Nuclear Spin-Dependent Parity Violation in Molecules

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Nature is governed by only four fundamentally different forces: electromagnetism, gravity, the strong force (which binds protons and neutrons together in the atomic nucleus), and the weak force (which governs certain radioactive decays and the interactions of some exotic particles). The goal of this project is to investigate certain properties of the weak force. The weak force has some features in common with the more familiar electromagnetic and gravitational forces, but also some differences. Elementary particles (such as electrons and quarks) have a particular electric charge, which determines the strength of electromagnetic force they exert, and a particular mass, which determines the strength of gravitational force they exert. By contrast, each elementary particle has two different numerical properties, analogous to electric charge, that determine the strength of the weak force they can exert. One of these numbers (known as the axial weak charge) gets modified whenever the particle is also being subjected to strong forces, such as when a proton is surrounded by other neutrons and protons inside an atomic nucleus. The precise way that the axial weak charge is modified in the presence of the strong force is a long-standing question in nuclear physics. Substantial effort has been made to understand this behavior, using large-scale experiments at accelerator facilities. In contrast, the approach funded by the present grant uses a room-sized apparatus staffed by only a few people in order to make precise measurements of the quantum mechanical energy levels in molecules. Tiny shifts in these energy levels signal the effect of the weak force, and the scientists can interpret these shifts in terms of the axial weak charge of the atomic nuclei inside the molecule. In a general sense, this project also advances the range of techniques for precision measurement science, which in the past has led to unexpected breakthroughs in technology; for example, in an earlier phase of this project the group invented a new type of magnetic resonance probe that may have broad applications. The supported group uses parity violation (PV) as a tool to isolate the effect of the weak force. To isolate the nuclear axial weak charge they measure how PV depends on the nuclear spin orientation. They use diatomic molecules to achieve a dramatic increase in sensitivity compared to similar experiments with atoms. The small energy splittings and narrow spectral lines associated with hyperfine/rotational structure in molecules enhance the size of nuclear spin-dependent (NSD) PV effects by orders of magnitude relative to those in atoms. The specific goal of this work is to measure the NSD-PV effect associated with the 137Ba nucleus in the molecule BaF. This will provide a measurement of the effective axial weak charge of 137Ba. It is expected that this quantity is dominated by an effect known as the nuclear anapole moment. The anapole moment describes a distribution of magnetic fields within a nucleus that can only be induced by the weak force acting within the nucleus. It has been observed in only one nucleus so far, with a size well outside the expected range based on other kinds of experiments that have probed how the weak force is modified in the presence of strong interactions. The proposed measurement with 137Ba should shed light on this discrepancy. The method used in the proposed work also has the promise to extend to measurements on a wide range of nuclei, which would provide a rich data set for comparison to theoretical models of this phenomenon.