Joint Theory Seminar: Callum Jones

Title: Smooth Splitting from On-Shell Recursion and Hidden Zeros in 4D Helicity Amplitudes

Abstract: Modern reformulations of scattering amplitudes have lead to the discovery of numerous remarkable properties hidden in plain sight. Recent developments of a "surfaceology" construction of color-ordered tree-level amplitudes in Tr(\phi^3), Yang-Mills and the NLSM by Arkani-Hamed et al. have lead to the discovery of an unexpected universal pattern of kinematic zeros, currently unexplained by any known symmetry principle. More mysteriously, near the zeros but away from any singularities, the amplitudes exhibit a new kind of factorization into a product of lower-point amplitudes, a property that has been dubbed "smooth splitting". Ordinary (singular) factorization of tree-level amplitudes is an expression of unitarity, does this new non-singular factorization hint at some generalization of this basic physical principle? In this talk, based on upcoming work with Shruti Paranjape, I will explain how these mysterious features can be understood using standard notions of unitarity and analyticity. The smooth splitting and zeros can be derived from an on-shell recursive contour integral argument, the non-trivial content of these properties is then identified as a new kind of improved high-energy growth that justifies dropping an arc at infinity. In the second part of this talk I will describe the consequences of these hidden zeros for four dimensional gluon helicity amplitudes. In this case many (but not all) of the hidden zeros are kinematically inaccessible due to dimensionality constraints and smooth splitting appears to have disappeared entirely. The accessible zeros can be understood by a similar recursive argument based on the familiar BCFW construction and this naturally leads to the discovery of a more generic class of “helicity zeros” peculiar to four dimensional gluon scattering.