Many-body Fock sectors in Wick-Cutkosky model
  In the model where two massive scalar particles interact by the ladder exchanges of massless scalar particles (Wick-Cutkosky model), we study in light-front dynamics the contributions of different Fock sectors (with increasing number of exchanged particles) to full normalization integral and electromagnetic form factor. It turns out that two-body sector always dominates. At small coupling constant $\alpha\ll 1$, its contribution is close to 100%. It decreases with increase of $\alpha$. For maximal value $\alpha=2\pi$, corresponding to the zero bound state mass, two-body sector contributes to the normalization integral 64%, whereas the three-body contribution is 26% and the sum of all higher contributions from four- to infinite-body sectors is 10%. Contributions to the form factor from different Fock sectors fall off faster for asymptotically large $Q^2$, when the number of particles in the Fock sectors becomes larger. So, asymptotic behavior of the form factor is determined by the two-body Fock sector. 
Dae Sung Hwang and V.A. Karmanov