Subthreshold pion dynamics as a source for hard photons beyond proton-neutron Bremsstrahlung in heavy-ion collisions

First measurement in heavy-ion collisions of hard photons of extremely high energies that extend the photon spectrum to five times the beam energy per nucleon is reported. The photon spectrum was measured for the systems 86Kr+natNi at 60 A MeV and 181Ta+197Au at 40 A MeV. The data are interpreted using the Dubna cascade model. The radiative channel of pion-nucleon interaction π + N → N + γ, involving subthreshold pions produced in the nuclear medium, dominates all other processes considered for the production of the very energetic photons. This channel was never considered so far at beam energies below the pion threshold. (author)