# Electron-Positron production by photon

• I
• Silviu
In summary: I don't know how to put this delicately. It must fail miserably. Obviously production of a pair from a photon and a massless chargeless photon traveling in a different direction must...well, I don't know how to put this delicately. It must fail miserably.
Silviu
Hello! I am a bit confused about the decay of a photon into a electron-positron pair. In the center of mass of the photon, isn't this decay violating the energy conservation?

What do you mean by "center of mass of the photon". First of all there's no center of mass, because the photon mass is 0. There's also no center-momentum-frame, because the photon mass is 0, i.e., you can never find a reference frame, where the photon is at rest. As you correctly noted a photon cannot decay to an electron-positron pair, because you cannot fulfill energy-momentum conservation and the on-shell conditions for the photon and the particles simultaneously. So there is no photon decay.

Silviu said:
Hello! I am a bit confused about the decay of a photon into a electron-positron pair. In the center of mass of the photon, isn't this decay violating the energy conservation?
As vanhees points out, there is no center of mass frame for a photon - that would be a frame in which the momentum of the photon is zero, and of course there is no such thing.

However, you are on to something here. An isolated photon cannot decay into an electron/positron pair, because there's no way that interaction can conserve both energy and momentum. (An easy way to see this is to think about how the interaction looks in the center of mass frame of the electron and positron after the collision).

Instead, pair production requires the involvement of some other massive charged particle, typically some nearby atomic nucleus. The reaction is properly written as ##\gamma+Z\rightarrow{Z}+e^++e^-## where Z is the other particle; its energy and momentum change in the interaction.

vanhees71
Silviu said:
Hello! I am a bit confused about the decay of a photon into a electron-positron pair. In the center of mass of the photon, isn't this decay violating the energy conservation?

To add to what have been mentioned already in the replies you received, we make e-p pair production by shooting high-energy photons into a material with high atomic number, such as Be. There is a reason for that, and that has been given in the two responses above.

Zz.

vanhees71
ZapperZ said:
by shooting high-energy photons into a material with high atomic number, such as Be
Beryllium has atomic number 4. The only solid target with an even lower number would be lithium, but that is too reactive (chemically) to be practical: Be is as low as you can get.
High atomic numbers would be lead (82) or tungsten (74). You need more material with lighter elements, but the produced electrons/positrons pass through the material easier as well, so low atomic numbers can be favorable.

mfb said:
Beryllium has atomic number 4. The only solid target with an even lower number would be lithium, but that is too reactive (chemically) to be practical: Be is as low as you can get.
High atomic numbers would be lead (82) or tungsten (74). You need more material with lighter elements, but the produced electrons/positrons pass through the material easier as well, so low atomic numbers can be favorable.

Sorry, I should have said tungsten. I had Be in my head because I've been shopping around for a Be window for one of our viewports.

Zz.

stoomart said:
Positrons and electrons (also neutrinos/antineutrinos) are released from the decay of W bosons after photon collisions.
Technically true, but that is an incredibly rare process. Direct electron and pair production from photon collisions is a very rare process already, but that is much more frequent than the indirect process via W boson production. Note the "collision" part, it doesn't happen with single photons as OP asked about.

mfb said:
Technically true, but that is an incredibly rare process. Direct electron and pair production from photon collisions is a very rare process already, but that is much more frequent than the indirect process via W boson production. Note the "collision" part, it doesn't happen with single photons as OP asked about.
Ahh thanks, didnt notice the OP was about a single photon.

Nugatory said:
However, you are on to something here. An isolated photon cannot decay into an electron/positron pair, because there's no way that interaction can conserve both energy and momentum. (An easy way to see this is to think about how the interaction looks in the center of mass frame of the electron and positron after the collision).
And neither can two or more photons traveling in exact same direction. Nor a photon and another massless particle traveling in exact same direction.
Nugatory said:
Instead, pair production requires the involvement of some other massive charged particle, typically some nearby atomic nucleus. The reaction is properly written as ##\gamma+Z\rightarrow{Z}+e^++e^-## where Z is the other particle; its energy and momentum change in the interaction.

Obviously production of a pair from a photon and a massless chargeless photon traveling in a different direction must be possible because that´s time reversal of annihilation.
Is it possible to produce a pair from a photon and a massive neutral particle, such as neutron?

snorkack said:
Is it possible to produce a pair from a photon and a massive neutral particle, such as neutron?
Neutrons have charged quarks inside. It is very unlikely in the MeV range, but becomes similar (within a factor of 2) to pair production at a proton at high energies. of a few hundred MeV.

## 1. What is the process of electron-positron production by photon?

Electron-positron production by photon is a phenomenon in which a high-energy photon interacts with a nucleus and produces an electron and a positron. This process is known as pair production and occurs when the energy of the photon is converted into the mass of the electron and positron pair.

## 2. How is the energy of the photon related to the production of electron-positron pairs?

The energy of the photon must be at least equal to the total mass of the electron-positron pair for pair production to occur. This minimum energy is known as the threshold energy and is given by the equation E = 2mc^2, where m is the mass of the electron or positron and c is the speed of light.

## 3. What is the role of a nucleus in electron-positron production by photon?

The nucleus serves as the target for the high-energy photon and provides the necessary momentum for the creation of the electron-positron pair. The photon interacts with the nucleus, transferring its energy and producing the pair.

## 4. Can electron-positron production by photon occur in vacuum?

No, electron-positron production by photon cannot occur in vacuum. This process requires the presence of a nucleus to serve as a target for the high-energy photon. In vacuum, there are no particles to interact with the photon and thus no pair production can occur.

## 5. What are some applications of electron-positron production by photon?

Electron-positron production by photon has several applications in the field of particle physics. It is used in particle accelerators to create high-energy electron-positron beams for experiments. It is also used in medical imaging techniques such as positron emission tomography (PET) scans, where the annihilation of electron-positron pairs is used to create images of the body.

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