HISQ

HISQ, or Highly Improved Staggered Quark, is a lattice fermion action used in lattice quantum chromodynamics to simulate quarks on a discretized space-time lattice. It is an advancement of staggered fermions designed to suppress taste-symmetry violations and reduce discretization errors, enabling more accurate simulations of light and charm quarks. The action was developed by the MILC collaboration and collaborators as part of the evolution from ASQTAD improvements, and has become a standard choice for high-precision QCD calculations.

Technical features include two levels of link smearing (Fat7) with reunitarization, followed by an additional smearing step, and an improved Naik term tuned to reduce O(a^2) errors for heavy quarks. The smearing and projection of links decrease coupling to high-momentum modes and reduce taste-symmetry breaking among the staggered fermion tastes. The formulation uses the fourth-root trick to reduce the four tastes per flavor to a single physical flavor, a common practice in staggered lattice QCD.

Impact and applications: HISQ reduces discretization errors compared with earlier staggered actions and is particularly effective for charm physics. It has been used to determine quark masses, decay constants such as f_K and f_D, and CKM matrix elements, among other quantities. Large ensembles of gauge configurations generated with HISQ quarks underpin many published results in lattice QCD and are used by collaborations including MILC and HPQCD.

Related topics include staggered fermions, taste-symmetry breaking, ASQTAD, and lattice QCD.