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# Elementary Particles

### Probing

Explore particle physics.

# Elementary Particles

After the discovery of electrons in 1897, protons, neutrons, pions, kaons, sigmas, rhos, and over 200 other particles have been discovered, some in cosmic rays and some generated in particle collision experiments.

Matter and energy are interchangable, and thus particles are also energy. A particle has a rest mass, but waves (photons) have zero rest mass. Thus, particles are frozen energy states, as are atoms. Many particles are unstable, and they disintegrate within a short period of time.

The particle advanture home page is a very interesting website for the general public to explore the zoo of particles.

## Electrons and Positrons

The electrons were discovered by Joseph J. Thomson in 1897. He determined the charge to mass ratio e/m = 1.759e8 C/g.

Robert A. Millikan determined the charge to be 1.602e-19 C, and thus the mass of the electron = 0.107e-31 kg.

The concept of antiparticles was proposed by Paul Dirac in 1930, and the first case of an antiparticle was the discovery of positrons by Carl D. Anderson in 1932.

Particles and their antiparticles have opposite spin, charge, and magnetic moments.

## Proton-antiproton and neutron-antineutron pairs

J.J. Thomson saw some positive particles in experiments that led to the discovery of electrons. This positive particles have about the same mass as that of a hydrogen atom. Thomson's student E. Rutherford positively identified them as the nuclei of hydrogen atoms, and called them protons. A proton has the same amount of charge as an electron, but of opposite kind.

After the discovery of the positron, able scientists looked for the antiprotons. Some looked for them in the cosmic rays, whereas the group in California built accelerators to make them. The group led by Segre looked for the negatively charge heavy particles, and they thaught antiprotons, -p, will anihilate protons, p, in their collisions, converting all the mass into energy. What they have found was a starburst of 8 pions p+ + p-, four of each.

p + -p = 4(p+ + p-)

In the mean time, there was a belief that antiprotons are present in cosmic rays. The following are some websites regarding antiprotons.

After the discovery of antiprotons, attention was directed to antineutrons, -n. A year later, Bruce Cork, also working in Berkley, thought that when an antiproton neutralized its charge with a proton, instead of annihilating it, the proton and antiproton pair will convert to a neutron and antineutron pair, p + -p -> n + -n. Cork observed The annihilation of an antineutron and a neutron also produces a star-burst of charged pions and photons. 114 antineutron annihilations. Particles and antiparticles have opposite electrical charge in the case of electrons and protons, but antineutrons, have opposite magnetic moment of neutrons.

## Particle Physics

The study of matter and forces holding particles together or forces for their transformation into each other is called particle physics If you are interested in this area, a visit of the CERN-European Laboratory for Particle Physics is worth while.