The electric dipole form factor of the nucleon stemming from the QCD θˉ term is calculated in chiral perturbation theory in leading order. To this order, the form factor originates from the pion cloud. Its momentum-dependence is proportional to a non-derivative time-reversal-violating pion-nucleon coupling, and the scale for momentum variation--appearing, in particular, in the radius of the form factor--is the pion mass.
@article{arxiv.nucl-th/0508012,
title = {The electric dipole form factor of the nucleon},
author = {W. H. Hockings and U. van Kolck},
journal= {arXiv preprint arXiv:nucl-th/0508012},
year = {2009}
}