What is the threshold energy for a neutrino reaction with a proton?

[jabers]

Im a little confused about how to calculate threshold energies. The threshold energy is when the energy is as low as it can possibly be and still do what it has to do, yes? So when a neutrino is detected its speed is refracted by water and so it slows down. If at the threshold the neutrino speed was c the refracted speed would be c'. How would I go about finding the threshold energy from this speed? I tried doing this by thinking about how the neutrino would scatter an electron but is there a way to find the threshold energy without doing this?

 

[fzero]

Are you supposed to be looking at neutrino refraction or a reaction \nu + p \rightarrow n + e^+? If it's the latter, you should determine the threshold energy by examining conservation of energy and momentum. Since the rest energy of the particles in the final state are larger than those in the initial state, there is a threshold energy for the neutrino, below which the reaction cannot occur.