Two-Photon Processes at Intermediate Energies

TL;DR

This paper discusses two-photon processes at intermediate energies, emphasizing their role in testing QCD, understanding hadron distribution amplitudes, and exploring various phenomena at low and high energies in photon-photon and electron-positron collisions.

Contribution

It highlights the importance of two-photon physics at intermediate energies for probing QCD, hadron structures, and resonance phenomena, especially in double-tagged reactions with polarization studies.

Findings

Potential measurements of hadron distribution amplitudes and skewed parton distributions.

Insights into the transition from low-energy effective theories to perturbative QCD.

Relevance of two-photon processes for muon g-2 and resonance studies.

Abstract

Exclusive hadron production processes in photon-photon collisions provide important tests of QCD at the amplitude level, particularly as measures of hadron distribution amplitudes and skewed parton distributions. The determination of the shape and normalization of the distribution amplitudes has become particularly important in view of their importance in the analysis of exclusive semi-leptonic and two-body hadronic B-decays. Interesting two-photon physics, including doubly-tagged $\gamma^* \gamma^*$ reactions, will be accessible at low energy, high luminosity $e^+ e^-$ colliders, including measurements of channels important in the light-by-light contribution to the muon $g$--2 and the study of the transition between threshold production controlled by low-energy effective chiral theories and the domain where leading-twist perturbative QCD becomes applicable. The threshold regime of hadron production in photon-photon and $e^+ e^-$ annihilation, where hadrons are formed at small relative velocity, is particularly interesting as a test of low energy theorems, soliton models, and new types of resonance production. Such studies will be particularly valuable in double-tagged reactions where polarization correlations, as well as the photon virtuality dependence, can be studied.