Electron and positron annihilate

In summary: The answer is that a photon with zero momentum and energy is a virtual photon and not a real one. It only exists as an intermediate step in the annihilation process.In summary, the conversation discusses the possibility of a photon having zero momentum and energy as described in handwritten lecture notes. However, without proper references and context, it is difficult to determine the validity of this statement. It is suggested to consult a textbook for accurate information on virtual photons and the annihilation process.
  • #1
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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  • #2
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.
 
  • #3
I can't provide references. It came from some handwritten lecture notes. Is it possible for a photon to have zero momentum or energy ?
 
  • #4
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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  • #5
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.
 
  • #6
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.
 
  • #7
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 "
 
  • #8
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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  • #9
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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1. What is electron-positron annihilation?

Electron-positron annihilation is a process in which an electron and a positron (its antimatter counterpart) collide and are converted into energy in the form of gamma rays.

2. How does electron-positron annihilation occur?

When an electron and positron collide, they can either annihilate each other completely, producing two or more gamma ray photons, or they can produce other particles before annihilation occurs.

3. What is the significance of electron-positron annihilation in particle physics?

Electron-positron annihilation is significant in particle physics because it provides a way to study the properties of the particles involved, such as their mass and charge, by examining the energy and momentum of the resulting gamma rays.

4. Can electron-positron annihilation occur in any environment?

Electron-positron annihilation can occur in any environment as long as there are electrons and positrons present. It is a common process in high-energy physics experiments and can also occur naturally in cosmic rays and certain types of radioactive decay.

5. Is electron-positron annihilation reversible?

No, electron-positron annihilation is not reversible. Once the particles have been converted into energy, they cannot be reformed into an electron and positron again.

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