  In this thesis, which is in the field of few-body systems, I am studying
low-energy electro-weak interactions in light nuclear systems whose number of
nucleons, $A$, is smaller than 4 ({\it i.e.,} $d$, $^3$H, $^3$He), where the
main purpose of this work is to calculate the proton-proton ($pp$) fusion rate.

Nuclear Theory