Forskel mellem versioner af "Annihilation"

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[[File:Mutual Annihilation of a Positron Electron pair.svg|200px|thumb|right|AEt [[Feynman diagram]] showing the mutual annihilation of a bound state [[electron]] [[positron]] pair into two photons. This bound state is more commonly known as [[positronium]]]]
'''Annihilation''' is defined as "total destruction" or "complete obliteration" of an object;<ref> [ - Dictionary Definition] (2006)</ref> having its root in the Latin ''nihil'' (nothing). A literal translation is "to make into nothing".
During a low-energy annihilation, [[photon]] production is favored, since these particles have no mass. However, high-energy [[particle collider]]s produce annihilations where a wide variety of exotic heavy particles are created.
== Examples of annihilation ==
This is an example of [[renormalization]] in [[quantum field theory]]&mdash; the field theory being necessary because the number of particles changes from one to two and back again.
=== Electron–positron annihilation ===
{{main|Electron–positron annihilation}}
: {{SubatomicParticle|Electron|link=yes}}&nbsp;+&nbsp;{{SubatomicParticle|Positron|link=yes}}&nbsp;→&nbsp;{{SubatomicParticle|Gamma|link=yes}}&nbsp;+&nbsp;{{SubatomicParticle|Gamma|link=yes}}
When a low-energy [[electron]] annihilates a low-energy [[positron]] (antielectron), they can only produce two or more [[gamma ray]] [[photon]]s, since the electron and positron do not carry enough [[mass-energy]] to produce heavier particles and conservation of energy and linear momentum forbid the creation of only one photon. When an electron and a positron collide to annihilate and create gamma rays, energy is given off. Both particles have a rest energy of 0.511 mega electron volts (MeV). When the mass of the two particles are converted entirely into energy, this rest energy is what is given off. The energy is given off in the form of the aforementioned gamma rays. Each of the gamma rays has an energy of 0.511 MeV. Since the positron and electron are both briefly at rest during this annihilation, the system has no momentum during that moment. This is the reason that two gamma rays are created. Conservation of momentum would not be achieved if only one photon was created in this particular reaction. Momentum and energy are both conserved with 1.022 MeV of gamma rays (accounting for the rest energy of the particles) moving in opposite directions (accounting for the total zero momentum of the system).<ref name=Fermilab>{{cite web|last=Cossairt|first=Don|title=Radiation from particle annihilation|url=|work=Inquiring Minds: Questions About Physics|publisher=Fermi Research Alliance, LLC|accessdate=17 October 2011}}</ref> However, if one or both particles carry a larger amount of kinetic energy, various other particle pairs can be produced. The annihilation (or decay) of an electron-positron pair into a ''single'' photon, cannot occur in free space because momentum would not be conserved in this process. The [[pair production|reverse reaction]] is also impossible for this reason, except in the presence of another particle that can carry away the excess momentum. However, in [[quantum field theory]] this process is allowed as an intermediate quantum state. Some authors justify this by saying that the photon exists for a time which is short enough that the violation of conservation of momentum can be accommodated by the [[uncertainty principle]]. Others choose to assign the intermediate photon a non-zero mass. (The mathematics of the theory are unaffected by which view is taken.) This opens the way for '''virtual pair''' production or annihilation in which a one-particle quantum state may fluctuate into a two-particle state and back again (coherent superposition). {{Citation needed|date=February 2007}} These processes are important in the [[vacuum state]] and [[renormalization]] of a quantum field theory. It also allows neutral particle mixing through processes such as the one pictured here.
===Proton-antiproton annihilation===
When a proton encounters its antiparticle, the reaction is not as simple as electron-positron annihilation. In general, a proton encountering an antiproton will turn into a number of mesons, mostly pions and kaons, which will fly away from the annihilation point. The newly created mesons are unstable, and will decay in a series of reactions that ultimately produce nothing but gamma rays, electrons, positrons, and neutrinos. This type of reaction will occur between any baryon (particle consisting of three quarks) and any antibaryon (consisting of three antiquarks). Antiprotons can and do annihilate with neutrons.
Here are the specifics of the reaction that produces the mesons. Protons consist of two up quarks and one down quark, while antiprotons consist of the corresponding antiquarks. The strong nuclear force provides a strong attraction between quarks and antiquarks, so when a proton and antiproton approach to within a distance where this force is operative (less than 1 fm), the quarks tend to pair up with the antiquarks, forming three pions. The energy released in this reaction is substantial, as the rest mass of three pions is much less than the mass of a proton and an antiproton. Energy may also be released by the direct annihilation of a quark with an antiquark. The extra energy can go to the kinetic energy of the released pions, be radiated as gamma rays, or into the creation of additional quark-antiquark pairs. When the annihilating proton and antiproton are at rest relative to one another, these newly created pairs may be composed of up, down or strange quarks. The other flavors of quarks are too massive to be created in this reaction, unless the incident antiproton has kinetic energy far exceeding its rest mass, i.e. is moving close to the speed of light. The newly created quarks and antiquarks join in the dance of pairing into mesons, producing additional pions and kaons. Reactions in which proton-antiproton annihilation produces as many as nine mesons have been observed, while production of thirteen mesons is theoretically possible. The generated mesons leave the site of the annihilation at moderate fractions of the speed of light, and decay with whatever lifetime is appropriate for their type of meson. <ref>"The Antinucleon-Nucleon Interaction at Low Energy: Annihilation Dynamics" by Eberhad Klempt, Chris Batty, and Jean-Marc Richard, arxiv:hep-ex/0501020v1 and Physics Reports, Volume 413, Issue 4-5, p. 197-317.</ref>
== References ==
* {{cite book | author=Kragh, Helge| title= Quantum Generations : A history of physics in the twentieth century | publisher= [[Princeton University Press]] | year=1999| isbn =0-691-01206-7 }}
==See also==
*[[Pair productionParproduction]]
*[[Electron-positron annihilation]]
*[[Proton-antiproton annihilation]]