Annihilation

Feynman diagram of the annihilation of an electron e - with a positron e + . The location is plotted from left to right in this diagram, the time from bottom to top. Electron and positron annihilate each other. Two photons remain after the annihilation .${\ displaystyle \ gamma}$

In elementary particle physics , annihilation ( Latin annihilatio "to destroy") is the process of pair annihilation (also: pair radiation ), in which an elementary particle and its antiparticle are transformed into other particles.

The opposite process to annihilation is pair creation , the formation of a particle-antiparticle pair from an energy other than that of pair annihilation , e.g. B. the conversion of a photon in the field of a heavy nucleus into an electron and a positron.

High energy physics research

For research purposes, experiments on collider systems allow electrons to collide with positrons with the same and very high kinetic energy , but in opposite flight directions. The same is possible with protons and antiprotons . Because of the favorable kinematics of such colliding beam experiments , in addition to the rest energies, almost the entire kinetic energy of the two particles is available for transformations.

Positron-electron annihilation in matter

Positrons of lower energy occur as beta radiation and as the decay product of positive muons of secondary cosmic rays . Such a positron is initially slowed down by impacts when it enters matter and can then form a positronium "atom" with one of the electrons present there . If the spin of the positron is opposite to that of the electron (parapositronium), then the positronium decays into two photons with a half-life of the order of 0.1 ns. However, annihilation is also possible directly without the formation of a bound positronium state.

If the momentum and kinetic energy of the positronium are negligibly small, then the angle between the emission directions of the two photons is exactly 180 ° and the energy of each photon is 511 keV , the rest energy of the electron or positron. However, if the system has an impulse before the annihilation , this is transferred to the photons so that they are not emitted at an angle of 180 °. The difference between the actual angle and 180 ° is the angle with ${\ displaystyle \ theta}$

${\ displaystyle \ tan {\ theta} = {\ frac {p_ {T}} {m_ {e} c}}}$,

where the transverse component of the momentum of the positronium before annihilation versus the emission direction is the electron mass and the speed of light. Since the positronium also has kinetic energy in this case, the Doppler effect occurs, so that the two photon energies are slightly shifted compared to 511 keV. In practice, this 511 keV line is always significantly broadened compared to other spectral lines when it is observed in a gamma spectrometer . ${\ displaystyle p_ {T}}$${\ displaystyle m_ {e}}$${\ displaystyle c}$

The orthopositronium does not break down into two, but three (or rarely more) photons. These do not have discrete energies, but a continuous energy spectrum.