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 $V_2=-g_A^eg_A^n\frac{ \hbar c}{ 4\pi}\vec\sigma_e\cdot\vec\sigma_n\frac{ e^{ -r/\lambda_c}}{ r}$, where $g_{ A}^{ e}$ and $g_{ A}^{ n}$ are the electron and neutron axial couplings, $\vec{ \sigma_e}$ and $\vec{ \sigma_n}$ are the electron and neutron spin, and $\lambda_c$ is the interaction range for an exotic axial vector interaction from massive spin-1 boson exchange of mass $\hbar c/\lambda_c$. The resulting average neutron spin rotation angle per unit length, $\frac{ d\bar{ \phi}_{ F5}}{ dz}=[0.41\pm6.30\ (stat.)\pm4.4\ (sys.)]\times10^{ -3}$ rad/m, is consistent with zero. Our novel approach improves the previous upper limits on the coupling constant product $g_A^eg_A^n$ by several orders of magnitude in the poorly explored $10^{ -8}\leq\lambda_c\leq10^{ -2}$ range.
Categories: nucl-ex cond-mat.mtrl-sci hep-ex quant-ph