Spin and Polarization in High-Energy Hadron-Hadron and Lepton-Hadron Scattering

TL;DR

This paper reviews the role of spin in high-energy scattering, emphasizing soft, non-perturbative effects, and discusses models based on Regge theory and the pomeron, proposing a universal pomeron concept.

Contribution

It introduces a unified view of the pomeron in high-energy scattering, integrating soft and hard regimes through Regge theory and complex trajectories.

Findings

Regge trajectories are non-linear and complex.

A single, universal pomeron explains various scattering phenomena.

Models can help recover generalized parton distributions.

Abstract

The role of spin degrees of freedom in high-energy hadron-hadron and lepton-hadron scattering is reviewed with emphasis on the dominant role of soft, diffractive, non-perturbative effects. Explicit models based on analyticity and Regge-pole theory, including the pomeron trajectory (gluon exchange in the $t$ channel) are discussed. We argue that there is a single, universal pomeron in Nature, manifest as relatively "soft" or "hard", depending on the kinematics considered. Regge trajectories are non-linear, complex functions, predicting a finite number of resonances. Explicit models and fits are presented that may be used in recovering generalized parton distributions from deeply virtual Compton scattering and electoproduction of vector mesons.