Universality of the transverse-photon soft function in twist-three DVCS

Determine whether the nonperturbative soft function entering the next-to-leading-power DVCS amplitude for a transversely polarized initial virtual photon is universal.

Background

At next-to-leading power, longitudinal DVCS amplitudes factorize onto twist-three GPDs, but transverse-photon amplitudes involve additional hard, jet, and soft functions rather than only GPDs. The soft function arises from ultrasoft modes and dynamical chiral-symmetry breaking, and its universality is necessary to establish whether the corresponding nonperturbative contribution can be transferred consistently between processes or kinematic settings.

References

It should be emphasized, however, that the universality of the soft function ${\cal S}$ remains unknown.

— Generalized parton distributions: Theory meets experiment  (2609.19544 - Guo et al., 17 Sep 2026) in Section 3.1.4, “Factorization of genuine twist-three correction”