose form the theoretical analysis of the duality of charged particles. P.A.M. Dirac’s work on the energy state of electrons implied the existence of identical particles aside from their charge. The positron is the antimatter equivalent to the electron and is not found as a stable particle in regular matter. In 1932 it was discovered as a byproduct of cosmic ray collisions, providing experimental evidence to back up Dirac’s theory. The lifespan of positrons in regular matter is incredibly short, unless the positron is moving very quickly. If the positron collides with or gets too close to an electron they will both disappear in a process called annihilation. The energy from the collision is released as gamma rays. It is possible to go through this process in reverse, in the right conditions, and produce a positron and an electron from gamma rays. The process is called electron-positron creation, or pair production. Dirac also theorized that a positron and an electron could combine to form an intermediate bound state similar to a proton and an electron forming H. The positron-electron bound system is called positronium. The lifetime of positronium depends on the orientation of the particles and is somewhere between 10^-10 and 10^-7 seconds. The Dirac wave equation also talks about protons and neutrons and predicts the existence of their counterpart particles. Antiprotons can be made by bombarding protons with protons, if the incident proton has enough energy, around 5.6 giga electron volts. Antiprotons were discovered at Berkeley using the Bevatron particle accelerator. Antineutrons were also discovered using the Bevatron PA by observing their annihilation with normal matter and the consequent release of gamma rays. All of the known subatomic particles are now known to have antiparticle counterparts. In 1995 physicists at CERN in Geneva created the first antiatom. The antihydrogen atom was made by firing antiprotons through a jet of xenon-gas. Some of the antiprotons caused pair-production and some of the resulting positrons combined with the anti protons to form antihydrogen. These antihydrogen particles only lasted 40-billionths of a second before annihilating.Since then CERN has produced larger quantities of antihydrogen that have lasted about 1000 seconds. In 2010 physicists using the Relativistic Heavy Ion Collider used a billion collisions between gold ions to make the heaviest anti molecule yet. Antihelium, which consists of 2 antiprotons and 2 positrons, is very rarely produced in nuclear reactions so detecting it in space would point to large amounts of antimatter in the universe. Although positrons are readily created in collisions between gamma rays there is no evidence of large amounts of antimatter in the universe. This however contradicts Dirac’s theory and the experiments done, because they show that matter and antimatter always form in equal parts. The big bang should’ve created equal amounts of matter-antimatter pairs, which would’ve annihilated leaving nothing but energy. However the number of protons outnumbers the amount of normal matter in the universe by a factor of 1 billion. This suggests that most of the matter annihilated with antimatter at the start of the universe but 1 in a billion particles didn’t have a partner antimatter particle and survived annihilation.
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Sources:
Sutton, Christine. "Antimatter." Encyclopædia Britannica. Encyclopædia Britannica, Inc., 24 Nov. 2015. Web. 18 Mar. 2017.
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