Generating tree amplitudes in N=4 SYM and N=8 SG

We study n-point tree amplitudes of N = 4 super Yang-Mills theory and N = 8 supergravity for general configurations of external particles of the two theories. We construct generating functions for n-point MHV and NMHV amplitudes with general external states. Amplitudes derived from them obey SUSY Ward identities, and the generating functions characterize and count amplitudes in the MHV and NMHV sectors. The MHV generating function provides an efficient way to perform the intermediate state helicity sums required to obtain loop amplitudes from trees. The NMHV generating functions rely on the MHV-vertex expansion obtained from recursion relations associated with a 3-line shift of external momenta involving a reference spinor |X]. When the shifted amplitude vanishes for large z for all |X], the sum of MHV-vertex diagrams is independent of |X] and gives the correct amplitude. If the shifted amplitude does not vanish for large z, Cauchy's theorem includes a term at infinity. Examples show that special choices of |X] eliminate this term and the MHV vertex expansion becomes valid at these values. We show that the MHV-vertex expansion of the n-graviton NMHV amplitude for n = 5, 6,..., 11 is independent of |X] and exhibits the asymptotic behavior z(n-12). Generating functions show how the symmetries of supergravity can be implemented in the quadratic map between supergravity and gauge theory embodied in the KLT and other similar relations between amplitudes in the two theories.