  Recent multi-dimensional simulations suggest that high-entropy buoyant plumes
help massive stars to explode. Outwardly protruding iron-rich fingers in the
galactic supernova remnant Cassiopeia A are uniquely suggestive of this
picture. Detecting signatures of specific elements synthesized in the
high-entropy nuclear burning regime (i.e., $\alpha$-rich freeze out) would be
among the strongest substantiating evidence. Here we report the discovery of
such elements, stable Ti and Cr, at a confidence level greater than 5$\sigma$
in the shocked high-velocity iron-rich ejecta of Cassiopeia A. We found the
observed Ti/Fe and Cr/Fe mass ratios require $\alpha$-rich freeze out,
providing the first observational demonstration for the existence of
high-entropy ejecta plumes that boosted the shock wave at explosion. The metal
composition of the plumes agrees well with predictions for strongly
neutrino-processed proton-rich ejecta. These results support the operation of
the convective supernova engine via neutrino heating in the supernova that
produced Cassiopeia A.
