Location: TRIUMF
Participating institutions: Manitoba, TRIUMF; Mexico, USA
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In atoms, extremely weak electric dipole transitions between states of the same parity are induced by the parity-violating exchange of Z-bosons between the electrons and the quarks in the nucleus, an effect known as atomic parity violation (APV). Measuring this amplitude, one can study neutral-current weak interactions with atomic physics methods and search for ‘new’ physics such as extra gauge bosons and leptoquarks. APV is strongly enhanced in heavy atoms (18 times larger in Fr compared to Cs), but is presently only sufficiently calculable in alkalis. While this makes Fr an obvious choice, its lack of a stable isotope requires work at a radioactive beam facility such as ISAC. There, the FrPNC collaboration has established a laser trap facility that can capture \(\approx 10^6\) atoms of a single Fr isotope in a volume of \(\approx 1\) mm\(^3\) at \(\mu\)K temperatures, an ideal environment for precision laser spectroscopy towards APV measurements.
Following initial commissioning work 1, focus has shifted on the observation of the highly forbidden \(7s - 8s\) transition on which an APV measurement could be based. It was first excited via two-photon spectroscopy 2, and recently observed as a Stark-induced single photon transition with oscillator strength of \(\approx 10^{-10}\) (see Figure 1). In the coming years, a program of spectroscopy of increasingly faint transition amplitudes will be carried out, in particular characterization of the relativistically induced M1, with the ultimate goal of observing the interference between the parity violating amplitude with the much larger, Stark-induced E1. By mid-decade, when ISAC/ARIEL will provide up to 3 beams simultaneously, statistics runs could take place for a competitive APV measurement. On the longer horizon, 2027-36, the Fr laser trap facility could merge into efforts with ultra-cold molecules, photo-associated from laser-cooled atoms, to which Fr would contribute its high sensitivity to new physics, wide range of isotopes, including deformed ones, and structural simplicity.