Collapse of Flow: Probing the Order of the Phase Transition

TL;DR

This paper reviews flow signals indicating a phase transition to quark-gluon plasma, highlighting a predicted collapse of flow patterns at specific energies, which may signal a first order phase transition at high baryon density.

Contribution

It emphasizes the significance of the flow excitation function from 1 to 50 A GeV and interprets the observed flow collapse as potential evidence for a first order phase transition.

Findings

Collapse of $v_1$-flow at ~10 A GeV

Collapse of $v_2$-flow at ~40 A GeV

Observation consistent with a first order phase transition

Abstract

We discuss the present collective flow signals for the phase transition to the quark-gluon plasma (QGP) and the collective flow as a barometer for the equation of state (EoS). We emphasize the importance of the flow excitation function from 1 to $50 A$ GeV: here the hydrodynamic model has predicted the collapse of the $v_1$-flow at $\sim 10 A$ GeV and of the $v_2$-flow at $\sim 40 A$ GeV. In the latter case, this has recently been observed by the NA49 collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy, we interpret this observation as potential evidence for a first order phase transition at high baryon density $\rho_B$.