Particle multiplicity dependence of high-energy photon production in a heavy-ion reaction

The production of high-energy photons in an intermediate energy heavy-ion reaction (65 MeV/nucleon ⁴⁰Ar+⁹³Nb) is studied by characterizing the events, which produce the photons, by the forward and backward, light and heavy, charged particle production. While the absolute yield of high-energy photons increases with increasing charged particle multiplicity, the spectral shape is found to be almost independent of multiplicity. This indicates that the fundamental photon production mechanism is insensitive to the impact parameter but that the production process is more probable for the more violent collisions. This is expected for an incoherent nucleon-nucleon bremsstrahlung mechanism but not for a statistical mechanism. This work also provides a comparison of two observables that have been suggested as impact parameter selectors: the high-energy photon yield and the charged particle multiplicity. We find that the photon yield, binned by particle multiplicity, does not scale as would be expected if both techniques truly measured the impact parameter. This observation can be explained by large fluctuations in the particle multiplicity at fixed impact parameter.