Electron and positron annihilate

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Discussion Overview

The discussion revolves around the annihilation of electrons and positrons, specifically addressing claims about the properties of the resulting photon, including its momentum and energy. Participants explore theoretical aspects of particle physics, including Feynman diagrams and the nature of virtual particles.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions the assertion that an electron and positron can annihilate to form a photon with zero momentum, referencing the de Broglie equation which suggests all photons should have momentum.
  • Another participant emphasizes the need for proper references to clarify the claims made in the handwritten notes, indicating that context is crucial for understanding the statements.
  • Some participants express skepticism about the possibility of a photon having zero momentum or energy, stating that without context, it is difficult to identify the misunderstanding.
  • A participant mentions a Feynman diagram illustrating the annihilation process, noting that the photon produced is virtual and does not need to adhere to the standard energy-momentum relationship.
  • There is a suggestion that the notes may be part of an explanation for why annihilation typically results in the production of two photons.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the claims regarding photons having zero momentum or energy. There are competing views on the validity of the statements made in the handwritten notes, and the discussion remains unresolved.

Contextual Notes

Limitations include the lack of references to the original source of the claims, which may lead to misunderstandings. The discussion also highlights the distinction between virtual and real photons, which affects the interpretation of energy and momentum in particle interactions.

dyn
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Hi.
I am just starting to self-study particle physics. Came across the following in some notes -
Electron and positron annihilate to form a photon with zero momentum. I thought all photons had momentum due to the de Broglie equation ?
Also 2 electrons scatter to form a photon with zero energy. Again I can't see how this can be true.
Any help would be appreciated.
Thanks
 
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Please provide proper references to where you have read this. Without proper references it is impossible to tell what the source has said and what you might have misunderstood from the source. In addition, we need to have more context to figure out what exactly you are looking at.
 
I can't provide references. It came from some handwritten lecture notes. Is it possible for a photon to have zero momentum or energy ?
 
dyn said:
I can't provide references.

Explain to me something I'm not going to show you is a hard question to answer.
 
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dyn said:
I can't provide references. It came from some handwritten lecture notes. Is it possible for a photon to have zero momentum or energy ?

It's probably part of an explanation why the annihilation always produces 2 photons.
 
dyn said:
Is it possible for a photon to have zero momentum or energy ?
No. But without any context it is impossible to tell what exactly went wrong.

You could write the relevant part of the notes here in the forum.
 
The Feynman diagram shows an electron and positron annihilating to form a photon which then decays into an electron and positron. The notes say " if an electron and a positron annihilate a photon is formed with zero charge , zero momentum and energy 2Ee and hence an apparent mass of 2Ee/c2 "
 
This is one of the two leading-order Feynman diagrams for electron-positron scattering (Bhabha scattering).

https://en.wikipedia.org/wiki/Bhabha_scattering

The photon is virtual, not real, so it need not obey the relationship ##E^2 - (pc)^2 = (mc^2)^2 = 0##.
 
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dyn said:
I can't provide references. It came from some handwritten lecture notes. Is it possible for a photon to have zero momentum or energy ?
If this was really stated in these lecture notes they are for sure not a "proper reference". So rather get a good textbook.
 
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