Measurement of the highly forbidden M1 transition in 7s-8s transition in francium 211.

Atomic parity violation (APV) in weak interaction provides a pathway to test the Standard Model at a uniquely low momentum scale. We are working towards a test of in radioactive francium at the ISAC radioactive beam facility at TRIUMF. We trap and cool francium atoms in a dual magneto optical trap and perform atomic spectroscopy by driving the highly forbidden 7s-8s transition in francium. The hyperfine interaction and relativistic effects give rise to a very faint M1 transition, which is 13 orders of magnitude weaker than allowed transitions. In a first experiment, we measured the M1 amplitude with an accuracy of better than 10 percent. To drive this transition, we intensify the 506 nm light in the interaction region ≈ 4000-fold using a power buildup cavity. Recently we demonstrated near-unity detection efficiency by driving atoms that have undergone the 7s-8s excitation on a cycling transition up to 5000 times. Overall, we have increased the signal rate more than million-fold. This puts us in terms of statistics in reach of taking APV data. We will present details of this work.