Wednesday, September 2, 2015

nuclear physics - Why are protons and neutrons the "right" degrees of freedom of nuclei?


This question may sound stupid but why do we visualize nuclei as composed of a bunch of neutrons and protons? Wouldn't the nucleons be too close together to be viewed as different particles? Isn't the whole nucleus just a complicated low energy state of QCD?




Answer



We can measure the form-factors of bound nucleons. For instance by doing quasi-elastic scattering of a proton out of the nucleus $A(e,e'p)$ at low energy loss (my dissertation work involved this reaction for deuterium, helium, carbon and iron).


The result are quite similar to (but measurably not identical to) the equivalent results on free protons. That similarity make the choice of nucleons as the degrees of freedom a good starting point.


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