Moments of Nucleon Light Cone Quark Distributions Calculated in Full Lattice QCD

TL;DR

This paper computes moments of nucleon quark distributions using unquenched lattice QCD, finding minimal sea quark effects and emphasizing the importance of instantons, but notes the need for chiral extrapolation to match experimental results.

Contribution

It presents the first unquenched lattice QCD calculations of nucleon distribution moments, highlighting the role of instantons and the necessity of chiral extrapolation.

Findings

Sea quark contributions are small at accessible quark masses.

Instantons play a dominant role in nucleon structure.

Chiral extrapolation including pion cloud effects is needed for experimental agreement.

Abstract

Moments of the quark density, helicity, and transversity distributions are calculated in unquenched lattice QCD. Calculations of proton matrix elements of operators corresponding to these moments through the operator product expansion have been performed on $16^3 \times 32$ lattices for Wilson fermions at $\beta = 5.6$ using configurations from the SESAM collaboration and at $\beta = 5.5$ using configurations from SCRI. One-loop perturbative renormalization corrections are included. At quark masses accessible in present calculations, there is no statistically significant difference between quenched and full QCD results, indicating that the contributions of quark-antiquark excitations from the Dirac Sea are small. Close agreement between calculations with cooled configurations containing essentially only instantons and the full gluon configurations indicates that quark zero modes associated with instantons play a dominant role. Naive linear extrapolation of the full QCD calculation to the physical pion mass yields results inconsistent with experiment. Extrapolation to the chiral limit including the physics of the pion cloud can resolve this discrepancy and the requirements for a definitive chiral extrapolation are described.