Electron–positron annihilation
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Electron–positron annihilationoccurs when anelectron(
e−
) and apositron(
e+
,the electron'santiparticle) collide. At low energies, the result of the collision is theannihilationof the electron and positron, and the creation of energeticphotons:
e−
+
e+
→
γ
+
γ
At high energies, other particles, such asB mesonsor theW and Z bosons,can be created. All processes must satisfy a number ofconservation laws,including:
- Conservation of electric charge.The netchargebefore and after is zero.
- Conservation oflinear momentumand totalenergy.This forbids the creation of a single photon. However, inquantum field theorythis process is allowed; seeexamples of annihilation.
- Conservation ofangular momentum.
- Conservation of total (i.e. net)lepton number,which is the number of leptons (such as the electron) minus the number of antileptons (such as the positron); this can be described as aconservation of (net) matterlaw.
As with any two charged objects, electrons and positrons may also interact with each other without annihilating, in general byelastic scattering.
Low-energy case
[edit]There are only a very limited set of possibilities for the final state. The most probable is the creation of two or more gamma photons. Conservation of energy and linear momentum forbid the creation of only one photon. (An exception to this rule can occur for tightly bound atomic electrons.[1]) In the most common case, two gamma photons are created, each withenergyequal to therest energyof the electron or positron (0.511MeV).[2]A convenientframe of referenceis that in which the system hasno net linear momentumbefore the annihilation; thus, after collision, the gamma photons are emitted in opposite directions. It is also common for three to be created, since in some angular momentum states, this is necessary to conservecharge parity.[3]It is also possible to create any larger number of photons, but the probability becomes lower with each additional gamma photon because these more complex processes have lowerprobability amplitudes.
Sinceneutrinosalso have a smaller mass than electrons, it is also possible – but exceedingly unlikely – for the annihilation to produce one or more neutrino–antineutrinopairs. The probability for such process is on the order of 10000 times less likely than the annihilation into photons. The same would be true for any other particles, which are as light, as long as they share at least onefundamental interactionwith electrons and no conservation laws forbid it. However, no other such particles are known.
High-energy case
[edit]If either the electron or positron, or both, have appreciablekinetic energies,other heavier particles can also be produced (such asD mesonsorB mesons), since there is enough kinetic energy in the relative velocities to provide therest energiesof those particles. Alternatively, it is possible to produce photons and other light particles, but they will emerge with higher kinetic energies.
At energies near and beyond the mass of the carriers of theweak force,theW and Z bosons,the strength of the weak force becomes comparable to theelectromagneticforce.[3]As a result, it becomes much easier to produce particles such as neutrinos that interact only weakly with other matter.
The heaviest particle pairs yet produced by electron–positron annihilation inparticle acceleratorsare
W+
–
W−
pairs (mass 80.385 GeV/c2× 2). The heaviest single-charged particle is theZ boson(mass 91.188 GeV/c2). The driving motivation for constructing theInternational Linear Collideris to produce theHiggs bosons(mass 125.09 GeV/c2) in this way.[citation needed]
Practical uses
[edit]The electron–positron annihilation process is the physical phenomenon relied on as the basis ofpositron emission tomography(PET) andpositron annihilation spectroscopy(PAS). It is also used as a method of measuring theFermi surfaceandband structureinmetalsby a technique calledAngular Correlation of Electron Positron Annihilation Radiation. It is also used for nuclear transition. Positron annihilation spectroscopy is also used for the study ofcrystallographic defectsin metals and semiconductors; it is considered the only direct probe for vacancy-type defects.[4]
Reverse reaction
[edit]The reverse reaction, electron–positron creation, is a form ofpair productiongoverned bytwo-photon physics.
See also
[edit]References
[edit]- ^ L. Sodickson; W. Bowman; J. Stephenson; R. Weinstein (1970). "Single-Quantum Annihilation of Positrons".Physical Review.124(6): 1851–1861.Bibcode:1961PhRv..124.1851S.doi:10.1103/PhysRev.124.1851.
- ^
W.B. Atwood, P.F. Michelson, S.Ritz (2008). "Una Ventana Abierta a los Confines del Universo".Investigación y Ciencia(in Spanish).377:24–31.
{{cite journal}}:CS1 maint: multiple names: authors list (link) - ^ab D.J. Griffiths (1987).Introduction to Elementary Particles.Wiley.ISBN0-471-60386-4.
- ^ F. Tuomisto and I. Makkonen (2013)."Defect identification in semiconductors with positron annihilation: Experiment and theory".Reviews of Modern Physics.85(4): 1583–1631.Bibcode:2013RvMP...85.1583T.doi:10.1103/RevModPhys.85.1583.hdl:10138/306582.S2CID41119818.
