Color–flavor locking (CFL) is a phenomenon that is expected to occur in ultra-high-density strange matter, a form of quark matter. The quarks form Cooper pairs, whose color properties are correlated with their flavor properties in a one-to-one correspondence between three color pairs and three flavor pairs. According to the Standard Model of particle physics, the color-flavor-locked phase is the highest-density phase of three-flavor colored matter.
Color-flavor-locked Cooper pairing
If each quark is represented as , with color index taking values 1, 2, 3 corresponding to red, green, and blue, and flavor index taking values 1, 2, 3 corresponding to up, down, and strange, then the color-flavor-locked pattern of Cooper pairing is 
This means that a Cooper pair of an up quark and a down quark must have colors red and green, and so on. This pairing pattern is special because it leaves a large unbroken[clarification needed] symmetry group.
The CFL phase has several remarkable properties.
- It breaks chiral symmetry.
- It is a superfluid.
- It is an electromagnetic insulator, in which there is a "rotated" photon, containing a small admixture of one of the gluons.
- It has the same symmetries as sufficiently dense hyperonic matter.
There are several variants of the CFL phase, representing distortions of the pairing structure in response to external stresses such as a difference between the mass of the strange quark and the mass of the up and down quarks.
- M. Alford; K. Rajagopal; T. Schäfer; A. Schmitt (2008). "Color superconductivity in dense quark matter". Reviews of Modern Physics. 80 (4): 1455. Bibcode:2008RvMP...80.1455A. arXiv: . doi:10.1103/RevModPhys.80.1455.
- M. Alford; K. Rajagopal; F. Wilczek (1998). "QCD at Finite Baryon Density: Nucleon Droplets and Color Superconductivity". Physics Letters B. 422: 247. Bibcode:1998PhLB..422..247A. arXiv: . doi:10.1016/S0370-2693(98)00051-3.