Feynman diagram for electron scattering. Image from HyperPhysics.
In 1932, Carl D. Anderson observed particles that behave just like electron but curved in the "wrong" direction under magnetic field. This positively charge electron are known as positron.
Feynman diagram for electron scattering. Image from HyperPhysics.
Minimum energy of a photon that is required to form a pair of particles can be determined using rest mass energy of that pair.
The "time-reverse" process of pair production is pair annihilation, in which an electron-positron pair forms a pair of photons. This can be used in positron emission tomography (PET, check problem 2 for detail).
(Harris) A stationary muon \(\mu^-\) annihilates with a stationary antimuon \(\mu^+\) (same mass, \(1.88 \times 10^{28} kg\). but opposite charge). The two disappear, replaced by electromagnetic radiation. (a) Why is it not possible for a single photon to result? (b) Suppose two photons result. Describe their possible directions of motion and wavelengths.
(Harris) In positron emission, an electron and positron annihilate, and two photons of a characteristic energy are detected. What is this energy, and what is the corresponding wavelength? The pair can be assumed to be essentially stationary before annihilation.