Transverse momentum dependence of directed particle flow at 160A GeV

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 v₁(y,Δp_t) changes sign from a negative slope in the low-p_t region to a positive slope at p_t>~0.6 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 v₁(p_t,Δy) 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.