12.3 Matter and Anti-Matter

12.3 Matter and Anti-Matter

It was predicted by Paul Dirac in 1928 that the fundamental particles have their anti-particles. The rest masses of the anti-particles are the same as that of their corresponding particles but with opposite charges and magnetic moments. For example, positron is the anti-particle of an electron. It is represented by $e^{+}$. The rest mass of the positron is the same as that of an electron but it carries positive charge with magnitude the same as that of an electron. It is noted that the positron was the first discovered anti-particle by Anderson in 1932 in a cloud chamber experiment. This was first experimental discovery of an anti-particle. After that a lot of anti-particles were discovered. Usually, the anti-particles are represented by a letter with a bar over it, e.g., anti-proton is represented by $\bar{p}$, anti-neutrino by $\bar{\nu}$.

The quarks and leptons have been recognized as the fundamental particles also known as elementary particles among the too many discovered particles. These elementary particles have also anti-particles.

For your information

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1. A particle accelerator is a huge machine that accelerates charged particles, such as electrons, protons, or ions, to extremely high energies and speeds, approaching the speed of light.

2. Linear Accelerators, Cyclotrons and Betatrons are important particle accelerators.

Interesting information

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For their work on this discovery, Dirac and Anderson received the Nobel Prize in Physics — Dirac in 1933, and Anderson in 1936. In 1955, Segre and Chamberlain discovered anti-proton using a particle accelerator and were awarded the Nobel Prize in physics in 1959 for their discovery of anti-proton.

Do you know?

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(i) The cosmic rays are high-energy particles coming from the outer space with unknown sources. Their source may be the Sun or the other stars. These particles consist mostly of protons, neutrons and heavier nuclei, which are continually bombarding the Earth. When these particles interact with the atoms of the gases of the Earth's atmosphere, they produce showers of secondary particles which rain down on us all the time.

(ii) When nuclei of unstable radioactive element say ${}^{238}\mathrm{U}$ undergo fission reactions in the nuclear reactors, they emit a variety of particles, such as neutrons, neutrinos, $\alpha$-particles, photons, electrons and positrons.

(iii) When the charged particles, such as electrons and protons are accelerated by an accelerator and then bombarded on the target material, which is hydrogen, these accelerated charged particles may also collide head-on with each other. As a result, the debris from these reactions contain particles like pions, kaons, muons and even anti-protons.