I'm trying to find the experimental evidence for quantum tunneling during a fusion reaction. Take the simple D + D -> T + P reaction. I know that the energies required to make this happen are much lower in actual experiment than what would be predicted by Coulomb's model of the electrical forces generated by the two +1 charges of the respective deuterons. That is to say we see fusion at energies much lower than that required to breach the Coulomb barrier. Is this a case where we say: "Well, the fusion takes place at lower energies, therefore it must be tunneling?" Or is there some other independent evidence for the tunneling? How do we know, for example, that Coulomb's law does not break down at very small distances as an alternative explanation?