Bel-Robinson as stress-tensor gradients and their extensions to massive matter

We show that the Bel–Robinson (BR) tensor is—generically, as well as in its original GR setting—an autonomously conserved part of the, manifestly conserved, double gradient of a system’s stress-tensor. This suggests its natural extension from GR to matter models, first to (known) massless scalars and vectors, then to massive ones, including tensors. These massive versions are to be expected, given that they arise upon KK reduction of massless D+1D+1 ones. We exhibit the resulting spin (0,1,2)(0,1,2) “massive” BR