Transverse momentum dependence of directed particle flow at160AGeV

The transverse momentum ${(p}_t)$ dependence of hadron flow at SPS energies is studied. In particular, the nucleon and pion flow in S+S and Pb+Pb collisions at $160A$ GeV is investigated. For simulations the microscopic quark-gluon string model is applied. It is found that the directed flow of pions $v_1(y,\Delta p_t)$ changes sign from a negative slope in the low-$p_t$ region to a positive slope at $p_t>~0.6\hspace{0.5em}\mathrm{GeV}/c$ as recently observed experimentally. The change of the flow behavior can be explained by early emission times for high-$p_t$ pions. We further found that a substantial amount of high-$p_t$ pions are produced in the very first primary nucleon-nucleon collisions at the surface region of the touching nuclei. Thus, at SPS energies high-$p_t$ nucleons seem to be a better probe for the hot and dense early phase of nuclear collisions than high-$p_t$ pions. Both in the light and in the heavy system the pion directed flow $v_1{(p}_t,\Delta y)$ exhibits large negative values when the transverse momentum approaches zero, as also seen experimentally in Pb+Pb collisions. It is found that this effect is caused by nuclear shadowing. The proton flow, on the contrary, shows the typical linear increase with rising $p_t.$