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Title: First measurement of the parity-violating energy difference between enantiomers
Plain: The left- and right-handed forms of a chiral molecule should differ in energy by a tiny amount because of the weak force. No experiment has yet been sensitive enough to see it.
Precise: $\Delta E_{\mathrm{PV}} = E(R) - E(S)$ between enantiomers, or the corresponding difference in a vibrational or rotational transition frequency, $\Delta \nu/\nu$. The best direct bound is at the $1e-13$ level of relative frequency for the C-F stretch of CHFClBr (Daussy et al., PRL 1999), several orders of magnitude above predictions for that molecule; heavy-atom candidates (Re, Os, Ru, W complexes, scaling roughly as $Z^5$) are predicted to be larger. An answer is a nonzero $\Delta \nu$ or $\Delta E_{\mathrm{PV}}$ with significance above $5\,\sigma$, with sign and magnitude compared to theory.
Settled by: Ultrastable-laser vibrational spectroscopy of both enantiomers of a heavy-atom chiral molecule, or a time-resolved measurement of parity evolution in parity-selected states, reaching the predicted $\Delta E_{\mathrm{PV}}$.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: Experiments with cold molecular beams and heavy-atom candidates were still described as progress toward detection in 2025 (Phys. Chem. Chem. Phys. 2025).
Posed since: (empty)