  Five-dimensional gauge and gravity theories are known to exhibit striking
properties. D=5 is the lowest dimension where massive tensor states appear
naturally, providing a testing ground for perturbative insights into
six-dimensional tensor theories. Five-dimensional supergravities are highly
constrained and admit elegant geometric and algebraic formulations, with global
symmetries manifest at the Lagrangian level.
  In this paper, we take a step towards the systematic investigation of
amplitudes in five dimensions, and present a five-dimensional version of the
spinor-helicity formalism, applicable to massless, massive and supersymmetric
states. We give explicit representations for on-shell spinor and polarization
variables such that the little-group symmetry and gauge redundancy are
manifest. Massive self-dual tensor states are discussed in some detail, as well
as all the on-shell supermultiplets that can appear in matter-coupled gauge and
supergravity theories. As a byproduct of considering supersymmetry in the
presence of central charge, we obtain massless ten-dimensional Majorana-Weyl
spinors as products of five-dimensional massive spinors.
  We present compact expressions for superamplitudes at multiplicity three and
four, including several novel superamplitudes that either do not
straightforwardly uplift to six dimensions, or have not appeared in the
six-dimensional literature. We discuss several examples of five-dimensional
double-copy constructions in the context of gravitational theories with massive
vectors and tensors, illustrating that the formalism we construct can also be
used to considerably streamline the double-copy construction of N=2
Maxwell-Einstein supergravities.
