Obtaining hadronic two-point functions is a central step in spectroscopy calculations in lattice QCD. This requires solving the Dirac equation repeatedly, which is computationally demanding. The distillation method addresses this difficulty by using the lowest eigenvectors of the spatial Laplacian to construct a subspace in which the Dirac operator can be fully inverted. This approach is efficient for nonlocal operators such as meson-meson and baryon-baryon operators. However, local multiquark operators with four or more (anti) quarks are computationally expensive in this framework: the cost of contractions scales with a high power of the number of Laplacian eigenvectors. We present a position-space sampling method within distillation that reduces this cost scaling by performing the momentum projection only over sparse grids rather than the full spatial lattice. We demonstrate the efficiency of this unbiased estimator for single-meson, single-baryon and local tetraquark operators. Using Wilson-clover fermions at the SU (3) -flavour-symmetric point, we apply this method to study the importance of local tetraquark operators for the finite-volume T₂₂ (3875) ^+ spectrum. To this end, we extend a large basis of bilocal DD^* and D^*D^* scattering operators by including local tetraquark operators. The inclusion of local operators leads to significant shifts in several energy levels. Finally, we show the effect of these shifts on the DD^* scattering phase shift from a single-channel s-wave Lüscher analysis.
Stump et al. (Wed,) studied this question.