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, Pz →∞. Since taking such limit is not feasible in lattice simulations, it is essential to provide guidance for what values of Pz 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 Pz & 1:5-2 GeV. On the other hand, for large x 1 much larger Pz > 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.
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