Natalia Toro (Stanford)
Constructing Tree-Level S-Matrices
November 11, 2008

The BCFW recursion relations define multi-point tree amplitudes for
gauge theory and gravity in terms of lower-point on-shell amplitudes,
so they furnish a purely on-shell description of spin-1 or spin-2
particle scattering; however, a BCFW construction is usually derived
from properties of field-theory amplitudes.  In this talk, I will
discuss directly building S-matrices that satisfy unitarity and
Lorentz invariance, bypassing local field theory altogether.   The
sufficient conditions for unitarity of BCFW amplitudes echo Weinberg's
soft photon/graviton construction; in the case of gravity, the
unitarity of BCFW amplitudes requires non-trivial scaling of
amplitudes at large complex momenta.