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QCD Functional Integrals for Systems with Nonzero Chemical Potential

This paper reviews some recent progress on QCD functional integrals at nonzero chemical potentials. One issue discussed is the use of QCD inequalities for this regime. In particular, the positivity of the integrand of particular Euclidean space functional integrals for two-flavor QCD with degenerate quark masses is used to demonstrate that the free energy per unit volume for QCD with a baryon chemical potential $μ_B$ (and zero isospin chemical potential) is necessarily greater than the free energy with isospin chemical potential $μ_I = \frac{2 μ_B}{N_c} $ (and zero baryon chemical potential). This result may be of use in model finite density systems. A corollary to this result is a rigorous {\it ab initio} bound on the nucleon mass. The second major issue addressed is the so-called ``Silver Blaze'' problem: the fact that at zero temperature and chemical potentials less than some critical value the free energy remains as that of the vacuum. This is puzzling in the context of a functional integral since a chemical potential affects the functional determinant of the Dirac operator and any nonzero $μ$ changes every eigenvalue of the Dirac operator compared to the $μ=0$ value. The isospin Silver Blaze problem is solved through the study of the spectrum of the operator $γ_0 (\dslash + m)$. The status of the baryon Silver Blaze problem is briefly discussed.