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While electrons are point particles in the sense that their position eigenstates are (as far as we know) $\delta$-like. Photons can't be said to be point particles in this sense, as you cannot transform to their rest-frame (although they are featureless with respect to small scales as far as we know). The correct way to thing about electrons and photons is ...


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There is optical theorem in quantum field theory and it enables you to study with the decay rates to find cross section. However there are some difficulties to calculate the cross section of explicit decay channel. $$ Im(\Sigma(m^2))=m\Gamma_{tot} $$ So the propagator becomes $$ iG(p^2)=\frac{i}{p^2-m^2+im\Gamma_{tot}} $$ You can jump up to cross section ...


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From an energy perspective, a free neutron sees a nucleus as a three-dimensional square well with a depth of a 5--10 MeV. The presence or absence of milli-eV thermal oscillations or eV-scale molecular bonds may change the details of the shape of that potential well, but in general the change is much less important than the uncertainty in the neutron's energy ...



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