Limits on possible new spin-spin interactions between neutrons from measurements of the Longitudinal Spin Relaxation Rate of Polarized $^{3}$He Gas

Understand

New particles with masses in the sub-eV range have been predicted by various theories beyond the Standard Model.

• Some can induce new spin-spin interactions between fermions.
• Existing constraints on such interactions between nucleons with mesoscopic ranges (millimeters to nanometers) are quite poor.
• Polarized $^{3}$He gas is an especially clean system to use to constrain these possible new spin-spin interactions because the spin-independent atomic potential between helium atoms is well-characterized experimentally.