change. For example, a question I find fascinating is simply: Why does the naive quark model work so well? In thinking about this question, it would be very useful to identify QCD-variants where the naive model begins to fail! (Perhaps simply QCD with many light quark flavors?)

Let me just mention one further potential application, to clear up a long-standing debate whose origins are in the prehistory of QCD but that remains contentious even now. This is the question of glueballs. When a particle is observed experimentally, it does not come labeled “quark-antiquark” or “glueball”, nor is there any strict objective criterion to distinguish them. The whole distinction is tied up with the naive quark model, which has no firm basis in quantum field theory, and ignores the inevitability of mixing. Nevertheless there is a simple objective question correlated with this classification: If we vary quark masses, how does the mass of the particle respond?

Challenges
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### Reduce Lattice Degrees of Freedom

Full-scale QCD simulations tend to be unwieldy, inflexible, and opaque. One would like to have procedures that fall somewhere between full-scale simulation of the microscopic theory and rough semiphenomenological modeling.

There are several suggestions regarding what are 