particles, i.e., discrete eigenstates of the Hamiltonian. Nucleons are much balder than black holes, for they have no hair, not only classically but even in quantum theory. Furthermore, there is no freedom to specify their mass and angular momentum arbitrarily. Also protons and neutrons can be well modeled in QCD Lite$^{\rm TM}$ (the idealization of QCD using only massless $u$ and $d$ quarks). In that approximation, they are entirely conceptually determined structures, allowing no continuous parameters whatsoever!

And they can be accessed experimentally. Indeed, the answer to many “practical” questions relies on understanding their structure. For example, structure functions are vital to the interpretation of accelerator experiments, since we use nucleons as projectiles. Notably, the primary mechanism for Higgs particle production at the LHC will be gluon fusion, $gg \rightarrow h$ through a top-quark loop. To predict the rates, and to check for possible deviations from standard model expectations for the $h$ coupling, we obviously need to know the gluon distribution in the proton accurately.

Another sort of canonical structure in QCD is the flux tube between heavy quark sources. There are very interesting questions concerning its spatial structure and modes of excitation, and specifically how accurately it can be 