In the standard model, the proton is made up of three quarks. The individual quarks are spin-1/2 particles. As elements of a QFT, the quarks are represented by propagators that satisfy the Dirac equation. What’s a bit odd is that the proton is also a spin-1/2 particle and is also represented by that same Dirac equation propagator.
Snuarks have a similar attribute. Three snuarks are (more or less) spin-1/2 particles and are represented by Dirac propagators. In the qubit representation, where we ignore spatial dependencies, their propagators are Pauli projection operators (i.e. density matrices). Somehow these three qubit objects combine to make a lepton or quark, which is also a qubit object whose (virtual) propagator is again a Pauli projection operator. In Feynman diagrams, what we are looking for is something like this:
What we would like is a way of combining the propagators of the three snuarks into a single object that can fill the requirement of representing the propagator of the quark or lepton they combine into. This is the “bound state propagator problem” which we will return to more completely when we analyze the quarks. For the moment, let us consider the problem of how a bound state of three snuarks can model the lepton mass interaction that converts left handed leptons to right handed leptons and vice versa.