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

Understand

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 $v_1(p_t)$.