Particle-antiparticle annihilation
In physics, particle-antiparticle annihilation refers to the encounter of a material particle with its respective antiparticle, in which all the mass of both particles is transformed into energy and/or other particles.
Description
If a particle and its antiparticle are in the appropriate quantum states, then they can annihilate each other and produce energy or other particles.
Positron-electron annihilation
The reaction e+ + e- → γ + γ is known as positron-electron annihilation. It consists of the total conversion of the mass of an electron and a positron into energy, it is the most observed form of particle-antiparticle annihilation.
Since pair annihilation is a process resulting from electromagnetic interaction, energy will always be emitted in the form of gamma rays. If the particles move at speeds much slower than light or are at rest, 2 photons emitted in the same direction but with opposite directions will be produced, each with an energy of 0.511 MeV, which coincides with the masses in rest of the electron and positron. Normally both particles will previously form a bound state known as positronium which is unstable and always ends with annihilation.
If the particles collide at speeds close to light, they will annihilate on the fly without forming a prior metastable state. The resulting photons will be able to form angles other than 180º in their exit trajectories and will be more energetic, even being able to generate pairs of particles with masses greater than those of the electron and the positron. This fact is used in particle accelerators, where these particles are annihilated with their respective antiparticles. The appearance of the record of a sequence of annihilations, creations and decays, which gives this sequence the name cascade.
Usually what happens, as can be seen in the figure, is that the positron, before annihilating itself, slows down with the environment until its energy is low enough to be captured by an electron to form positronium. Thus annihilation in flight is rare in practice and most of the photons will go out in opposite directions and with exactly 511 keV of energy each.
The annihilation of an electron-positron pair in a single photon: e+ + e- → γ cannot occur, since the principle of conservation of energy and momentum is violated, the reverse reaction is also impossible due to the same reason; however, this phenomenon is observed in nature, where an electron pair can be created -virtual positron from a single photon with an energy of at least the mass of both particles (1.022 MeV). The truth is that according to quantum field theory, this process is allowed as an intermediate quantum state for sufficiently short times in which the violation of the conservation of energy can accommodate the Heisenberg uncertainty principle. This opens the way for the production of virtual pairs or their annihilation where the single-particle quantum state can fluctuate in a two-particle quantum state and back to its initial state.. These processes are important in the vacuum state and the renormalization of a quantum field theory. It also opens the way for a mixture of neutral particles through processes like the one shown here, which is a complicated example of mass renormalization.
Positron annihilation is used by PET medical diagnostic devices to generate three-dimensional images of certain regions of the body.