Neutron-Neutrino collisions and the W bosons

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Hi, I have my AS physics exam tomorrow and the only thing on the syllabus I'm unsure of is the reaction between a neutron and a neutrino and also the reaction between an antineutrino and a proton.

A Ve is an antineutrino, I can't work the overline codes :/

I've seen various feynman diagrams showing different exchange particles for the reaction below:
Ve + n > e- + p

In my notes, the feynman diagram shows a W+ boson being given off by the Ve, but I've also seen diagrams with a W- being given off by the neutron. What does this mean? Can either happen? If I were asked what exchanged particle mediated this collision and answered W+ or W-, would I be right?

The same problem with an anti-neutrino proton collision:

Ve + p > e+ + n

Again, I've seen a W- going from the Ve and I've seen a W+ going from the proton...

Why is this? Which is right? Any help would be greatly appreciated, thanks in advance :)
 
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It is a virtual W boson. It doesn't have a proper "time direction" or well-defined charge type. If you draw it from the neutron/proton to the electron/neutrino side it is a W-, if you draw it in the other direction it is a W+, in both cases from charge conservation. This is purely a graphical choice, it has no impact on the physics.
 

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