Axial-metric/polar-gauge perturbations of the Maldacena-Milekhin-Popov wormhole

Abstract: We study linear perturbations of the Maldacena-Milekhin-Popov (MMP) wormhole in the parity sector that pairs axial metric perturbations with polar gauge perturbations. It contains the axial gravitational mode and the electric field along the magnetic field lines. The lowest Landau level (LLL) of the charged fermions responds through its charge. The gauge components $a_τ$, $a_ρ$ form a two-dimensional gauge field on the worldsheet of each field line, and the fermions respond with the exact Schwinger current. Lifted over the sphere, it gives the electric field the MMP screening mass $m_S<sup>2=αq<sup>2$, which is not small and survives in the mouth. In the throat the perturbation tilts the worldsheets, and the Casimir stress tensor acquires mixed components. Force balance then requires a current across the field lines, and charge conservation fixes an accompanying polarisation current, the counterpart of the Hall current of the polar sector. These currents close the Bianchi identities at first order in the fermionic backreaction $α$. The system reduces to three coupled master equations, given in closed form and matched to the fermion-free mouth. Consistency alone does not select the fermionic constitutive relation, but locality, covariance and the index theorem do. At $α=0$ the throat reduces to Poschl-Teller problems with masses $l(l-1)$ and $(l+1)(l+2)$ at $m_S<sup>2=0$. At finite $m_S<sup>2$ one mode is screened, one tends to the mass $l<sup>2+l+2$, and one neutral charge-density wave stays exactly massless. The $O(α)$ shifts of the massive normal frequencies are real and negative. The two parity sectors are isospectral only without fermions. The neutral channel sees an attractive region in each mouth, and a numerical solution coupled to the gravitational channel in the exact mouth potential finds no bound state. The sector has no growing mode for $l\ge2$ at this order.