Trapped chiral molecular ions for precision sensing of parity violation - a new experiment

The prediction that weak force parity violation (PV) breaks the symmetry between the left and right-handed chiral molecules has eluded detection for decades. Although, the field is dominated by neutral molecule experiments, the recent success of trapped molecular ions in eEDM searches presents a different avenue toward this goal. Despite the potential, the lack of theory on chiral molecular ions makes it challenging to select a species to initiate an experiment. Importantly, the ideal candidate, must be preparable at internally cold temperatures and have efficient detection methods, in addition to exhibiting a large PV shift.

We have found several intriguing candidates to search for PV, the most promising of which is CHDBrI⁺. Ab initio theory predicts a >1 Hz shift between L and R molecules for the C-H bend vibrational transition, where the transition’s linewidth is >10x narrower than the shift. Additionally, the C-H stretch transition is also predicted to exhibit a similarly large shift, but with a shorter natural lifetime. Our plan is to prepare cold CHDBrI⁺ through state-selective, near-threshold photoionization of neutral CHDBrI. Resolved detection of the molecular ion's internal state will be performed through sensing photofragment velocities in our ion trap that is integrated with velocity map imaging.

We plan to extract the PV signature from a mixed chirality ensemble of trapped CHDBrI⁺, using vibrational Ramsey spectroscopy that is embedded within the 3-wave mixing (3WM) framework. So far 3WM has been demonstrated using microwaves to separate molecules according to their handedness using asymmetry of the chiral molecules’ transition dipole moment components. We propose to extend these ideas for differential precision spectroscopy between the two chiralities, which is critical to suppress noise and sources of systematic uncertainty.

Crucially, comparison between the two chiralities of the molecule isolates all PV interactions, which arise from the Standard Model and beyond. We will discuss the possibility of using trapped chiral molecular ions to search for new physics.