Nuclear Spin-Dependent Parity Violation in Diatomic Molecules

Nuclear spin-dependent parity violation (NSD-PV) effects arise from exchange of the $Z^0$ boson (parametrized by the electroweak coupling constants $C_{2P,N})$ between electrons and the nucleus, and from the interaction of electrons with the nuclear anapole moment, a parity-odd magnetic moment. The latter scales with the nucleon number $A$ of the nucleus as $A^{3/2}$, while the $Z^0$ coupling is independent of $A$; the former will be the dominant source of NSD-PV in nuclei with $A$ greater than 20. NSD-PV effects can be dramatically amplified in diatomic molecules by bringing two levels of opposite parity close to degeneracy in a strong magnetic field. This opens the prospect for measurements across a broad range of nuclei. As a precursor to the measurement of the nuclear anapole moment of $^{137}Ba$, we have experimentally observed and characterized opposite-parity level crossings in $^{138}BaF$. These are found to be in excellent agreement with parameter-free predictions and indicate that the sensitivity necessary for NSD-PV measurements should be within reach.