Particle-Antiparticle Pairs: When High Energy Photons Split

In summary, high energy photons can become a particle/antiparticle pair when their energy is above twice the particle mass. This process, known as pair production, requires both the photon energy to exceed 1.02 MeV and for momentum to be transferred to a nucleus or other heavy, electrically charged particle. Alternatively, pair production can also occur when two photons collide with each other, such as in the early stages of the universe. However, it is not possible for a single photon to split into two massive particles and still conserve momentum.
  • #1
kashiark
210
0
when do high energy photons become a paritcle/antiparticle pair?
 
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  • #2
it can become when its energy is above twice the particle mass.

e.g. pair production of electron + positron is possible when the photon has energy above 2*m_e = 1022keV
 
  • #3
malawi_glenn said:
it can become when its energy is above twice the particle mass.

e.g. pair production of electron + positron is possible when the photon has energy above 2*m_e = 1022keV

Your description is somewhat incomplete. In order for pair production to take place the photon has to interact with a nucleus so that momentum conservation will hold. Alternatively if two photons collide (such as just after the big bang) pair production will also be possible.
 
  • #4
Pair production requires BOTH the photon energy exceeding 1.02 MeV, AND something momentum can be transferred to, like a nucleus. The cross section is proportional to Z2 Ln(E) from 3 MeV to over 1 GeV. (E = hv)
 
  • #5
why can't the momentum be transferred to to the particle/antiparticle pair? and what is it transferred to when they collide?
 
  • #6
kashiark said:
why can't the momentum be transferred to to the particle/antiparticle pair? and what is it transferred to when they collide?

photon -> e+ e- violates momentum conservation, as mathman told you.

The photon must 'interact' with a nucleus or other heavy, electrically charged particle, to make pair production possible.

http://en.wikipedia.org/wiki/Pair_production

here is feynman diagram (the process to the left)

http://musr.physics.ubc.ca/~jess/p200/emc2/img48.gif
 
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  • #7
what about the photons colliding after the big bang?
 
  • #8
they can 'collide' with each other, as mathman told you also. Don't you read the answers you've got?
 
  • #9
"Alternatively if two photons collide (such as just after the big bang) pair production will also be possible."
"The photon must 'interact' with a nucleus or other heavy, electrically charged particle, to make pair production possible."
they seem to contradict each other don't they? i was asking about the physics of the first and i would appreciate it if you would not insult me
 
  • #10
because in post #5 you are referring to post #4 where the "pair production in matter" was discussed by Bob_S.
 
  • #11
i don't understand if they can conserve momentum by colliding why can't they conserve it otherwise without a nucleus?
 
  • #12
you cannot have a massless thing split to two massive things and still conserve momentum. just work it out.

you CAN, on the other hand, have two (massless or massive) things go to two massive things and conserve momentum and energy.

so as to the DIFFERENT processes that you are referring to:

1. photon -> e+ e- : that violates momentum conservation.

2. photon + Nucleus -> e+ e- + nucleus: that is allowed by momentum conservation (think of the nucleus as a "catalyst" that allows the process to occur).

3. photon + photon -> e+ e- : that is also allowed by momentum conservation.

mometum conservation is nothing deep: it's straight out of first-year physics.
 
  • #13
ah ok i get it thanks everyone
 

1. What are particle-antiparticle pairs?

Particle-antiparticle pairs are pairs of subatomic particles that have opposite charges and other properties such as spin. They are created when high energy photons split into two particles of equal mass and opposite charge.

2. How are particle-antiparticle pairs formed?

Particle-antiparticle pairs are formed through a process called pair production, where a high energy photon interacts with a nucleus or another particle and converts into a pair of particles with opposite charges. This process follows the law of conservation of energy and mass, where the combined energy and mass of the two particles must be equal to the energy and mass of the initial photon.

3. What is the significance of particle-antiparticle pairs?

Particle-antiparticle pairs are significant because they provide insight into the fundamental nature of matter and antimatter. They also play a crucial role in understanding the early universe and the processes that led to the formation of matter.

4. Can particle-antiparticle pairs be observed in experiments?

Yes, particle-antiparticle pairs have been observed in experiments such as particle accelerators and cosmic ray detectors. These experiments have confirmed the existence of various types of particle-antiparticle pairs, including electron-positron pairs and quark-antiquark pairs.

5. How do particle-antiparticle pairs relate to the concept of annihilation?

Particle-antiparticle pairs can annihilate each other, releasing energy in the form of gamma rays. This process is the reverse of pair production and follows the law of conservation of energy. The annihilation of particle-antiparticle pairs is also a crucial concept in the development of technologies such as positron emission tomography (PET) scans in medical imaging.

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