Improved Limits on Spin-Mass Interactions

Very light particles with $CP$-violating couplings to ordinary matter, such as axions or axionlike particles, can mediate long-range forces between polarized and unpolarized fermions. We describe a new experimental search for such forces between unpolarized nucleons in two 250 kg Pb weights and polarized neutrons and electrons in a ${}^3\mathrm{He}\text{−}\mathrm{K}$ comagnetometer located about 15 cm away. We place improved constraints on the products of scalar and pseudoscalar coupling constants, $g_p^n{g}_s^N<4.2\times {10}^{-30}$ and $g_p^e{g}_s^N<1.7\times {10}^{-30}$ (95% C.L.) for axionlike particle masses less than ${10}^{-6}\mathrm{eV}$, which represents an order of magnitude improvement over the best previous neutron laboratory limit.

Figure 1

Probe and pump optical setup in the $x\text{−}y$ plane. Bottom left insert: Experimental schematic in the $y\text{−}z$ plane.

Figure 2

Measured signal correlation. $B-$ runs are denoted with triangles pointing down while $B+$ runs are denoted with triangles pointing up. The weighted average is given by the square point. For 4 degrees of freedom, the ${\tilde{\chi }}^2$ is 0.73.

Figure 3

Constraints (95% C.L.) on $g_p^n{g}_s^N$ for the neutron. The solid red line (1) is from this work. Limits in [17, 19] were multiplied by 2 to obtain a 95% C.L. Reference [20] was extrapolated by integrating over the Earth’s mass density for heavier axions. Reference [45] uses astrophysical constraints to limit $g_p^n$.

Figure 4

Constraints (95% C.L.) on $g_p^e{g}_s^N$ for the electron. The solid red line (1) is from this work. Limits in [19] were multiplied by 2 to obtain a 95% C.L. Reference [45] uses astrophysical observations.