Chiral dynamics and partonic structure at large transverse distances
Abstract
We study largedistance contributions to the nucleon’s parton densities in the transverse coordinate (impact parameter) representation based on generalized parton distributions (GPDs). Chiral dynamics generates a distinct component of the partonic structure, located at momentum fractions x≲M_{π}/M_{N} and transverse distances b̃1/M_{π}. We calculate this component using phenomenological pion exchange with a physical lower limit in b (the transverse “core” radius estimated from the nucleon’s axial form factor, R_{core}=0.55fm) and demonstrate its universal character. This formulation preserves the basic picture of the “pion cloud” model of the nucleon’s sea quark distributions, while restricting its application to the region actually governed by chiral dynamics. It is found that (a) the largedistance component accounts for only ̃1/3 of the measured antiquark flavor asymmetry d̄ū at x̃0.1; (b) the strange sea quarks s and s̄ are significantly more localized than the light antiquark sea; (c) the nucleon’s singlet quark size for x<0.1 is larger than its gluonic size, ⟨b^{2}⟩_{q+q̄}>⟨b^{2}⟩_{g}, as suggested by the tslopes of deeplyvirtual Compton scattering and exclusive J/ψ production measured at HERA and FNAL. We show that our approach reproduces the general N_{c}scaling of parton densities in QCD, thanks to the degeneracy of N and ∆ intermediate states in the largeN_{c} limit. We also comment on the role of pionic configurations at large longitudinal distances and the limits of their applicability at small x.
 Publication:

Physical Review D
 Pub Date:
 December 2009
 DOI:
 10.1103/PhysRevD.80.114029
 arXiv:
 arXiv:0906.3267
 Bibcode:
 2009PhRvD..80k4029S
 Keywords:

 13.60.Hb;
 11.15.Pg;
 12.39.Fe;
 14.20.Dh;
 Total and inclusive cross sections;
 Expansions for large numbers of components;
 Chiral Lagrangians;
 Protons and neutrons;
 High Energy Physics  Phenomenology
 EPrint:
 24 pages, 11 figures. Minor corrections in discussion of timeordered formulation