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# feynman diagram help watch

1. I don't really understand them 100%. Charge must be conserved at the vertices, but what does that mean exactly? I've posted a diagram from a past paper. It asks what type of reaction is (I'd say weak) but then asks which exchange particle's involved. It would probably be really obvious if I completely understood the diagram! Thanks for any help.
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2. (Original post by Zapsta)
I don't really understand them 100%. Charge must be conserved at the vertices, but what does that mean exactly? I've posted a diagram from a past paper. It asks what type of reaction is (I'd say weak) but then asks which exchange particle's involved. It would probably be really obvious if I completely understood the diagram! Thanks for any help.
Not exactly sure about the charge conservation, but if you knew which force was involved, you'd know the exchange particle (strong - gluons, weak - W and Z, electromagnetic - photon, gravity - graviton).
3. (Original post by Nylex)
Not exactly sure about the charge conservation, but if you knew which force was involved, you'd know the exchange particle (strong - gluons, weak - W and Z, electromagnetic - photon, gravity - graviton).
I think they want you to decide between W+, W- and Z.
4. vertices are where two lines meet. charge being conserved at the vertices means charge going in must equal charge going out.

in the diagram yuo've given, we've got a happy little anti-neutrino coming along, and it emits something (an exchange particle) and then becomes an positron. We've got charge of netural -> something + postiive So the something must be negative. The exchange particle is thus negative.

At the bottom you have a proton flying in, absorbing our exchange particle and becoming a neutron. That makes sense because positive proton + negative exchange particle = netural neutron.

The interaction is indeed weak (leptons everyhwere)
5. (Original post by Willa)
vertices are where two lines meet. charge being conserved at the vertices means charge going in must equal charge going out.

in the diagram yuo've given, we've got a happy little anti-neutrino coming along, and it emits something (an exchange particle) and then becomes an positron. We've got charge of netural -> something + postiive So the something must be negative. The exchange particle is thus negative.

At the bottom you have a proton flying in, absorbing our exchange particle and becoming a neutron. That makes sense because positive proton + negative exchange particle = netural neutron.

The interaction is indeed weak (leptons everyhwere)
Okay, I get that. Thanks a lot .
6. Although to add to this, if the direction of travel of the exchange particle was not known could it be either W+ or W-?
7. yes - feynman diagrams are reversible
8. (Original post by Zapsta)
I don't really understand them 100%. Charge must be conserved at the vertices, but what does that mean exactly? I've posted a diagram from a past paper. It asks what type of reaction is (I'd say weak) but then asks which exchange particle's involved. It would probably be really obvious if I completely understood the diagram! Thanks for any help.
It's a w+ boson from L to R
9. (Original post by zazy)
It's a w+ boson from L to R
W-?
10. (Original post by Zapsta)
W-?
But isn't it a beta plus decay?If so, it is a positive boson...
11. (Original post by zazy)
But isn't it a beta plus decay?If so, it is a positive boson...
Rather than a neutrino being formed though one is interacting with the proton initially, therefore it's not beta plus. It could be W- but the diagram shows that the exchange particle is travelling from the neutrino to the proton rather than the other way round.
12. (Original post by Zapsta)
Rather than a neutrino being formed though one is interacting with the proton initially, therefore it's not beta plus. It could be W- but the diagram shows that the exchange particle is travelling from the neutrino to the proton rather than the other way round.
Ah, it depends on the angle you look at it..got you know
13. Technically it counts as a Beta plus reaction here, in terms of reversibility, because there is an incoming anti-neutrino which is equivilent to an outgoing neutrino. But that assumes the proton emits the exchange particle (W+)

it is a W- if the exchange particle goes from the neutrino to the proton!

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