Subtracted Dispersion Relations for Virtual Compton Scattering off the Proton

Abstract

We present a once-subtracted dispersion relation (DR) formalism for the virtual Compton scattering (VCS) process from threshold up to the $Δ(1232)$ energy region. The formalism aims at extracting the nucleon's electric and magnetic generalized polarizabilities from the $e^- p \to e^- γp$ process, in view of the precision goals of the present and near future experiments at Jefferson Lab. The present work improves upon the existing unsubtracted DR formalism in several ways. The required $s$- and $t$-channel discontinuities in the subtracted dispersion integrals are obtained in a largely data-driven manner in this energy region. The $s$-channel dispersive integrals are reconstructed from $γ^\ast p \to πN \to γp$ using pion photo- and electro-production data, while the $t$-channel dispersion integrals are evaluated from $γ^\ast γ\to ππ\to N \bar N$ using recent dispersive analyses of both the $γ^\ast γ\to ππ$ and $ππ\to N \bar N$ processes. We compare our results to VCS data and show the sensitivity of the observables to the nucleon's scalar generalized polarizabilities, which enter the present formalism as subtraction constants.

Figures (21)

• Figure 1: The virtual Compton scattering process on a nucleon.
• Figure 2: The $e N \to e \gamma N$ process: Bethe-Heitler process (upper diagrams) and VCS process (lower diagram).
• Figure 3: The $s$-channel $\pi N$ cut contribution in the dispersive formalism for VCS on the nucleon.
• Figure 4: The $t$-channel $\pi \pi$ right-hand cut contribution in the subtracted dispersive formalism for VCS on the nucleon.
• Figure 5: The $t$-channel $\Delta(1232)$-resonance exchange left-hand cut contribution in the subtracted dispersive formalism for VCS on the nucleon.