Introduction to Elementary Particles

The branch of physics which deals with property, interaction and structure of elementary particles is called particle physics. Particle physics deals with smallest things in the universe whereas cosmology deals with the biggest thing in the universe.

Elementary Particles

The particles which are structureless, indivisible and not regarded as made up of some other particles is known as elementary particles. In 1897, after the discovery t electron by JJ Thomson, it was assumed that atoms were considered as fundamental particles of all the matter. Thomson’s discovery of electron and Rutherford’s discovery of atomic nucleus and proton in 1811 made it apparent that atom where not fundamental in the sense that they have an internal structure.

A brief description of some important particles is as follows:

1. Electron
   The electron is the first fundamental particle to be discovered and it revolves around the nucleus of an atom in different orbits. Its charge is -1.6×10^-19 C and mass is 9.1×10^-31 C^.
2. Proton
   Proton was discovered by Rutherford in 1911. Its charge is +1.6×10^-19 C and mass is 1.6726×10^-27 kg which is 1836 times the electronic mass.
3. Neutron
   Neutron was discovered by Chadwick in 1932. It carries no charge but its mass is 1.6749×10^-27 kg which is 1839 times the electronic mass. In free state the neutron is unstable, but it constituents a stable nucleus along with proton.
4. Positron
   Positron was discovered by Anderson. Its charge and mass are same of the electron, the only difference is it is positively charged. Whereas electron is positively charged.
5. Antiproton
   It was discovered in 1955. Its charge and mass are same as those of proton, the only difference is it is negatively charged whereas proton is positively charged.
6. Antineutron
   It was discovered in 1956. It has no charge and its mass is equal to the mass of the neutron. The only magnetic moments will be in opposite direction.
7. Neutrino and antineutrino
   The existence of these particles was predicted by Pauli while explaining the emission of β-particles from radioactive nuclei, but they were observed in 1956. Their rest mass and charge both are zero but they have energy and momentum. Both neutrino and antineutrino are stable particles. The only difference between them is their spins are in opposite directions.
8. Pi-mesons
   The existence of these particles was predicted by Yukawa in 1935 as originator exchange forces between the nucleons, but they were actually discovered in 1947 in cosmic rays. Pi-mesons are of three types: 1) positive pi-mesons, 2) negative pi-mesons and 3) neutral pi-meson.
9. Photons
   These are the bundles of electromagnetic energy and travel with the speed of light. If the frequency of waves is v, then the energy of a photon id hv and momentum is hv/c. its symbol is γ.

Classification of Elementary Particles

Elementary particles can be classified on the basis of different properties of particles. They can be classified on the basis of mass (massless, light, intermediate and heavy), charge (positive, negative, neutral), spin or statistics (Bosons and Fermions), interaction (Gravitational, strong, weak and electromagnetic), lifetimes (stable and resonance).

The classification of massive elementary particles.

Characteristic Properties of Elementary Particles

Mass: The elementary has always the same rest mass. The magnitude of the rest mass serves as the principle label which identifies the particles uniquely.
Charge: All known elementary particles have charge positive negative or zero. Further, the charge is always conserved in any collision process.

Average lifetime: All known elementary particles except photon, electron, proton and neutrinos are unstable and undergo decay into elementary particles of similar mass. The decay probability of a particular particle is, however, independent of the length of the time has lived.

Spin: Many elementary particles spin in a manner analogous to that of the earth on its axis, but with certain differences. The spin property forms a basis for the classification of elementary particles.

Interactions: Four kinds of interactions between elementary particles are known: gravitational, weak, electromagnetic and strong.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, et.al Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.