eta/s is critical (at phase transitions)

TL;DR

The paper discusses how the viscosity over entropy density ratio (eta/s) can serve as an indicator of phase transitions in the hadron phase diagram, showing potential minima or discontinuities at critical points.

Contribution

It presents the argument that eta/s exhibits characteristic features at phase transitions, providing a possible experimental signature in relativistic heavy ion collisions.

Findings

eta/s has a minimum at phase transitions or crossovers

Divergence in eta/s is unlikely due to low critical exponents

Experimental data may show discontinuities or smooth minima in eta/s

Abstract

The viscosity over entropy density ratio, or KSS number, can help isolate the critical point in the hadron phase-diagram in Relativistic Heavy Ion Collisions. We argue that this quantity does have a minimum at a phase transition or crossover. Although indications from conventional non-relativistic gases point out to even a divergence in eta/s when the phase-transition is first-order, since the critical exponent is rather low, this will be more difficult to ascertain in RHIC or FAIR. The experimental data are more likely to reveal a discontinuity for a first order phase transition or a smooth minimum at a crossover.