My textbooks lists the exchange particle for a neutron-neutrino interaction as being the W- boson. Is this the only option, or can it also be a W+ boson? Nothing jumps out at me that would suggest it being impossible. In the scenario that it can only be a W- boson though, why?
And as a followup question, could anyone explain what exactly the role of the W boson is in this interaction? In beta decay, for example, it's fairly intuitive, as it's just the one particle (a neutron) that transforms into other particles, and so it's easy to visualise the W- boson as simply being "emitted" by the neutron. But with a neutron-neutrino interaction, it's hard to conceptualise what exactly is happening. Does the neutron "give" a W boson to the neutrino, or what?
Answer
In the reaction that you ask about the exchange boson is space-like (meaning that for that particle $E^2 - (pc)^2$ takes on a negative value.
In cases like that there is no unique way to decide if you have a $W^-$ going from the nucleon to the lepton or a $W^+$ going from the lepton to the nucleon, and the drawing is usually annotated only with a $W$.
Short answer: yes, both types are allowed, but you can't actually tell what type you had.
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