System-size independence of directed flow at the Relativistic Heavy-Ion Collider

We measure directed flow ($v_1$) for charged particles in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}=$ 200 GeV and 62.4 GeV, as a function of pseudorapidity ($\eta$), transverse momentum ($p_t$) and collision centrality, based on data from the STAR experiment. We find that the directed flow depends on the incident energy but, contrary to all existing models, not on the size of the colliding system at a given centrality. We extend the validity of the limiting fragmentation concept to different collision systems, and investigate possible explanations for the observed sign change in $v_1(p_t)$.