Gapless Color-Flavor-Locked Quark Matter

In neutral cold quark matter that is so dense that the strange quark mass M_s is unimportant, all three quark flavors pair in a color-flavor locked (CFL) pattern, and all nine fermionic quasiparticles have a gap Δ (or 2Δ). We argue that, as the density decreases (or M_s increases), there is a quantum phase transition (at M_s²/μ≈2Δ) to a new “gapless CFL phase” in which only seven quasiparticles have a gap. There is still an unbroken U(1)_Q˜ gluon/photon, but, unlike CFL, gapless CFL is a Q˜ conductor with gapless (charged) quasiquarks and a nonzero electron density at zero temperature, so its low energy effective theory and astrophysical properties are qualitatively new. At the transition, the dispersion relations of both gapless quasiparticles are quadratic, but for larger M_s²/μ, one becomes conventionally linear while the other remains quadratic, up to tiny corrections.