# Charged-current: Why does the neutrino interact with the down-quark?

I'm revising for an exam and looking at a few exercises, one of which starts with

Consider the charged-current interaction between a muon neutrino with one of the valence quarks of the proton.

We did not discuss things like in detail in the course, so I looked at the solution to get a hint. The first sentence in the solution is

At first order, the charged-current process must be $\nu_\mu d\rightarrow \mu^-u$

• Why's that? Why can't the neutrino interact with an up-quark?

• Where do the flavour changes in both sectors come from?

• That means that the quark involved must be changed to another quark with a charge that is 1 unit more positive than it started, so you have to interact with a $-1/3$ quark.