Feynman Diagrams: Weak Interaction Help

[Monarch]

Hi all

Just a basic question really; I'm at university and have been tutoring someone through a couple of A level physics modules, and I'm covering some basic particle physics at the moment, which is all alright, but I've come to an issue with Feynman diagrams.

Basically, for interactions with either the W or Z bosons, how do you know which one to use? I know these bosons correspond to weak interactions, but is there any kind of methodology for working out which one? For the purpose of the exam back when I did it I just committed the few interactions we needed to know to memory, and this isn't something I've really met at uni yet!

Hope that makes sense

Cheers

 

[Chopin]

The W bosons carry charge, and the Z boson does not, so you should be able to use conservation of charge to determine which boson takes part in any given process.

 

[Monarch]

Thanks for the reply!

That makes sense but I'm struggling to work it out in my head properly. The main ones required are beta minus and beta plus decays. In beta minus, with this diagram
http://www.barnsley.org/penistone-grammar/science/Images/feynman_1.jpg

the exchange particle is a W-, so where does the charge conservation come from (other than the overall conservation with the proton and electron cancelling each other out)?

Apologies if this is basic, it was taught very badly at my school and it's not something I've really had to do since.

 

[Chopin]

Charge conservation happens at every vertex. You have a neutron coming in on the left, which is charge 0. A proton exits, which is of charge +1, so the boson must have charge -1 in order for things to balance. So you know it must be a W-. It then decays into an electron (charge -1), and an electron antineutrino (charge 0), meaning that charge is conserved at that vertex as well.

It wouldn't make sense for the proton-neutron vertex to give off a W+ or a Z, because then the total charge wouldn't be conserved. Similarly, it wouldn't make sense for the W- to decay into a positron + electron neutrino, for the same reason.