Nuclear anapole moments from molecular spectroscopy
In plain words
The weak force inside a nucleus produces a tiny magnetic feature called the anapole moment, measured so far only for cesium atoms. Polar molecules amplify this effect and could measure it for other nuclei, testing how the weak force acts between protons and neutrons.
Precise statement
Nuclear-spin-dependent parity violation in diatomic molecules (BaF with Ba-137, TlF, RaF and others) measured by tuning opposite-parity rotational-hyperfine levels near degeneracy with a magnetic field; the observable is the matrix element $W_A k_A$, with $W_A$ computed by electronic structure and $k_A$ dominated by the anapole moment. The only measured anapole (Cs-133, Wood et al. 1997) yields weak meson-nucleon couplings that disagree with other hadronic parity-violation data. An answer is $k_A$ for at least one more nucleus with uncertainty below about 20 percent and a test of the Cs inconsistency.
What would settle it
A molecular-beam or laser-cooled-molecule measurement of the parity-violating mixing of near-degenerate levels giving a nonzero $W_{A} k_{A}$ for a nucleus other than Cs-133.
Status in the literature
Unverified note
Sensitivity of the method was demonstrated in 2018 (Altuntas et al., PRL); laser-cooled BaF (Kogel et al., Phys. Rev. Research 2025) and TlF schemes (2023) are in development.