Two-photon exchange corrections to the pion form factor

TL;DR

This paper calculates two-photon exchange corrections to the pion's electromagnetic form factor, revealing small effects at low momentum transfer but significant corrections at higher Q^2 and backward angles, emphasizing the importance of hadron structure.

Contribution

It provides the first detailed computation of two-photon exchange effects on the pion form factor including finite size effects, extending beyond the soft-photon approximation.

Findings

Corrections are less than 1% for Q^2 < 0.1 GeV^2.

Corrections increase to several percent for Q^2 > 1 GeV^2.

Significant effects at backward angles for high Q^2.

Abstract

We compute two-photon exchange corrections to the electromagnetic form factor of the pion, taking into account the finite size of the pion. Compared to the soft-photon approximation for the infrared divergent contribution which neglects hadron structure effects, the corrections are found to be < 1% for small Q^2 (Q^2 < 0.1 GeV^2), but increase to several percent for Q^2 > 1 GeV^2 at extreme backward angles.