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

TL;DR

This study measures directed flow in heavy-ion collisions at different energies and system sizes, revealing a surprising independence from system size at fixed centrality, challenging existing models.

Contribution

It demonstrates that directed flow is independent of system size at a given centrality, extending the limiting fragmentation concept across different collision systems.

Findings

Directed flow depends on incident energy but not on system size.

The limiting fragmentation concept applies across different collision systems.

Observed sign change in $v_1(p_t)$ is explored with possible explanations.

Abstract

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 $v_1(p_t)$.