New Upper Bounds on Exotic Neutron-Spin–Electron-Spin Interactions via Neutron-Spin-Rotation Measurements in a Compensated Ferrimagnet

We report a search for exotic spin-spin interactions between neutrons and electrons which could signal new physics beyond the standard model using slow neutron polarimetric imaging through a dense medium of polarized electrons. Our dense polarized electron target is a ferrimagnet held at its magnetic compensation temperature, which realizes a polarized electron ensemble with zero net magnetization. We sought the spin rotation of transversely polarized neutrons from a neutron-spin–electron-spin interaction of the form V2=-gAegAn(ℏc/4π)σ→e·σ→n(e-r/λc/r), where gAe and gAn are the electron and neutron axial couplings, σe→ and σn→ are the electron and neutron spin, and λc is the interaction range for an exotic axial vector interaction from massive spin-1 boson exchange of mass ℏc/λc. The resulting average neutron-spin-rotation angle per unit length, (dϕ¯F5/dz)=[0.41±6.3(stat)±4.4(sys)]×10-3 rad/m, is consistent with zero. Our novel approach improves the previous upper limits on the coupling constant product gAegAn by several orders of magnitude in the poorly explored 10-8≤λc≤10-2 m range.