Hg-Cs Comagnetometer Sets Record Limits on Long-Range Axial Couplings at 3e-48

The experiment mounted the comagnetometer on a precision rotation stage to modulate the relative orientation between polarized electrons in Earth and the neutron-proton spins in the sensor, isolating exotic spin-spin potentials V2 and V11. Data were collected across multiple azimuths and inclinations, with systematic errors controlled through magnetic shielding and co-magnetometry that cancels ordinary magnetic fields to parts in 10^12. The resulting constraints directly limit hypothetical bosons that could mediate interactions between dark matter and ordinary matter while preserving Lorentz invariance at laboratory scales.

These limits probe parameter space relevant to ultralight axion-like particles and certain Lorentz-violating extensions that predict spin-dependent fifth forces with ranges from kilometers to infinity. Unlike torsion-balance or atomic-clock searches, the comagnetometer technique accesses axial-vector couplings that flip sign under parity, offering orthogonal sensitivity to models favored by some neutron lifetime and cosmic birefringence anomalies. The work improves on the 2015 Hunter et al. result by nearly an order of magnitude at λ > 10^3 km through better rotation control and longer integration.

Future runs with larger ensembles and cryogenic operation could push the electron-neutron bound below 10^{-49}, closing remaining windows for certain dark-photon and majoron scenarios. Parallel efforts at other laboratories using polarized noble gases will be essential to confirm any signal or further exclude ranges below 100 km.