The light Roberge-Weiss tricritical endpoint at imaginary isospin and baryon chemical potential

TL;DR

This paper investigates the phase diagram of QCD at imaginary baryon and isospin chemical potentials, identifying a tricritical point and analyzing the dynamics of Polyakov loop clusters through perturbation theory and lattice simulations.

Contribution

It provides the first determination of the QCD phase diagram at arbitrary imaginary chemical potentials and explores the tricritical endpoint using both analytical and numerical methods.

Findings

Identified a nontrivial structure of Roberge-Weiss phase transitions.

Established a lower bound for the light quark mass where the transition line ends.

Found that the tricritical mass increases compared to purely baryonic cases.

Abstract

Imaginary chemical potentials serve as a useful tool to constrain the QCD phase diagram and to gain insight into the thermodynamics of strongly interacting matter. In this study, we report on the first determination of the phase diagram for arbitrary imaginary baryon and isospin chemical potentials at high temperature using one-loop perturbation theory, revealing a nontrivial structure of Roberge-Weiss (RW) phase transitions in this plane. Subsequently, this system is simulated numerically with $N_{\rm f}=2$ unimproved staggered quarks on $N_{\tau}=4$ lattices at a range of temperatures at one of the RW phase transitions. We establish a lower bound for the light quark mass, where the first-order transition line terminates in a tricritical point. It is found that this tricritical mass is increased as compared to the case of purely baryonic imaginary chemical potentials, indicating that our setup is more advantageous for identifying critical behavior towards the chiral limit. Finally, the dynamics of local Polyakov loop clusters is also studied in conjuction with the RW phase transition.