The Isospin Influence on Squeeze-out Phenomena in Heavy Ion Collisions

Equilibration of the isospin degree of freedom, the isospin dependence of the sidewards flow, balance energy, preequilibrium nucleon emission and subthreshold pion production are phenomena proven to be sensitive to the isospin-dependent nuclear equation of state and in-medium nucleon-nucleon cross section [1]. In this contribution we will report on preliminary experimental evidence on the influence of the N/Z content of the spectator matter on the azimuthal distributions. We used the experimental data obtained with the Phase II of the FOPI detector at GSI Darmstadt for Ru + Zr and Zr + Ru collisions at 400 A·MeV [2]. The charge multiplicity in the central drift chamber was used in order to select the collision geometry. The impact parameter value was extracted from a standard geometrical sharp cut-off approximation for the reaction cross section. The reaction plane reconstruction is based on the transverse momentum method. For an impact parameter of 5 fm, a geometrical estimate based on straight trajectory approximation gives for the fireball three times lower left-right N/Z asymmetry relative to 1.18 : 1.4 corresponding to the spectators for Ru + Zr combination. In such a situation, even if the chemical equilibrium of the fireball is not completely reached the remaining asymmetry can be neglected in a first approximation and we have to do with an expanding object in the presence of spectator matter asymmetric in N/Z.