Pair production

Under pairing, also pair production, is understood in particle physics, the formation of a real, observable particle-antiparticle pair. The ever-present virtual, only indirectly observable pairs (see vacuum polarization) are not included within the term in general.

In a narrower sense is under the pairing - understood from a high-energy photon production of an electron- positron pair - historically the first known. Today, however, the formation of muon antimuon or proton -antiproton pairs are known, for example. In any case, the available energy of the sum of the rest of the energy to be produced, the particles must be at least equal, so that the operation occurs.

The opposite process, in which a particle-antiparticle pair is destroyed, is called Annihilation.

Electron-positron production

The creation of an electron - positron pair from a high-energy photon was detected in 1933 as the first pair educational process experimentally by Irène Curie and Frédéric Joliot. This pairing is an important process of interaction of photons with matter Represent leads, for example, in bubble chambers to characteristic traces. We distinguish two cases: the pair formation can occur by interaction of a photon with the electric field of an atomic nucleus or an electron shell.

Interaction of a photon with an atomic nucleus

Find the pair production in the field of an atomic nucleus instead, the entire energy of the photon is almost converted into the rest mass of the two resulting particles and their kinetic energy. The energy of the photon must therefore be at least the sum of the rest energies of the electron and positron meet. In a detailed energy balance in addition to the recoil of the nucleus is an even, in the field runs the pairing. The threshold energy for pair production is therefore

With the mass of the interacting nucleus, as the rest mass of the electron and the speed of light. The term can be often neglected. The gamma spectroscopy using a germanium detector is available with the mass of the germanium core example

Thus amounts to approximately 1.022 MeV (gamma radiation). Does the photon has a higher energy, it will be converted into the kinetic energy of the electron and positron. The probability of pair production increases in proportion to the atomic number of the nucleus and to the logarithm of photon energy.

The fact that the formation of an electron-positron pair is ( the atomic nucleus here), but not only observed in the interaction of the photon with a particle in vacuum, can be explained by the general momentum conservation. To this end, the following thought experiment for the limiting case of a photon energy of straight, so that the kinetic energy of the particles produced is zero: have the rest frame of the two caused by pairing particles when taken together, a pulse of zero. A photon has but in any reference system the same vacuum speed of light c, and thus in this system also has a pulse size. Therefore, the two particles with their total momentum zero can not be the only available after the trial particles. Rather, the pairing is carried out only if an additional particles, which receives the impulse in this case, the nucleus.

Interaction of a photon with an electron

Find the pair production in the electric field of an electron of the atomic shell instead, this electron is with its low mass greatly accelerated by the transmitted pulse and released from the atom. Because of the three free particles ( two electrons and a positron ), this process is known as the triplet formation.

The necessary threshold energy can be calculated from the energy - momentum relation

Derived. Assuming that all three particles move according to the pair production at the same speed in the same direction, thus are relatively at rest, corresponds to the rest energy of the system with just the rest energy of the three particles. Since the total energy is composed of the energy of the photon and of the rest energy of the bound electron before pairing, it follows

This resolves the equation with respect to, the threshold energy is obtained

Of the amount for the formation of the electron - positron pair is required, while the rest of the energy transfers from also as kinetic energy of the three particles.

Interaction between photons

Also by collisions very energy photons with each other can real electron-positron pairs are produced. This was first demonstrated in 1997 with an experiment at the Stanford Linear Accelerator Center. The photons of a Nd: glass laser were in turn caused by scattering of electrons of 47 Gigaelektronvolt ( GeV ) in GeV energies. Such production of matter- photon-photon collisions is assumed for some star formation.

Quantum mechanical description

In quantum mechanics, the process of pair production is described by the four-fold differential scattering cross section. By integration over two angles one obtains the double differential scattering cross -section which can be used very well for it to put it in Monte Carlo simulations. An analysis of this expression shows that positrons are emitted mainly in the direction of the incoming photon.