Radioactive Atoms and Molecules for Fundamental Physics

Abstract

The Standard Model (SM) of particle physics and the theory of General Relativity represent two of the greatest achievements in physics in the past century. However, despite their success, many experimental observations remain unanswered: What is the nature of Dark Matter and Dark Energy? Why is there so little antimatter in the Universe? Why is gravity so weak compared to the other fundamental forces? These questions point to the existence of new phenomena waiting to be discovered. High-precision laser spectroscopy experiments using atoms and molecules emerged as a fruitful approach for searching for new physics effects. Recently, atoms and molecules containing short-lived radioactive isotopes have been proposed as particularly sensitive laboratories to search for physics beyond the SM, especially at the nuclear level. However, many atoms containing very short-lived isotopes are still out of reach for spectroscopic investigations, while radioactive molecules have been completely inaccessible experimentally until recently.

In this thesis, I will present a series of pioneering experiments aimed at harnessing the power of radioactive atoms and molecules to explore nuclear phenomena, both within and beyond the SM. I will start by describing the first-ever precision laser spectroscopy investigation of a radioactive molecule, radium monofluoride (RaF). I will present measurements of the vibrational, rotational, and hyperfine spectrum of RaF, proving its high sensitivity to minuscule nuclear effects. These experiments allowed the quantification of a feasible laser-cooling scheme for RaF and the observation of the effect of the distribution of nuclear magnetization inside the Ra nucleus on the energy levels of RaF. To our knowledge, this is the first time this effect was observed in a molecule, opening the way for using molecules to benchmark ab initio nuclear theory. Finally, I will present measurements of the ionization potential of RaF, showing its suitability for Rydberg states studies and precise quantum control using external electric fields.

I will then present the theoretical calculations and the status of an experiment aiming to measure hadronic parity violation using single molecular ions inside a Penning trap. The experiment's goal is to use the external magnetic field provided by the trap to fine-tune molecular energy levels of opposite parity close to degeneracy, thus increasing the signal produced by parity violating nuclear properties. The sensitivity to the sought-after signal is expected to be increased by more than twelve orders of magnitude compared to atoms. This amplification will allow the observation of yet-to-be-measured parity violating effects in a molecule. These measurements will be critical to guide our understanding of electroweak nuclear phenomena.

Finally, I will show preliminary results obtained from a novel experiment with the goal of enabling laser spectroscopy studies of atoms and molecules containing radioactive nuclei with lifetimes of 1 ms and below. Such isotopes can't be currently studied spectroscopically. Using an event-by-event Doppler reconstruction, our approach could overcome most of the challenges encountered by state-of-the-art experimental techniques, allowing us to extend our reach toward unexplored regions of the nuclear chart. Such short-lived isotopes are of great importance for our microscopic understanding of nuclei as well as for constraining the properties of nuclear matter.