Critical Opalescence: An Optical Signature for a QCD Critical Point

TL;DR

This paper proposes using optical opacity measurements, indicative of critical opalescence, as a novel experimental signature to locate the QCD critical point in heavy ion collisions, and discusses the theoretical and experimental criteria involved.

Contribution

It introduces optical opacity as a new observable for identifying the QCD critical point and discusses how to measure critical exponents and universality class in this context.

Findings

Optical opacity peaks at the QCD critical point.

Estimated nuclear attenuation length is approximately 2.9 fm in central collisions.

Nearly identical length scales are obtained from different measurement methods.

Abstract

Four possible scenarios are considered for a transition from a quark-gluon matter to hadronic matter, and their corresponding correlation signatures are discussed. Four criteria are highlighted for a definitive experimental search for a QCD critical point. An old-new experimental measure, the optical opacity (or its inverse the nuclear attenuation length) is determined, in terms of a combination of nuclear suppression factors and a measurement of the relevant fireball length scales. Length scale estimates using either the Hanbury Brown -- Twiss radii or that of the initial nuclear geometry for measurements of optical opacity with respect to the reaction plane yield, somewhat surprizingly, nearly the same nuclear attenuation lenght in 0-5 % most central 200 GeV Au+Au collisions, corresponding to 2.9 $\pm$ 0.3 fm. The necessity and the possibility of measuring critical exponents is also discussed in the context of determination of the universality class of the QCD critical point. Critical opalescence is proposed to locate such a critical point on the QCD phase diagram, corresponding to a maximum of optical opacity in heavy ion experiments.