A study of quasi-parton distribution functions in the diquark spectator model

To facilitate lattice QCD calculations of nucleon structute, a set of quasi-parton distributions were recently introduced. These quasi-PDFs were shown to reduce to standard PDFs when the nucleon is boosted to high energies, P_z →∞. Since taking such limit is not feasible in lattice simulations, it is essential to provide guidance for what values of P_z the quasi-PDFs are good approximations of standard PDFs. Within the framework of the spectator diquark model, we evaluate both the up and down quarks' quasi-PDFs and standard PDFs for all leading-Twist distributions (unpolarized distribution fP₁, helicity distribution P₁, and transversity distribution hP₁). We find that, for intermediate parton momentum fractions x, quasi-PDFs are good approximations to standard PDFs (within 20-30%) when P_z & 1:5-2 GeV. On the other hand, for large x 1 much larger P_z > 4 GeV is necessary to obtain a satisfactory agreement between the two sets. We further find that the Soffer positivity bound does not hold in general for quasi-PDFs.