Transverse momentum dependence of directed particle flow at 160A GeV - art. no. 034902

The transverse momentum (p(t)) dependence of hadron how at SPS energies is studied. In particular, the nucleon and pion flow in S+S and Pb+Pb collisio ns at 160A GeV is investigated. For simulations the microscopic quark-gluon string model is applied. It is found that the directed flow of pions nu (1 )(y,Deltap(t)) changes sign from a negative slope in the low-p(t) region to a positive slope at p(t)greater than or equal to0.6 GeV/c as recently obse rved experimentally. The change of the flow behavior can be explained by ea rly emission times for high-p(t) pions. We further found that a substantial amount of high-p(t) pious are produced in the very first primary nucleon-n ucleon collisions at the surface region of the touching nuclei. Thus, at SP S energies high-p(t) nucleons seem to be a better probe for the hot and den se early phase of nuclear collisions than high-p(t) pions. Both in the ligh t and in the heavy system the pion directed flow nu (1)(p(t),Deltay) exhibi ts large negative values when the transverse momentum approaches zero, as a lso seen experimentally in Pb+Pb collisions. It is found that this effect i s caused by nuclear shadowing. The proton flow, on the contrary, shows the typical linear increase with rising p(t).