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 V_{2}=-g_{A}^{e}g_{A}^{n}(ℏc/4π)σ[over →]_{e}·σ[over →]_{n}(e^{-r/λ_{c}}/r), where g_{A}^{e} and g_{A}^{n} are the electron and neutron axial couplings, σ_{e}[over →] and σ_{n}[over →] are the electron and neutron spin, and λ_{c} is the interaction range for an exotic axial vector interaction from massive spin-1 boson exchange of mass ℏc/λ_{c}. The resulting average neutron-spin-rotation angle per unit length, (dϕ[over ¯]_{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 g_{A}^{e}g_{A}^{n} by several orders of magnitude in the poorly explored 10^{-8}≤λ_{c}≤10^{-2} m range.