  Neutrino interactions with stellar material are widely believed to be
fundamental to the explosion of massive stars. However, this important process
has remained difficult to confirm observationally. We propose a new method to
verify it using X-ray observations of the supernova remnant Cassiopeia A. The
elemental composition in its Fe-rich ejecta that could have been produced at
the innermost region of the supernova, where neutrinos are expected to
interact, allows us to examine the presence of neutrino interactions. Here we
demonstrate that the amount of Mn produced without neutrino nucleosynthesis
processes (i.e., the $\nu$- and $\nu$p-process) is too small to explain the
Mn/Fe mass ratio we measure (0.14--0.67\%). This result supports the operation
of significant neutrino interactions in the Cassiopeia A supernova. If the
observed Mn/Fe mass ratio purely reflects the production at the innermost
region of the supernova, this would be the first robust confirmation of
neutrino-matter interactions in an individual supernova. We further show that
the Mn/Fe mass ratio has the potential to constrain supernova neutrino
parameters (i.e., total neutrino luminosity, neutrino temperature). Future
spatially-resolved, high-resolution X-ray spectroscopy will allow us to
investigate the details of neutrino-supernova astrophysics through its
signatures in elemental composition not only in Cassiopeia A but also in other
remnants.
