Speaker
Description
Magnetic fields explicitly break the non-singlet flavour symmetry of QCD, since the light quarks carry unequal electric charges. The conventional mesonic partner relations used to diagnose chiral and $U(1)_A$ restoration at vanishing field must therefore be reconsidered before they can be applied at $eB\neq0$. We show that the neutral channels retain this diagnostic role: the splitting between the $\pi^0-\sigma$ susceptibilities remains a probe of chiral symmetry restoration, and the $\pi^0-\delta^0$ splitting a probe of the effective restoration of $U(1)_A$.
We compute both splittings in (2+1)-flavours lattice QCD with the highly improved staggered quark (HISQ) action, at a fixed lattice spacing, physical strange quark mass $m_s$, and light quark masses $m_l=m_s/10$ ($m_\pi\approx220$ MeV), spanning $T = 17-281$ MeV and $eB \leq 2.51$ GeV$^2$ . The chiral splitting is enhanced by the field at low temperatures and suppressed at sufficiently large $eB$ near the crossover, providing susceptibility-level counterparts of magnetic catalysis and inverse magnetic catalysis. The $U(1)_A$ splitting behaves analogously, but its suppression sets in at larger $eB$ and remains milder, indicating that the anomaly is more resilient to the magnetic field than chiral symmetry breaking in this temperature range.
The talk is based on [1].
[1] Ding, Heng-Tong and Hernández Hernández, José Javier and Zhang, Dan; Chiral and $U(1)_A$ symmetries in background magnetic fields from lattice QCD; arXiv:2607.11625