Transitions of collective flow observables at intermediate energies

The comparison of experimental collective flow studies to model calculations has suggested various qualities of nuclear matter, such as the momentum dependent nuclear mean field and the reduced in-medium nucleon-nucleon cross section. Transitions in collective flow observables are particularly beneficial, since these transitions are mostly independent of experimental biases and therefore better suited for relating to theory. Three known transitions in collective flow observables exist, two of which occur at NSCL energies: the disappearance of directed flow (balance energy) and the disappearance of elliptic flow (transition energy).;The disappearance of directed flow in intermediate energy heavy-ion collisions is measured for the 197Au + 197Au system using the Michigan State University 4pi Array. Previous experiments have extrapolated values of the Au + Au balance energy, but a recent quantum molecular dynamics study suggested that the balance energy cannot be measured directly for Au + Au due to a prohibitively large Coulomb force. Boltzmann-Uehling-Uhlenbeck (BUU) model calculations are performed to show that the Coulomb interaction lowers the experimental balance energy significantly but does not suppress the observable completely. Also, BUU calculations show that the dual dependence of the balance energy on nuclear compressibility and in-medium cross section seen for light- and medium-sized systems reduces to a single dependence on the nuclear compressibility, allowing for the first time a characterization of the nuclear equation of state using the balance energy.;The disappearance of elliptic flow is measured for 40Ar + 45Sc, 58Ni + 58Ni, 86Kr + 93Nb and 197Au + 197Au. The transition energy for Au + Au exhibits a strong impact parameter dependence, in contrast to the balance energy for the same system which is nearly independent of impact parameter. However, the transition energy appears to decrease with increasing system mass, which is in accordance with the mass dependence of the balance energy. Comparison of experimental 40Ar + 45Sc data to published model calculations of 48Ca + 48Ca allows one to study the nuclear EOS using the transition energy for the first time. For the two independent cases of Au + Au directed flow and Ar + Sc elliptic flow, comparison of data with BUU calculations indicates a soft equation of state and a reduced in-medium cross section.