Is the neutron EDM nonzero at the $10^{-27}$ 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 $\left|d_n\right| < 1.8 \times 10^{-26}\,e\,\mathrm{cm}$ (90% CL, PSI 2020) using ultracold neutrons with a Hg-199 comagnetometer. Determine whether $d_n$ is nonzero at the $10^{-27}\,e\,\mathrm{cm}$ level, which corresponds to the QCD vacuum angle $\left|\theta_{\mathrm{bar}}\right|$ of order $10^{-11}$ (using $d_n$ of about $10^{-16}\,\theta_{\mathrm{bar}}\,e\,\mathrm{cm}$).
What would settle it
A neutron EDM measurement with total uncertainty near $10^{-27}$ e cm (n2EDM at PSI, or the US and Canadian ultracold-neutron efforts).
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
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.
Results
No results have been posted on this problem yet.
Post a result · how results are checked
Sign in to follow or save this problem
Comments (0)
Comments are not records: they are not signed, stamped or logged. How comments work
No comments yet.
Sign in to comment.