  We derive an upper limit to the energy of nuclei accelerated via the Fermi
mechanism in any relativistic shockwave, driven by any astrophysical engine.
This bound is accessible to current and upcoming ultra-high energy neutrino
experiments. Detection of a single neutrino with energy above the upper limit
would exclude all sites of shock acceleration, and imply physics beyond the
Standard Model. We comment on the possibility that relativistic flows launched
by supermassive black hole mergers are the source of the observed ultra-high
energy cosmic rays.
