  We report new detections of the hotspots in Cygnus A at 4.5 and 8.0 microns
with the Spitzer Space Telescope. Together with detailed published radio
observations and synchrotron self-Compton modeling of previous X-ray
detections, we reconstruct the underlying electron energy spectra of the two
brightest hotspots (A and D). The low-energy portion of the electron
distributions have flat power-law slopes (s~1.5) up to the break energy which
corresponds almost exactly to the mass ratio between protons and electrons; we
argue that these features are most likely intrinsic rather than due to
absorption effects. Beyond the break, the electron spectra continue to higher
energies with very steep slopes s>3. Thus, there is no evidence for the
`canonical' s=2 slope expected in 1st order Fermi-type shocks within the whole
observable electron energy range. We discuss the significance of these
observations and the insight offered into high-energy particle acceleration
processes in mildly relativistic shocks.
