Inverse Beta Decay: Why is it Forbidden for a Free Proton?

[earthmark]

Homework Statement

Why is the inverse Beta decay forbidden for a free proton?

Homework Equations

p -> n + e(+)(positron) + v (neutrino)

The Attempt at a Solution

Is it because, the change in the Q value must be the same for each decay?

 

[vela]

The charge balances out, so it's not a problem with charge conservation. Think energy.