Lattice QCD at Imaginary Chemical Potential in the Chiral Limit

TL;DR

This study investigates the nature of Roberge-Weiss and chiral transitions in (2+1)-flavor QCD with imaginary chemical potential, establishing the universality class of the RW endpoint and examining implications for the chiral phase transition.

Contribution

The paper demonstrates that the RW endpoint belongs to the 3D Z2 universality class with HISQ action and explores the sensitivity of chiral observables to RW transitions across different quark masses.

Findings

RW endpoint is in the 3D Z2 universality class.

Chiral observables are energy-like for the Z2 transition.

Constraints on the critical pion mass for first-order transition.

Abstract

We report on an ongoing study on the interplay between Roberge-Weiss (RW) and chiral transitions in simulations of (2+1)-flavor QCD with an imaginary chemical potential. We established that the RW endpoint belongs to the 3-$d$, $Z_2$ universality class when calculations are done with the Highly Improved Staggered Quark (HISQ) action in the RW plane with physical quark masses. We also have explored a range of quark masses corresponding to pion mass values, $m_\pi\geq40$~MeV and found that the transition is consistent with $Z_2$ universality class. We argue that observables that were usually used to determine the chiral phase transition temperature, e.g. the chiral condensate and chiral susceptibility, are sensitive to the RW transition and are energy-like observables for the $Z_2$ transition, contrary to the magnetic-like (order parameter) behavior at vanishing chemical potential. Moreover the calculations performed at $m_\pi\sim40$~MeV also put a stringent constraint for a critical pion mass at zero chemical potential for a possible first-order chiral phase transition.