SIDEWARD FLOW IN AU+AU COLLISIONS AT 400 A.MEV

We present new experimental data obtained with the FOPI detector at SI S, for the Au + Au heavy-ion collisions at 400A MeV incident energy. T he sideward flow, determined from a method without reaction-plane reco nstruction, and the nuclear stopping are studied as a function of the centrality of the collisions. In order to study the nuclear in-medium effects, which act on the NN cross sections and potential and hence on experimental observables like the nuclear-matter flow and stopping, t hese results are compared with the predictions of two different QMD ve rsions. The first one offers a fully microscopic calculation of the cr oss sections and potential in the G-matrix formalism and naturally inc ludes the in-medium effects (this version is for the first time confro nted with experiment). The second one uses a standard Skyrme potential plus a momentum-dependent term in order to mimic the in-medium effect s.