Phase-diagram of two-color lattice QCD in the chiral limit

TL;DR

This paper investigates the phase transitions in two-color lattice QCD with massless fermions, revealing a change from weak first order to second order transitions at finite chemical potential and identifying the universality classes involved.

Contribution

The study introduces a new cluster algorithm to analyze the thermodynamics of the model and characterizes the nature and universality classes of the phase transitions.

Findings

Weak first order transition at zero chemical potential

Second order transition at finite chemical potential

Quantum phase transition is second order mean field type

Abstract

We study thermodynamics of strongly coupled lattice QCD with two colors of massless staggered fermions as a function of the baryon chemical potential $\mu$ in 3+1 dimensions using a new cluster algorithm. We find evidence that the model undergoes a weak first order phase transition at $\mu=0$ which becomes second order at a finite $\mu$. Symmetry considerations suggest that the universality class of these phase transitions should be governed by an $O(N)\times O(2)$ field theory with collinear order, with N=3 at $\mu=0$ and N=2 at $\mu \neq 0$. The universality class of the second order phase transition at $\mu\neq 0$ appears to be governed by the decoupled XY fixed point present in the $O(2)\times O(2)$ field theory. Finally we show that the quantum (T=0) phase transition as a function of $\mu$ is a second order mean field transition.