Speaker
Description
Understanding how external magnetic fields affect chiral symmetry and the axial anomaly is an important question in QCD under extreme conditions. At vanishing magnetic field, the light-quark chiral condensate serves as the standard order-parameter-like observable for chiral symmetry breaking, while the restoration of chiral $SU(2)_L\times SU(2)_R$ symmetry and the effective restoration of $U(1)_A$ symmetry can also be characterized through degeneracy patterns among scalar and pseudoscalar mesonic correlators. In a background magnetic field, however, this familiar picture has to be revisited. Since up and down quarks carry different electric charges, the magnetic field explicitly breaks isospin symmetry and induces different responses in the light-quark condensates. As a result, the conventional mesonic partner relations are modified, and one must carefully identify which mesonic channels and susceptibility differences provide proper probes of chiral symmetry restoration and the $U(1)_A$ anomaly in this case.
In this talk, I will first discuss the status of chiral and axial symmetries in nonzero magnetic fields, with particular attention to the role of light-quark condensates and mesonic susceptibility differences. I will then present an analysis of which observables remain suitable diagnostics of chiral symmetry restoration and of the $U(1)_A$ anomaly in the presence of magnetic-field-induced flavor breaking. Based on this theoretical consideration, I will present lattice-QCD results in $(2+1)$-flavor QCD using the highly improved staggered quark action. The calculations are performed at a fixed lattice spacing with a physical strange quark mass and light quark masses corresponding to $m_\pi\simeq 220~\mathrm{MeV}$, over a wide range of temperatures and background magnetic fields. The results provide insight into how the hierarchy and pattern of chiral and $U(1)_A$ symmetry restoration are modified by external magnetic fields.