Is the neutron EDM nonzero at the 1e-27 e cm level?
In plain words
Slow neutrons stored in a bottle precess in electric and magnetic fields, and a dipole would shift their precession rate when the electric field is flipped. No shift has yet been seen.
Precise statement
The best bound is $\left|d_n\right| < 1.8e-26\,e\,\mathrm{cm}$ (90% CL, PSI 2020) using ultracold neutrons with a Hg-199 comagnetometer. Determine whether $d_n$ is nonzero at the $1e-27\,e\,\mathrm{cm}$ level, which corresponds to the QCD vacuum angle $\left|\theta_{\mathrm{bar}}\right|$ of order 1e-11 (using $d_n$ of about $1e-16\,\theta_{\mathrm{bar}}\,e\,\mathrm{cm}$).
What would settle it
A neutron EDM measurement with total uncertainty near 1e-27 e cm (n2EDM at PSI, or the US and Canadian ultracold-neutron efforts).
Status in the literature
Unverified note
The n2EDM apparatus at PSI was commissioned in 2024-2026 with its magnetic shielding and field uniformity characterized; no new limit as of mid-2026.