The usual approximation of simply ignoring the fermion determinant fails dramatically in this case. Instead of resembling nuclear matter, the lowest density non-vacuum phase is dominated by unphysical hadrons which have a mass equal to that of the pion but which carry baryon number. Because of the failure of Monte Carlo simulations, our understanding of the phase diagram derives from QCD-based models and qualitative arguments, rather than from controlled calculations in the full theory. These models and many others support the long-held expectation that above some chemical potential, hadronic matter gives way to a phase in which chiral symmetry is restored. It was realized some time ago that this phase must be a color superconductor, if there is any channel in which the interaction between pairs of quarks is attractive.
Many questions remain, however.
The combination of fascinating new proposed phenomena and a much clearer understanding of why and how standard lattice techniques fail make this an opportune time for a 3-month program, in which we propose to mount a concerted theoretical assault on these problems. A brief workshop in Bielefeld this year was very beneficial, and we think that all there would agree that a longer program is called for. Short workshops are planned for Brookhaven and Heidelberg during the coming year. The INT seems the perfect venue for a longer program. One part of our effort would be further discussion of the problems with lattice simulations, and possible directions toward solutions.