On the universality of the $J=0$ fixed pole contribution in DVCS

Abstract

S. Brodsky, F.J. Llanes-Estrada, and A. Szczepaniak formulated the $J=0$ fixed pole universality hypothesis for (deeply) virtual Compton scattering. We show that in the Bjorken limit this hypothesis is equivalent to the validity of the inverse moment sum rule for the $D$-term form factor. However, any supplementary $D$-term added to a generalized parton distribution (GPD) results in an additional $J=0$ fixed pole contribution that violates universality. Unfortunately, one can not provide any reliable theoretical argument excluding the existence of such supplementary $D$-term. Moreover, the violation of $J=0$ fixed pole universality was revealed in field theoretical GPD models. Therefore, $J=0$ fixed pole universality hypothesis remains just an external assumption and probably will never be proven theoretically.

Comments: Talk 6 pages, 1 figure, contribution to the 6th International Conference on Physics Opportunities at an EleCtron-Ion Collider (POETIC 2015)