Possible new phase of thermal QCD

Andrei Alexandru,Ivan Horváth

doi:10.1103/physrevd.100.094507

Andrei Alexandru, Ivan Horváth

Open Access

https://doi.org/10.1103/physrevd.100.094507

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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.

Highlights

The study of strongly interacting matter as a function of temperature and baryon density is an active area of theoretical and experimental research
At high energies of colliding heavy nuclei, such as those studied at LHC and the high end of RHIC, baryon densities are small enough so that the results are generally expected, among other things, to shed light on the nature and properties of thermal QCD transition in the early universe
We propose a hierarchy of thermal effects in QCD, based on scale invariance properties at long distances, which adds new detail to the existing picture, and ties with it in a natural manner

Summary

INTRODUCTION

The study of strongly interacting matter as a function of temperature and baryon density is an active area of theoretical and experimental research (see [1] for recent review). At high energies of colliding heavy nuclei, such as those studied at LHC and the high end of RHIC, baryon densities are small enough so that the results are generally expected, among other things, to shed light on the nature and properties of thermal QCD transition in the early universe In this regime, it has become widely accepted, largely due to the matured power of lattice QCD [2], that increasing temperature leads to a smooth crossover in properties of thermal strongly interacting matter. At T ≳ TIR, we detect the onset of scale invariant 1=λ behavior of Dirac spectral density ρðλÞ (number of eigenmodes per unit volume and spectral interval) for λ ≲ T We propose that this arises due to the onset of effective IR scale invariance of glue fields dominating the thermal state.. Its precise determination in lattice simulations is challenging in part because the minimal system size needed to detect the IR phase grows with temperature (Appendix A)

LATTICE EVIDENCE

IR-UV SEPARATION AND BANKS-ZAKS FIXED POINT

SYNTHESIS AND MAIN POINTS