Abstract
It would be very useful to find a way of reducing excited-state effects in lattice QCD calculations of nucleon structure that has a low computational cost. We explore the use of hybrid interpolators, which contain a nontrivial gluonic excitation, in a variational basis together with the standard interpolator with tuned smearing width. Using the clover discretization of the field strength tensor, a calculation using a fixed linear combination of standard and hybrid interpolators can be done using the same number of quark propagators as a standard calculation, making this a cost-effective option. We find that such an interpolator, optimized by solving a generalized eigenvalue problem, reduces excited-state contributions in two-point correlators. However, the effect in three-point correlators, which are needed for computing nucleon matrix elements, is mixed: for some matrix elements such as the tensor charge, excited-state effects are suppressed, whereas for others such as the axial charge, they are enhanced. The results illustrate that the variational method is not guaranteed to reduce the net contribution from excited states except in its asymptotic regime, and suggest that it may be important to use a large basis of interpolators capable of isolating all of the relevant low-lying states.
Highlights
One of the most challenging sources of systematic uncertainty faced by lattice QCD calculations of nucleon structure is excited-state contamination: the failure to isolate the ground-state nucleon from the tower of higher-energy states to which the interpolating operator can couple
The full 3 × 3 matrix of two-point correlators was computed in the full-statistics run, allowing for a more detailed analysis
The use of a variational basis comprising a standard interpolating operator and hybrid ones presents the possibility of reducing excited-state contamination in nucleon structure calculations at low computational cost
Summary
One of the most challenging sources of systematic uncertainty faced by lattice QCD calculations of nucleon structure is excited-state contamination: the failure to isolate the ground-state nucleon from the tower of higher-energy states to which the interpolating operator can couple. [15], which used the distillation method to enable the use of interpolators with a variety of local structures including covariant derivatives. In these cases, the variational setup was more computationally expensive than a standard calculation because of the need for additional quark propagators with different smeared sources or (for time-evolved operators) additional source-sink separations.. We present a study of a variational setup that requires the same number of quark propagators as a standard calculation, for a fixed choice of optimized. It should be stressed that this study, along with all previous ones in the nucleon sector, uses only local interpolating operators, which are poor at isolating multiparticle states.
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