Physics around the QCD (tri)critical endpoint and new challenges for femtoscopy

TL;DR

This paper explores the physics near the QCD (tri)critical endpoint using solvable models, highlighting the role of surface tension and bag surface properties in phase transitions and cross-over phenomena.

Contribution

It introduces a novel model linking surface tension vanishing to phase transition order change and predicts fractal bag surfaces at the cross-over, advancing understanding of QCD phase structure.

Findings

Surface tension vanishes at the (tri)critical endpoint.

Power law distribution of QGP bags appears at the endpoint.

Negative surface tension leads to non-spherical, fractal bag surfaces.

Abstract

On the basis of exactly solvable models with the tricritical and critical endpoints I discuss the physical mechanism of endpoints formation which is similar to the usual liquids. It is demonstrated that the necessary condition for the transformation of the 1-st order deconfinement phase transition into the 2-nd order phase transition at the (tri)critical endpoint is the vanishing of surface tension coefficient of large/heavy QGP bags. Using the novel model of the confinement phenomenon I argue that the physical reason for the cross-over appearance at low baryonic densities is the negative value of QGP bag surface tension coefficient. This implies the existence of highly non-spherical or, probably, even fractal surfaces of large and heavy bags at and above the cross-over, which, perhaps, can be observed via some correlations. The model with the tricritical endpoint predicts that at the deconfinement transition line the volume (mass) distribution of large (heavy) QGP bags acquires the power law form at the endpoint only, while in the model with the critical endpoint such a power law exists inside the mixed phase. The role of finite width of QGP bags is also discussed.