Exploration of the Tree-Level S-Matrix of Massless Particles

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In recent years, the BCFW construction provided a very powerful tool for computing scattering amplitudes as well as it shed light on the perturbation theory structure. In this talk, we discuss the long-standing issue of the boundary term arising when the amplitudes do not vanish as some momenta are taken to infinity along some complex direction. In particular, we provide a new set of on-shell recursion relations valid for such theories and discuss its consequences on our understanding on the perturbation theory structure of the S-Matrix.