Microscopic study of the $p_T$-dependence of directed particle flow at SPS

Directed flow of nucleons and pions is studied in different $p_t$ intervals in S+S and Pb+Pb collisions at 160 AGeV. For simulations the microscopic quark-gluon string model (QGSM) is applied. It is shown that the behaviour of the directed flow of hadrons with high transverse momenta diverges considerably from that of low-$p_t$ particles. Both light and heavy systems of colliding nuclei exhibit negative values of the coefficient $v_1(p_t)$ of the azimuthal distribution when the transverse momentum approaches zero. Since the formation of a rapidly expanding thermal source is very unlikely for S+S and peripheral Pb+Pb collisions, the most plausible explanation of the effect is shadowing.