Microscopic study of the p T - dependence of directed particle flow at SPS

Directed flow of nucleons and pions is studied in different pt intervals in S+S and Pb+Pb collisions at 160A GeV. For simulations the microscopic quarkgluon 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-pt particles. Both light and heavy systems of colliding nuclei exhibit negative values of the coefficient v1(pt) 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. PACS numbers: 25.75.-q, 25.75.Ld, 24.10.Lx, 24.10.Jv Typeset using REVTEX 1 The collective flow of hadrons in ultrarelativistic heavy-ion collisions is a very useful signal to probe the evolution of hot and dense nuclear matter from the onset of its formation [1–4]. Since the development of flow is closely related to the equation of state (EOS) of nuclear matter, the investigation of the flow can shed light on the transition to a new phase of matter, the so-called quark-gluon plasma (QGP), and its subsequent hadronization [5–10]. At present, the Fourier expansion technique is widely employed to study collective flow phenomenon [11–13]. Namely, the invariant distribution E dN dp is presented as E dN dp = 1 π dN dp2tdy [