Particle Phyiscs/Quantum Electrodynamics

>>8882093
It's just charge conservation and by the CPT theorem a flip in time T is equivalent to a flip in charge and parity (an antiparticle moving backwards in time is the same as a particle moving forward in time). Parity for a vector particle just flips the sign of the amplitude so it's not too interesting here.

>>8882093
Oh and another thing, W bosons (or any other bosons for that matter except in some exotic Higgs sectors of Standard Model extensions) don't carry lepton number so there is no lepton number current flowing in these diagrams.

>>8882292
*except for the external particles of course.

>>8882290
In both cases the W boson is moving forward in time, plus the decay products are the same :s I don't get it.

>>8883212
Think of how the actual process is going on. In the first one, an electron emits a W- and becomes a neutrino. Then, the W- interacts with the incoming mu-neutrino and converts into a muon.

In the second process, it's the muonic neutrino the one that first emits the W+, and then the electron the one that receives it.

One usually doesn't think in these terms, but remember that the interactions are not inmediate, the bosons take some time from one point to the other.

In general you just put a W because of course both are equivalent.

>>8883215
So the W boson will be of the same charge as the particle emitting it?

>>8883215
>>8883222
Wait ignore that, neutrinos don't have charge

>>8883215
OH I get it now! Charge conservation lol :) Thank you.

>>8883223
An important trick for understanding Feynman diagrams is that all conserved quantities must also be conserved in every vertex. In particular, electric charge is conserved in every vertex. Incoming electric charge = outgoing electric charge.

Since, indeed, neutrinos are neutral, it's clear that in the first diagram the W- must be going *out* of the inferior vertex and going *into* the superior vertex, while it's the other way around in the 2nd diagram. Thus, electric charge conservation automatically lets you know how the temporal succesion of events I wrote before.

I hope I'm being more or less clear.

>>8883224
Yes :)

>>8883231
Yes that was very clear, thank you. I forgot neutrinos are neutral and that the boson is transmitting charge :)

I'm doing past papers in preperation for my last physics exam tomorrow (second year). It's on electricity and magnetism, nuclear and particle physics, optics and classical and quantum waves. Each paper I do makes me realise how much arbitrary bullshit information there was in nuclear physics that I've forgotten :s

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