  Form factors in planar N=4 Super-Yang-Mills theory admit a type of
non-perturbative operator product expansion (OPE), as we have recently shown in
arXiv:2009.11297. This expansion is based on a decomposition of the dual
periodic Wilson loop into elementary building blocks: the known pentagon
transitions and a new object that we call form factor transition, which encodes
the information about the local operator. In this paper, we compute the
two-particle form factor transitions for the chiral part of the stress-tensor
supermultiplet at Born level; they yield the leading contribution to the OPE.
To achieve this, we explicitly construct the Gubser-Klebanov-Polyakov
two-particle singlet states. The resulting transitions are then used to test
the OPE against known perturbative data and to make higher-loop predictions.
