Hybrid nature of the abnormal solutions of the Bethe-Salpeter equation in the Wick-Cutkosky model

In the Wick-Cutkosky model, where two scalar massive constituents interact by means of the exchange of a scalar massless particle, the Bethe-Salpeter equation has solutions of two types, called normal and abnormal. In the non-relativistic limit, the normal solutions correspond to the usual Coulomb spectrum, whereas the abnormal ones do not have non-relativistic counterparts - they are absent in the Schrodinger equation framework. We have studied, in the formalism of the light-front dynamics, the Fock-space content of the abnormal solutions. It turns out that, in contrast to the normal ones, the abnormal states are dominated by the massless exchange particles (by 90 % or more), what provides a natural explanation of their decoupling from the two-body Schrodinger equation. Assuming that one of the massive constituents is charged, we have calculated the electromagnetic elastic form factors of the normal and abnormal states, as well as the transition form factors. The results on form factors confirm the many-body nature of the abnormal states, as found from the Fock-space analysis. The abnormal solutions have thus properties similar to those of hybrid states, made here essentially of two massive constituents and several or many massless exchange particles. They could also be interpreted as the Abelian scalar analogs of the QCD hybrid states. The question of the validity of the ladder approximation of the model is also examined.

Automated summary

In the Wick-Cutkosky model, abnormal solutions of the Bethe-Salpeter equation are dominated by massless exchange particles and exhibit many-body nature, similar to hybrid states. These abnormal solutions could be interpreted as Abelian scalar analogs of QCD hybrid states, and their validity in the ladder approximation of the model is also examined.