  Features of dense groups, or spikes, of negative pions produced in Mg-Mg
collisions at 4.3 GeV/c/nucleon are studied to search for a coherent,
Cerenkov-like, mechanism of particle production process. We investigate the
distributions of spike centers and, for the first time, the energy spectra of
particles in spikes. The spike-center distributions are obtained to exhibit the
structure due to the coherent gluon-jet emission dynamics. This structure is
similar to that observed recently for all-charged-particle spikes in hadronic
and nuclear interactions. The energy distribution within spikes is found to
have a significant peak over the inclusive background, while the inclusive
spectrum shows exponential decrease with two characteristic values of average
kinetic energy. The value of the peak energy and its width are in a good
agreement with those expected for pions produced in a nuclear medium in the
framework of the Cerenkov quantum approach. The peak energy obtained is
consistent with the value of the cross-section maximum observed in coincidence
experiments of nucleon-nucleus interactions.
