Electron–positron annihilation
− +
Electron–positron annihilation occurs when an electron (e ) and a positron (e , the electron's
antiparticle) collide. At low energies, the result of the collision is the annihilation of the electron and
positron, and the creation of energetic photons:
− +
e +e →γ+γ
At high energies, other particles, such as B mesons or the W and Z bosons, can be created. All
processes must satisfy a number of conservation laws, including:
Conservation of electric charge. The net charge before and after is zero.
Natura
Conservation of linear momentum and total energy. This forbids the creation of a single photon.
result
However, in quantum field theory this process is allowed; see examples of annihilation.
Conservation of angular momentum.
Conservation of total (i.e. net) lepton number, which is the number of leptons (such as the electron) minus th
positron); this can be described as a conservation of (net) matter law.
As with any two charged objects, electrons and positrons may also interact with each other without annihilating
Low-energy case
There are only a very limited set of possibilities for the final state. The most probable is the creation of two or m
and linear momentum forbid the creation of only one photon. (An exception to this rule can occur for tigh
common case, two gamma photons are created, each with energy equal to the rest energy of the electron or po
reference is that in which the system has no net linear momentum before the annihilation; thus, after collision
directions. It is also common for three to be created, since in some angular momentum states, this is nece
possible to create any larger number of photons, but the probability becomes lower with each additional g
processes have lower probability amplitudes.
Since neutrinos also have a smaller mass than electrons, it is also possible – but exceedingly unlikely – fo
neutrino–antineutrino pairs. The probability for such process is on the order of 10000 times less likely than th
be true for any other particles, which are as light, as long as they share at least one fundamental interaction wi
it. However, no other such particles are known.
High-energy case
If either the electron or positron, or both, have appreciable kinetic energies, other heavier particles can also be
since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. Al
and other light particles, but they will emerge with higher kinetic energies.
At energies near and beyond the mass of the carriers of the weak force, the W and Z bosons, the strength of
electromagnetic force.[3] As a result, it becomes much easier to produce particles such as neutrinos that interact
+ −
The heaviest particle pairs yet produced by electron–positron annihilation in particle accelerators are W –W p
single-charged particle is the Z boson (mass 91.188 GeV/c2). The driving motivation for constructing the Int
Higgs bosons (mass 125.09 GeV/c2) in this way.
Practical uses
The electron–positron annihilation process is the physical phenomenon relied on as the basis of positron
− +
Electron–positron annihilation occurs when an electron (e ) and a positron (e , the electron's
antiparticle) collide. At low energies, the result of the collision is the annihilation of the electron and
positron, and the creation of energetic photons:
− +
e +e →γ+γ
At high energies, other particles, such as B mesons or the W and Z bosons, can be created. All
processes must satisfy a number of conservation laws, including:
Conservation of electric charge. The net charge before and after is zero.
Natura
Conservation of linear momentum and total energy. This forbids the creation of a single photon.
result
However, in quantum field theory this process is allowed; see examples of annihilation.
Conservation of angular momentum.
Conservation of total (i.e. net) lepton number, which is the number of leptons (such as the electron) minus th
positron); this can be described as a conservation of (net) matter law.
As with any two charged objects, electrons and positrons may also interact with each other without annihilating
Low-energy case
There are only a very limited set of possibilities for the final state. The most probable is the creation of two or m
and linear momentum forbid the creation of only one photon. (An exception to this rule can occur for tigh
common case, two gamma photons are created, each with energy equal to the rest energy of the electron or po
reference is that in which the system has no net linear momentum before the annihilation; thus, after collision
directions. It is also common for three to be created, since in some angular momentum states, this is nece
possible to create any larger number of photons, but the probability becomes lower with each additional g
processes have lower probability amplitudes.
Since neutrinos also have a smaller mass than electrons, it is also possible – but exceedingly unlikely – fo
neutrino–antineutrino pairs. The probability for such process is on the order of 10000 times less likely than th
be true for any other particles, which are as light, as long as they share at least one fundamental interaction wi
it. However, no other such particles are known.
High-energy case
If either the electron or positron, or both, have appreciable kinetic energies, other heavier particles can also be
since there is enough kinetic energy in the relative velocities to provide the rest energies of those particles. Al
and other light particles, but they will emerge with higher kinetic energies.
At energies near and beyond the mass of the carriers of the weak force, the W and Z bosons, the strength of
electromagnetic force.[3] As a result, it becomes much easier to produce particles such as neutrinos that interact
+ −
The heaviest particle pairs yet produced by electron–positron annihilation in particle accelerators are W –W p
single-charged particle is the Z boson (mass 91.188 GeV/c2). The driving motivation for constructing the Int
Higgs bosons (mass 125.09 GeV/c2) in this way.
Practical uses
The electron–positron annihilation process is the physical phenomenon relied on as the basis of positron
