What is meant by pair annihilation? Write a balanced equation for the same.

Hint: It is a process in particle physics, where a pair of particles collides. In this process both momentum and energy are conserved. Most reactions occur at low temperatures with low mass, however, if the energy is high, the reaction undergoes a secondary process.

Complete step by step answer:
The word annihilation roughly means interaction between a pair of particles. Pair annihilation occurs due to the collision between the subatomic particle and its antiparticle, which has the same mass and opposite charge. Since there are two particles that collide, it is known as a pair. Also, the antiparticles of the species have exactly opposite quantum nature and their quantum sum is always zero if the momentum and energy of the reaction is conserved. The principle behind this production is the photoelectric and Compton effect.
For example, say an electron $e^{-}$ collides with a positron $e^{+}$, which is the antiparticle of the electron. This occurs at low energy, with almost negligible mass, they produce two gamma photons of energy \[0.51\text{ }MeV\] each.
Balanced equation: $e^{-}+e^{+}=2\gamma$
Here, both momentum and energy are conserved. If the charged particles carry high energy, then other particles similar to gamma photons are released, which are exotic in nature or highly unstable.
Other examples include proton-antiproton annihilation and Higgs production, where meson and baryons produce several different final states.

Additional Information:
The inverse process of pair annihilation is called pair production. Pair annihilation is the principle behind positron emission tomography, which is used for the medical diagnostic of the internal structure of the organ.

Note:
This production is based on the photoelectric and Compton effect. The inverse process of pair annihilation is called pair production. Pair annihilation is the principle behind positron emission tomography. Balanced equation: $e^{-}+e^{+}=2\gamma$ and other examples include proton-antiproton annihilation and Higgs production.