Why must an electron-positron collision produce gamma rays?

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Discussion Overview

The discussion revolves around the outcomes of electron-positron collisions, specifically focusing on the production of gamma rays and the factors that determine the frequency of emitted photons. It encompasses theoretical considerations, conservation laws, and potential alternative products of such collisions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the frequency of photons emitted in an electron-positron collision is influenced by the spin and energy states of the initial particles.
  • Others argue that energy and momentum conservation are critical factors in determining the outcomes of the collision.
  • It is noted that the collision does not necessarily have to produce gamma rays, as other products may emerge if the energy is sufficient, including elastic scattering.
  • Some participants mention that if the particles are moving at relatively low speeds, the annihilation typically results in a pair of photons at 511 keV.
  • There is a discussion about the role of spin and parity in the collision, with some asserting that these do not determine branching fractions between equally allowed processes.
  • One participant highlights that there is always enough energy to potentially produce neutrino/antineutrino pairs or even gravitons, although the branching fractions for these processes are very small.
  • A reference to the Wikipedia page on electron-positron annihilation is provided as a source for further information.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the necessity of gamma ray production in electron-positron collisions, with some asserting that other outcomes are possible. The discussion remains unresolved with respect to the definitive outcomes of such collisions.

Contextual Notes

Limitations include the dependence on specific energy levels and the conditions under which the collisions occur, as well as the unresolved nature of branching fractions for various potential products.

animagaletix
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Or, more specifically, what determinates the frequency of the photons emitted by a such a collision. I know that the number of photons produced depends on the spin and energy states of the initial particles.
 
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Energy and momentum conservation.

The collision does not have to produce gamma rays, other products are possible if the energy is sufficient, and elastic scattering is always possible.
 
If the particles are going at relatively low speeds, then the annihilation will result in a pair of photons at 511 kev.
 
mfb said:
Energy and momentum conservation.
And spin and parity.
Which does not determine branching fractions between equally allowed processes.
mfb said:
The collision does not have to produce gamma rays, other products are possible if the energy is sufficient,
There is always enough energy to produce neutrino/antineutrino pairs, of three flavour options, or gravitons.
 
With tiny branching fractions, yes (especially for the gravitons).
Decays of Positronium - 10-18 for electron neutrino + antineutrino, even lower for the other flavors.
 

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