Up Learn – A Level physics (AQA) – particles and interactions
How pair production results in a particle-antiparticle pair from a high-energy photon.
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Particles and Interactions
So far we’ve seen that a particle and its antiparticle annihilate each other and produce high energy photons
But, what about the opposite?
Can highly energetic photons create particles and antiparticles?
Well, as it happens… this process is indeed possible…
For instance, this is a very high energy photon
And, in the right conditions, it can create an electron and its antiparticle the positron
Next, let’s take this even more energetic photon
And, in the right conditions, it can create the more massive muon and its antiparticle the antimuon
Finally, let’s take this even more energetic photon
And once again, in the right conditions, it can create the even more massive proton and its antiparticle the antiproton
High energy photons can create particle-antiparticle pairs
So now, which of these particles can be created by high energy photons?
These can because they’re particle-antiparticle pairs
But these can’t since high energy photons can only produce particle-antiparticle pairs.
Now, each time it’s pairs of particles that are produced by photons
And so we call this process pair production.
But… why pairs?
Well, as we saw in the conservation laws section… charge has to be conserved in any interaction – so that’s one reason
And another more complex reason is that momentum needs to be conserved
When two particles are created, momentum is conserved, but that wouldn’t be the case if only one was created.
Next, we’ve seen that a high energy photon can create an electron-positron pair
An even more high energy photon can create the more massive muon-antimuon pair
And an even higher energy photon can create an even more massive proton-antiproton pair
Higher energy photons can create more massive particles
So now, which of these particles could have been created by the most energetic photon?
This particle-antiparticle pair is the most massive and so it would require the most energetic photon to be produced.
To sum up, pair production is a process where…
Pair production is a process where a high energy photon creates a particle-antiparticle pair.
Where the greater the photon energy…
In pair production, the greater the photon energy, the more massive the particle-antiparticle pair that can be produced.