Regge description of spin-spin asymmetry in photon diffractive dissociation

TL;DR

This paper investigates the potential to extract the gluon helicity distribution from spin asymmetry measurements in photon-proton diffractive scattering, identifying kinematic regions where perturbative QCD applies and soft process contributions are minimal.

Contribution

It demonstrates that at high energies and specific kinematic conditions, reggeon exchange contributions to spin asymmetry are negligible, enabling reliable pQCD analysis of experimental data.

Findings

Reggeon exchange contributions to $A_{LL}$ are below 10^{-4} in specified kinematics.

pQCD predictions are applicable for $s_{\gamma p} \\geq 10^3 GeV^2$, $M_X \\leq 10 GeV/c^2$, and $Q^2 \\geq 4 (GeV/c)^2.

Soft process contributions are suppressed in the identified kinematic region.

Abstract

We explore the possibility whether the gluon helicity distribution $\Delta G(x)$ can be extracted from a comparison of experimental data on the longitudinal spin-spin asymmetry $A_{LL}$ in $\gamma p$ diffractive deep inelastic scattering with calculations performed within the framework of perturbative QCD (pQCD). The data could be obtained at the future HERA collider in scattering of polarized electrons/positrons off polarized protons. In this paper we look for such kinematical regions where contributions to $A_{LL}$ from soft processes (reggeon exchanges) are suppressed to guarantee an applicability of pQCD. It is shown that for the square of the center-of-mass energy $s_{\gamma p} \geq 10^3 GeV^2$, the hadronic diffractive mass $M_X \leq 10 GeV/c^2$, the momentum transferred to the proton $\Delta_T\leq 0.5 GeV/c$, and $Q^2\geq4 (GeV/c)^2$ the longitudinal spin-spin asymmetry due to reggeon exchanges is less than 10^{-4}. This value is presumably lower than the asymmetry which can be measured with modern experimental technique. This means that the pQCD prediction can be reliably compared with data in this kinematical region.