Nuclear Spin-Dependent Parity Violation in Molecules

自然界只有四种根本不同的力：电磁力、引力、强力(在原子核中将质子和中子结合在一起)和弱力(控制某些放射性衰变和一些奇异粒子的相互作用)。这个项目的目标是研究弱力的某些性质。弱力与更常见的电磁力和引力有一些共同之处，但也有一些不同。基本粒子(如电子和夸克)有一个特定的电荷，这决定了它们所施加的电磁力的强度，以及一个特定的质量，它决定了它们所施加的引力的强度。相比之下，每个基本粒子都有两种不同的数值特性，类似于电荷，这两种特性决定了它们所能施加的弱力的强度。每当粒子也受到强力时，其中一个数字(称为轴向弱电荷)就会被修改，例如当一个质子被原子核内的其他中子和质子包围时。轴向弱电荷在强力作用下的精确修正方式是核物理学中一个长期存在的问题。通过在加速器设施进行的大规模实验，人们已经做出了很大的努力来理解这种行为。相比之下，目前拨款资助的方法使用的是一个房间大小的仪器，只有几个人操作，以便精确测量分子中的量子力学能级。这些能级的微小变化表明了弱力的影响，科学家们可以用分子内原子核的轴向弱电荷来解释这些变化。在一般意义上，该项目还推进了精密测量科学的技术范围，这在过去导致了技术上意想不到的突破；例如，在该项目的早期阶段，该小组发明了一种可能具有广泛应用的新型磁共振探头。被支持的组使用奇偶破坏(PV)作为工具来隔离弱力的影响。为了分离核轴向弱电荷，他们测量了PV如何依赖于核自旋方向。与类似的原子实验相比，他们使用双原子分子实现了敏感度的显著提高。分子中与超精细/转动结构相关的小能量分裂和窄谱线使核自旋相关(NSD)PV效应的大小比原子中的增强了一个数量级。这项工作的具体目标是测量与BaF分子中的137Ba核相关的NSD-PV效应。这将为137Ba的有效轴向弱电荷提供测量。预计这个量是由一种被称为核极极矩的效应所主导的。极点力矩描述了原子核内的磁场分布，这种分布只能由原子核内的弱作用力引起。到目前为止，它只在一个原子核中观察到，其大小远远超出了预期的范围，这是基于其他类型的实验，这些实验探索了在强相互作用存在的情况下，弱力是如何被修正的。拟议中的137BA测量应能揭示这一差异。拟议工作中使用的方法还有望扩展到对大范围原子核的测量，这将提供丰富的数据集，用于与这一现象的理论模型进行比较。

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.