SheikholeslamiWohlert

Sheikholeslami-Wohlert refers to an improvement term added to the Wilson fermion action in lattice quantum chromodynamics (QCD). Introduced by Alireza Sheikholeslami and R. Wohlert in 1985, it is commonly known as the clover term because the lattice field-strength tensor is approximated by a four-plaquette clover-leaf pattern around each lattice site. The term was proposed to reduce discretization artifacts that arise from the Wilson formulation.

The clover term adds a dimension-five operator to the Wilson action, proportional to the lattice field-strength tensor F_hat_mu nu contracted with the spin matrix sigma_mu nu. In practical form, the improvement term is written with a dimensionless coefficient c_SW, which can be tuned. The discretized F_hat_mu nu is constructed from the surrounding plaquettes in a clover pattern. The resulting action, often called the Wilson-clover action, reduces leading lattice spacing errors from O(a) to O(a^2) for on-shell quantities, improving the approach to the continuum limit.

The coefficient c_SW can be set at tree level (c_SW = 1 in the simplest case) or determined nonperturbatively to cancel remaining O(a) effects across a range of lattice spacings and gauge actions. Nonperturbative determinations have been carried out by collaborations aiming to optimize improvement for precise QCD calculations.

The Sheikholeslami-Wohlert term is a standard component in modern lattice QCD simulations employing Wilson-type fermions and is part of the broader Symanzik improvement program. It enables more accurate studies of hadron spectra, matrix elements, and other nonperturbative QCD observables.