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

Abstract

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 $V_2=-g_A^e{g}_A^n\frac{\hslash c}{4\ddot{I}€}\vec{\ddot{I}}{ƒ}_e\cdot \vec{\ddot{I}}{ƒ}_n\frac{e^{-r/\hat{I}{»}_c}}{r}$, where $g_A^e$ and $g_A^n$ are the electron and neutron axial couplings, $\overrightarrow{\ddot{I}{ƒ}_e}$ and $\overrightarrow{\ddot{I}{ƒ}_n}$ are the electron and neutron spin, and $\hat{I}{»}_c$ is the interaction range for an exotic axial vector interaction from massive spin-1 boson exchange of mass $\hslash c/\hat{I}{»}_c$. The resulting average neutron spin rotation angle per unit length, $\frac{d\bar{\ddot{I}}{•}_{F5}}{dz}=[0.41\pm 6.30(stat.)\pm 4.4(sys.)]\times 10^{-3}$ 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 $10^{-8}\le \hat{I}{»}_c\le 10^{-2}$ range.