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#### Particles and Antiparticles

A subatomic particle that has the same mass as another particle but opposite value of some other properties is called antiparticle. For example, the antiparticle of the electron is a positron, which has the same mass as that of the electron but has a positive charge equal to the proton's positive charge. The existence of antiparticles is predicted by relativistic quantum mechanics. When a particle and its antiparticle collide annihilation takes place. When an electron meets a positron the energy produced due to annihilation is 2mc2 where m is the mass of electron or positron and c is the velocity of light.

 particles Symbols antiparticles mass (incomparision to that of electrons) Averagelifes(s) Proton p+ Antiprotons $$\overline{p}$$ 1836 Stable Electron e- positron e+ 1 stable Neutron n Antineutron $$\overline{n}$$ 1839 1010 Neutrino v Antineutrino $$\overline{v}$$ 0 stable Pion π+ π- 274 2.6 $$\times$$108
##### Pair-production and Pair-Annihilation

When an energetic γ-ray photon falls on heavy substance, it is absorbed by some nucleus of the substance, and its energy gives rise to the production of an electron and a positron. This phenomenon, in which energy is converted into mass, is called ‘pair-production’ and represented by following equation:

\begin{matrix} &hv &= &-1e^0 &+ &1e^0 \\ &(\gamma -\text {photon}) &&(\text {electron}) &&(\text {positron}) \end{matrix}

According to Einstein’s energy-mass relation, a body in the state of rest also has some energy, called its rest-mass energy. If the rest mass of the body be m0, then its rest-mass energy is $$E_0 = m_0c^2$$.

The rest-mass of each of the electron and the positron is 9.1 × 10-31 kg. so, the rest-mass energy of each of them is

\begin{align*} E_0 = m_0c^2 &= (9.1 \times 10^{-31} kg ) \times (3.0 \times 10^8 s^{-1})^2 \\ &= 8.2 \times 10^{-14} = \frac {8.2 \times 10^{-14}}{1.6 \times 10^{-13}} Mev = 0.51 Mev \\ \end{align*}

For pair production, it is essential that the energy of γ-photon must be at least 2× 0.51 = 1.02 MeV. If the energy of γ photon is less than this, it would cause photoelectric effect or Compton effect on striking the matter. If the energy of γ-photon is more than 1.02 MeV, then electron and positron are produced and the energy in excess of 1.02 MeV is obtain as a kinetic energy of these particles. The converse phenomenon of pair-annihilation is also possible. Whenever an electron and a positron come very close to each other, they annihilate each other by combing together and two γ-photons are produced. This phenomenon, in which mass is converted into energy, is called ‘pair-annihilation’, and s represented by following equation:
\begin{matrix}&-1e^0 &+ & +e^0 &= hv &+ &hv \\&(\text {electron})&&(\text {positron})&(\gamma-\text {photon}) &&(\gamma-\text {photon})&\end{matrix}

##### Fundamental Forces/Interactions

There are four types of forces/interactions in the nature. They are:

1. Strong Force:
This force acts between hadrons/quarks and is mediated by mesons/gluons. This force is charge and mass independent and saturative. Its range is small and it is responsible for stability of nucleus.
2. Electromagnetic Force:
The force which acts in between all charged particles is called an electromagnetic force. It is stronger than gravitational force but weaker than strong force. This force is attractive for unlike charge and repulsive for like charge. It is responsible for the stability of atoms, binding atoms in a matter and chemical reaction.
3. Gravitational Force:
This force acts between the particles having mass and is always attractive. It is the weakest force of nature (1039 times weaker than strong force). It is mediated by graviton and is responsible for the stability of the universe.
4. Weak Force:
This force acts between leptons and hadrons. It is stronger than gravitational force but weaker than electromagnetic and strong force. This force is responsible for decay.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, Bhesha Raj Adhikari, Arjun Kumar Gautam, Parashu Ram Poudel.Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

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

A subatomic particle that has the same mass as another particle but opposite value of some other properties is called antiparticle.

According to Einstein’s energy-mass relation, a body in the state of rest also has some energy, called its rest-mass energy.

The phenomenon, in which energy is converted into mass, is called ‘pair-production’

The phenomenon, in which mass is converted into energy, is called ‘pair-annihilation’,

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