Isospin tracing: A probe of nonequilibrium in central heavy-ion collisions

Four different combinations of ⁹⁶₄₄Ru and ⁹⁶₄₀Zr nuclei, both as projectile and target, were investigated at the same bombarding energy of 400AMeV using a 4π detector. The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space using two different isospin-tracer observables, the number of measured protons and the tritium to ³He yield ratio. The experimental results show that the global equilibrium is not reached even in the most central collisions. Quantitative measures of stopping and mixing are extracted from the data. They are found to exhibit a quite strong sensitivity to the in-medium (n, n) cross section used in microscopic transport calculations.