Pion Electromagnetic Form Factor in Virtuality Distribution Formalism

TL;DR

This paper applies the Virtuality Distribution Amplitudes formalism to study the pion electromagnetic form factor, analyzing soft transverse momentum effects, form factor data, and the transition to perturbative QCD predictions at high momentum transfer.

Contribution

It introduces the VDA approach and pion TMDA, models soft TMDAs, and assesses the onset of pQCD behavior and the relevance of the Feynman mechanism for the pion form factor.

Findings

Late transition to pQCD asymptotics at Q^2 > 20 GeV^2

No sensitivity to pion distribution amplitude shape below 10 GeV^2

One-gluon exchange contribution is much smaller than experimental data

Abstract

We discuss two applications of the {\it Virtuality Distribution Amplitudes} (VDA) formalism developed in our recent papers. We start with an overview of the main properties of the pion distribution amplitude emphasizing the quantitative measures of its width, and possibility to access them through the pion transition form factor studies. We formulate the basic concepts of the VDA approach and introduce the pion transverse momentum distribution amplitude (TMDA) which plays, in a covariant Lagrangian formulation, a role similar to that of the pion wave function in the 3-dimensional Hamiltonian light-front approach. We propose simple factorized models for soft TMDAs, and use them to describe existing data on the pion transition form factor, thus fixing the scale determining the size of the transverse-momentum effects. Finally, we apply the VDA approach to the one-gluon exchange contribution for the pion electromagnetic form factor. We observe a very late $Q^2 \gtrsim 20$ GeV$^2$ onset of transition to the asymptotic pQCD predictions and show that in the $Q^2 \lesssim 10$ GeV$^2$ region there is essentially no sensitivity to the shape of the pion distribution amplitude. Furthermore, the magnitude of the one-gluon exchange contribution in this region is estimated to be an order of magnitude below the Jefferson Lab data, thus leaving the Feynman mechanism as the only one relevant to the pion electromagnetic form factor behavior for accessible $Q^2$.