KogutSusskind

Kogut-Susskind fermions, commonly called staggered fermions, are a formulation of lattice fermions in quantum chromodynamics (QCD) that aims to address the fermion doubling problem while preserving a remnant of chiral symmetry. They were introduced by Kenneth G. Kogut and Leonard Susskind in 1975.

Staggered fermions distribute the components of a Dirac spinor over the sites of a hypercubic lattice, reducing the 16 naive fermion doublers in four dimensions to four tastes per flavor. In practice one works with one-component Grassmann fields and uses a set of phase factors along with gauge links to maintain gauge invariance.

The formulation preserves a remnant chiral symmetry at finite lattice spacing, which helps control additive mass renormalization and improves the treatment of light quarks. However, the four tastes are not exactly degenerate at nonzero lattice spacing, leading to taste-symmetry breaking that vanishes in the continuum limit.

To represent a physical number of quark flavors, simulations often apply a rooting procedure that reduces the effective number of tastes. This approach has been widely used because of its computational efficiency, though it has sparked debate about locality and universality at finite lattice spacing.

Efforts to reduce taste breaking have produced improved staggered actions, such as Asqtad and HISQ, increasing the accuracy of hadron spectroscopy and thermodynamic studies. Staggered fermions remain a popular choice for large-scale lattice QCD calculations when computational cost is a major concern, while other formulations address different theoretical priorities.