Shape of collective flow in highly central Au(150 A MeV)+Au collisions

Using the FOPI facility at GSI, charged particles (1 ≤Z≤6) produced in the Au(150 A MeV)+Au reaction have been measured at laboratory angles 1.20 < Θlab < 300. Highly central collisions have been selected with two criteria, both dealing with the longitudinal and transverse degrees of freedom of the reaction. The relevance of this selection method is supported by QMD calculations which indicate that such criteria are able to select mean impact parameters less than 2 fm. Bias effects introduced by the criteria have been evaluated. The centre-of-mass polar angle distributions of low energy clusters emitted in these central collisions, have been extracted: the intensity ratio deduced for a transverse to longitudinal emission is found to be R= 1.4 −0.4 +0.2 . Model comparisons using QMD are presented. The value of R appears to depend sensitively on the nucleon-nucleon cross section, σnn. Within this model, a value of σ=25+ - 5 mb is derived.