The title of this section would lead one to believe that we know the low-energy theory of QCD, when in reality we do not. We know that the hadrons that are cataloged in the particle data book[3] are the asymptotic states of the theory, either mesons or baryons, but in addition all nuclei are also the asymptotic states of QCD. If we could solve QCD we would know the two-body, three-body and n-body interactions, and with these interactions proceed to compute the ground state energy, spin and quadrupole moment of all nuclei. We are not even at the stage where we can use lattice gauge theory to make any predictions at all for systems involving more than one nucleon.
All this sounds very daunting, yet there are still more lectures in this
course.
For many years, even before the discovery of QCD, nuclear physics was able to
make predictions, and allow understanding of phenomena at a relatively good
level of precision.
If one is interested in observable of a system where the typical, or maximum
momentum
of the system is much less than
,
which corresponds to about
,
the wavelength of all interacting particles is so large that
they cannot resolve the composite structure of each other.
What this means is that at low-energies, we need only know ``global''
properties of the protons and neutrons, like their mass, magnetic moments,
charge radii, etc, and this will be sufficient to do a reasonable job in
computing
many observables to a given precision.
We do not need to know anything about their structure in terms of quarks
and gluons, if we are willing to fit some parameters,
as opposed to predict everything.
However, this approach is still predictive.
Clearly, it will not be sufficient to compute quantities to arbitrary
precision, but as we will see, one can do pretty well.
Therefore, we are going to study nucleons and their interactions with
themselves and other hadrons.
This can be done with no knowledge of QCD.
However, we must keep in mind that our low-energy theory (picture) will fail
for momentum transfers
or so.