# Possible New Phase of Thermal QCD

## TL;DR

This paper presents lattice simulation evidence for a new infrared phase of thermal QCD characterized by a specific spectral density behavior, suggesting coexistence of different scale invariances and implications for quark-gluon plasma properties.

## Contribution

It introduces the concept of an IR phase in thermal QCD with unique scale invariance features, expanding the understanding of QCD phases beyond the standard chiral crossover.

## Key findings

- Spectral density $
ho(\lambda)$ scales as 1/$\lambda$ in the IR phase.
- IR phase exists between 200 MeV and 250 MeV temperature range.
- Potential connection to massless glueballs and fluidity properties at RHIC and LHC.

## Abstract

Using lattice simulations, we show that there is a phase of thermal QCD, where the spectral density $\rho(\lambda)$ of Dirac operator changes as $1/\lambda$ for the infrared eigenvalues $\lambda<T$. This behavior persists over the entire low energy band we can resolve accurately, over three orders of magnitude on our largest volumes. We propose that in this "IR phase", the well-known non-interacting scale invariance at very short distances (UV, $\lambda \rightarrow \infty$, asymptotic freedom), coexists with very different interacting type of scale invariance at long distances (IR, $\lambda<T$). Such dynamics may be responsible for the unusual fluidity properties of the medium observed at RHIC and LHC. We point out its connection to the physics of Banks-Zaks fixed point, leading to the possibility of massless glueballs in the fluid. Our results lead to the classification of thermal QCD phases in terms of IR scale invariance. The ensuing picture naturally subsumes the standard chiral crossover feature at $"\!T_c\!" \,\approx 155$ MeV. Its crucial new aspect is the existence of temperature $T_{IR}$ (200 MeV $< T_{IR} < $ 250 MeV) marking the onset of IR phase and possibly a true phase transition.