Accessing baryon-antibaryon generalized distribution amplitudes in $e^{\pm} γ\to e^{\pm} B \bar{B} $

TL;DR

This study develops a collinear-factorization framework to access baryon–antibaryon generalized distribution amplitudes (GDAs) via the exclusive process $e^{\pm}\gamma \to e^{\pm} B \bar B$, including spin correlations. Baryon GDAs are defined at twist-2 and related to timelike Compton form factors, which in turn connect to timelike electromagnetic form factors; the formalism accounts for both the GDA (two-photon) and bremsstrahlung subprocesses and their interference, with polarization observables enabling access to the imaginary parts of GDA-related amplitudes. Numerical estimates for $e^- \gamma \to e^- p \bar p$ using a simple GDA model and proton EM FFs indicate a kinematic region, accessible at Belle II, where the GDA contribution can dominate, and where lepton-charge and single-spin asymmetries provide practical handles for an initial extraction of baryon GDAs. The work thus extends hadron tomography by offering a pathway to measure baryon GDAs, including unstable baryons, and complements existing GPD programs with new spin-sensitive observables and experimental opportunities.

Abstract

$γ^* γ\to B \bar{B}$ is the golden process to access chiral-even di-baryon generalized distribution amplitudes (GDAs) as deeply virtual Compton scattering has proven to be for the generalized parton distributions. In the framework of colinear QCD factorization where the leading twist amplitude is the convolution of GDAs and a perturbatively calculable coefficient function, we study the scattering amplitude for the baryonic channels where $B$ is a spin $1/2$ baryon such as the nucleon or hyperon $Λ, Σ$. Taking into account the interfering QED amplitude, we calculate the cross section of the $e^\pm γ\to e^\pm B \bar B$ process that can be experimentally studied in $e^- e^+$ as well as in electron-ion facilities. We explore both the final state polarization summed case and the polarization dependent effects. Numerical estimates are presented for $e^- γ\to e^- p \bar{p}$, using motivated models for GDAs. Our results show that a first extraction of baryon-antibaryon GDAs from experimental measurements is feasible at Belle II.

Figures (7)

• Figure 1: The center-of-mass frame of the baryon-antibaryon pair for the process $e^{\pm}\gamma \to e^{\pm} B \bar{B}$.
• Figure 2: The Feynman diagram for the QCD subprocess (a) $\gamma^{\ast} \gamma \to B \bar{B}$; the crossed diagram is obtained by interchanging the photon-quark vertices.
• Figure 3: The Feynman diagram for the bremsstrahlung subprocess (b) where the photon-baryon-antibaryon coupling is described by the timelike baryon EM FFs, the crossed diagram is obtained by interchanging the photon-lepton vertices.
• Figure 4: The differential cross section of the bremsstrahlung process in $e^- \gamma \to e^- p \bar{p}$.
• Figure 5: