该研究团队探讨了电弱相互作用过程中角动量如何守恒的问题。为了尽量简化数学描述，研究人员首先以圆极化光子与原子的弹性散射为例提出这一问题，其中散射后的光子自旋方向与原始光子不同，并指出这一现象也存在于全相对论框架下自旋1/2粒子与中心势的散射中。
接着，研究人员考虑了逆β衰变过程，即一个中微子被原子核捕获后发射出一个电子。在这个过程中，入射的中微子和最终的电子的自旋都与它们的动量方向相反，但电子的最终自旋方向与中微子的不同，看似出现了角动量的变化。然而，在所有这些情况下，最终粒子被测量之前，角动量实际上是守恒的。
角动量看似不守恒的现象出现在量子测量过程中，这是因为测量装置本身在测量前并没有一个明确定义的角动量，但它却能够在外部世界中被定位。研究人员将讨论推广到具有质量的中微子和电子，并同样地研究了核β衰变和电子-正电子湮灭过程，从而能够物理上直观地推导出这些反应中的角度和螺旋度分布。
附：英文原文
Title: Understanding the puzzle of angular momentum conservation in beta decay and related processes
Author: Baym, Gordon, Peng, Jen-Chieh, Pethick, C. J.
Issue&Volume: 2024-11-19
Abstract: We ask the question of how angular momentum is conserved in electroweak interaction processes. To introduce the problem with a minimum of mathematics, we first raise the same issue in elastic scattering of a circularly polarized photon by an atom, where the scattered photon has a different spin direction than the original photon, and note its presence in scattering of a fully relativistic spin-1/2 particle by a central potential. We then consider inverse beta decay in which an electron is emitted following the capture of a neutrino on a nucleus. While both the incident neutrino and final electron spins are antiparallel to their momenta, the final spin is in a different direction than that of the neutrino—an apparent change of angular momentum. However, prior to measurement of the final particle, in all these cases angular momentum is indeed conserved. The apparent nonconservation of angular momentum arises in the quantum measurement process in which the measuring apparatus does not have an initially well-defined angular momentum, but is localized in the outside world. We generalize the discussion to massive neutrinos and electrons, and examine nuclear beta decay and electron-positron annihilation processes through the same lens, enabling physically transparent derivations of angular and helicity distributions in these reactions.
DOI: 10.1073/pnas.2416768121
Source: https://www.pnas.org/doi/abs/10.1073/pnas.2416768121