I just read something about the creation of deuteron in the first step of the pp cycle. Given that you have the reaction: p + p -> d + e^+ + v_e, where e^+ is a positron and v_e is an electron neutrino. Since there is a neutrino present, it is a weak interaction. In addition, as the interaction is weak (and at low energies?) the effective coupling is small. My question is, do weak interactions only proceed at low energies? I have read about 'low energy limits' and how this means only virtual W's can be made. How is it then that this process can proceed at roughly 1.5 x 10^7 K in the Sun's core?