Fundamental symmetry tests with the francium laser trap facility at ISAC

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The fundamental forces of nature are one of the fascinating subjects that can be studied at the ISAC radioactive beam facility at TRIUMF, Canada's national particle accelerator laboratory. In particular, the weak force, causing the beta -decay of nuclei, is unique in exhibiting the phenomenon of parity- violation: the mirror images of physical processes mediated by the weak force are not observed in nature. ISAC offers the possibility to study atomic parity- violation in francium (Fr), an element combining a very heavy nucleus (Z=87) with the simple, essentially hydrogen-like, valence structure of an alkali. Fr nuclei are the least stable among the first 103 elements in the periodic table, the longest--lived isotope decays in only 23 minutes. To make optimum use of the minute sample, the Fr atoms are slowed down with laser light and captured into a `magneto--optical' laser trap, suspended in free space as a millimeter-sized cloud of up to one millions atoms, at temperatures millions of times colder than room temperature. This permits manipulation of individual atoms with unheard-of precision. To harness these capabilities, we have established the francium trapping facility at ISAC and are now using this francium source to carry out a program of precision experiments using laser and microwave spectroscopy. Previously, amongst other work, we have investigated the distribution of magnetism inside the nuclei in a chain of francium isotopes. Recently we started to excite francium's outer valence electron from its ground state, the 7s orbital, to the 8s state. In the absence of applied external electric fields, this highly forbidden transition is roughly 13 orders of magnitude harder to excite than a typical atomic transition. We now succeeded in observing this process for the first time, by amplifying the available laser light 4000-fold in what is known as a power buildup cavity, where the light bounces between two mirrors whose distance must be kept constant to less than the size of an atom. This device has to reside inside our ultra-high vacuum chamber mounted on a beamline at ISAC, which proved very challenging. This work provides the basis for measuring the signature of the weak interaction: The likelihood to excite 8s is slightly different when observing the mirrored process. By putting the atoms in orthogonal electric and magnetic fields, we can create the mirror flip by reversing either one of the two fields' direction, while keeping the other the same. In the coming years, we will work towards preparing the francium atomic parity violation experiment. Supported by NSERC, NRC/TRIUMF, the University of Manitoba, and the University of Maryland.