Neutrons from multiplicity-selected Au-Au collisions at 150A, 250A, 400A, and 650A MeV

We measured neutron triple-differential cross sections from multiplicity-selected Au-Au collisions at 150A, 250A, 400A, and 650A MeV. The reaction plane for each collision was estimated from the summed transverse velocity vector of the charged fragments emitted in the collision. We examined the azimuthal distribution of the triple-differential cross sections as a function of the polar angle and the neutron rapidity. We extracted the average in-plane transverse momentum 〈P_x〉 and the normalized observable 〈P_x/P_⊥〉, where P_⊥ is the neutron transverse momentum, as a function of the neutron center-of-mass rapidity, and we examined the dependence of these observables on beam energy. These collective flow observables for neutrons, which are consistent with those of protons plus bound nucleons from the Plastic Ball Group, agree with the Boltzmann-Uehling-Uhlenback (BUU) calculations with a momentum-dependent interaction. Also, we calculated the polar-angle-integrated maximum azimuthal anisotropy ratio R from the value of 〈P_x/P_⊥〉.