Scattering equations: from projective spaces to tropical grassmannians

A bstract We introduce a natural generalization of the scattering equations, which connect the space of Mandelstam invariants to that of points on ℂℙ 1 , to higher-dimensional projective spaces ℂℙ k − 1 . The standard, k = 2 Mandelstam invariants, s ab , are generalized to completely symmetric tensors s_a₁a₂ aₖ s a 1 a 2 … a k subject to a ‘massless’ condition s_a₁a₂ aₖ₋₂bb=0 s a 1 a 2 … a k − 2 b b = 0 and to ‘momentum conservation’. The scattering equations are obtained by constructing a potential function and computing its critical points. We mainly concentrate on the k = 3 case: study solutions and define the generalization of biadjoint scalar amplitudes. We compute all ‘biadjoint amplitudes’ for ( k, n ) = (3 , 6) and find a direct connection to the tropical Grassmannian. This leads to the notion of k = 3 Feynman diagrams. We also find a concrete realization of the new kinematic spaces, which coincides with the spinor-helicity formalism for k = 2, and provides analytic solutions analogous to the MHV ones.