QCD at finite isospin density: from pion to quark-antiquark condensation

QCD at finite isospin chemical potential mu_I is studied. This theory has no fermion sign problem and can be simulated on the lattice using present-day techniques. We solve this theory analytically in two limits: low mu_I where chiral perturbation theory is applicable, and asymptotically high mu_I where perturbative QCD is at work. At low isospin density the ground state is a superfluid pion condensate. At very high density it is a Fermi liquid with Cooper pairing. The pairs carry the same quantum numbers as the pions. Motivated by this observation, we put forward a conjecture that the transition from hadron to quark matter is smooth. The conjecture passes several nontrivial tests. Our results imply a nontrivial phase diagram in the space of temperature and chemical potentials of isospin and baryon number. At asymtotically large values of mu_I and small values of baryon chemical potential the ground state is in a phase similar to Fulde-Ferrell-Larkin-Ovchinnikov phase. It is characterized by a spatially modulated superfluid order parameter <ubar*gamma_5*d> and may be the asymptotic limit of the inhomogeneous pion condensation phase advocated by Migdal and others.