Collisionless Resonances Set the Size of a Weyl Exceptional Ring

Abstract

Eliminating continuum degrees of freedom converts conservative dynamics into a dispersive, non-Hermitian response whose topology need not represent the causally selected kinetic dynamics. In collisionless magnetized plasma, retarded Landau and cyclotron resonances damp two Weyl modes differently and replace a cold Weyl point by an exceptional ring. Axial roots and root-normalized polarization mixing predict its off-axis radius, $R_{\mathrm{EP}}^{\mathrm{pred}}=|\Delta \Gamma |/(2|v_{\mathrm{mix}}|)$, before any double root is computed. Across five nonrelativistic scans spanning a 310-fold range of differential damping, $R_{\mathrm{EP}}^{\mathrm{pred}}$ agrees with the nonlinear double-root radius $R_{\mathrm{EP}}^{\mathrm{full}}$ within $0.12\%$. The selected retarded kinetic-response bundle is a continuous graph over the electric root line bundle and has the same first Chern class under stated conditions. The cold interface mode continues to a localized Keldysh pole with one oriented crossing in the tested line-gap window, consistent with the inherited charge.

6 pages, 2 figures; Supplemental Material appended. Data and code are available at https://doi.org/10.5281/zenodo.22020911