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arXiv · 2507.18234

New superconvergence relations for spin and tensor structure functions of $γγ$ fusion

Vladimir Pascalutsa
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Abstract

The Burkhardt--Cottingham sum rule is an exact superconvergence relation for a spin-structure function, derived from general principles of light absorption and scattering, and valid at any momentum transfer $Q^2$. I illustrate how a class of such relations emerges from the Siegert point, an unphysical kinematical point where both the probe and the target are at rest. From light-by-light scattering, new sum rules for $γ^\ast γ^\ast$ fusion are emerging, valid for arbitrary photon virtualities. Regarding the convergence of these relations, there is a simple argument for the suppression of longitudinal photon polarizations at high energy. Among its consequences is the prediction of $σ_L/ σ_T \to 0$ at high energy, for the ratio of unpolarized nucleon photoabsorption cross sections.

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Vladimir Pascalutsa. 2025-10-21. New superconvergence relations for spin and tensor structure functions of $γγ$ fusion. https://arxiv.org/abs/2507.18234

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