Speaker
Description
Understanding how external magnetic fields affect chiral and singlet $U(1)_A$ symmetry breaking is an important question in QCD under extreme conditions. In a pure magnetic background, the unequal electric charges of the light quarks explicitly reduce the non-singlet flavor symmetry, making it necessary to identify the appropriate neutral mesonic partner channels. We clarify the neutral-sector symmetry structure and identify $\chi_{\pi^0}-\chi_\sigma$ as the chiral-partner splitting associated with the surviving neutral non-singlet axial symmetry, and $\chi_{\pi^0}-\chi_{\delta^0}$ as the singlet $U(1)_A$ partner splitting.
Using fixed-scale $(2+1)$-flavor HISQ ensembles with $m_\pi\simeq220\,\mathrm{MeV}$ at vanishing magnetic field, we study these observables over a broad range of temperatures and magnetic fields. At low temperatures, both splittings increase with $eB$. At intermediate temperatures, both splittings decrease with increasing $eB$ once the field is sufficiently strong. The decrease of the singlet $U(1)_A$ partner splitting sets in at larger $eB$ and is milder than in the chiral channel. These results provide fixed-scale lattice-QCD evidence for distinct magnetic responses of the neutral chiral and singlet $U(1)_A$ partner susceptibility splittings.
