YbF, BOB and the eEDM: probing zero with diatomic molecules

The shape of the electron is a fascinating mystery which could prove to unlock a path to physics beyond the standard model. The key to finding this shape, or charge distribution, is a measurement of the electron Electric Dipole Moment, or $e$EDM. The standard model predicts an $e$EDM of less than $10^{-40}e\cdot$cm, but other models, such as string theory, predict values that are orders of magnitude higher. The current experimental upper limit is $\approx 10^{-28}e\cdot$cm, measured with YbF, a diatomic chosen due to huge internal fields in the Yb nucleus. These large fields are essential to resolving the energy level splittings resulting from the $e$EDM. We have performed both Fourier transform microwave spectroscopy and pump-probe optical microwave double resonance spectroscopy on YbF in pursuit of further constraining the Yb nuclear wavefunction, a necessary parameter for the $e$EDM experiments. We present a robust method for fitting multi-isopologue data sets which allows quantitative determination of effects such as the breakdown of approximating the nucleus as a point charge and the breakdown of treating electronic and nuclear wavefunctions as separable.