  The form factor program for the regularized space-time S-matrix in planar
maximally supersymmetric gauge theory, known as the pentagon operator product
expansion, is formulated in terms of flux-tube excitations propagating on a
dual two-dimensional world-sheet, whose dynamics is known exactly as a function
of 't Hooft coupling. Both MHV and non-MHV amplitudes are described in a
uniform, systematic fashion within this framework, with the difference between
the two encoded in coupling-dependent helicity form factors expressed via
Zhukowski variables. The nontrivial SU(4) tensor structure of flux-tube
transitions is coupling independent and is known for any number of charged
excitations from solutions of a system of Watson and Mirror equations. This
description allows one to resum the infinite series of form factors and recover
the space-time S-matrix exactly in kinematical variables at a given order of
perturbation series. Recently, this was done for the hexagon. Presently, we
successfully perform resummation for the seven-leg tree NMHV amplitude. To this
end, we construct the flux-tube integrands of the fifteen independent Grassmann
component of the heptagon with an infinite number of small fermion-antifermion
pairs accounted for in NMHV two-channel conformal blocks.
