Max Planck Institute of Quantum Optics, Garching, Germany.
QUANTUM, Institut für Physik & Exzellenzcluster PRISMA, Johannes Gutenberg-Universität Mainz, Mainz, Germany.
Nature. 2021 Jan;589(7843):527-531. doi: 10.1038/s41586-021-03183-1. Epub 2021 Jan 27.
The energy levels of hydrogen-like atomic systems can be calculated with great precision. Starting from their quantum mechanical solution, they have been refined over the years to include the electron spin, the relativistic and quantum field effects, and tiny energy shifts related to the complex structure of the nucleus. These energy shifts caused by the nuclear structure are vastly magnified in hydrogen-like systems formed by a negative muon and a nucleus, so spectroscopy of these muonic ions can be used to investigate the nuclear structure with high precision. Here we present the measurement of two 2S-2P transitions in the muonic helium-4 ion that yields a precise determination of the root-mean-square charge radius of the α particle of 1.67824(83) femtometres. This determination from atomic spectroscopy is in excellent agreement with the value from electron scattering, but a factor of 4.8 more precise, providing a benchmark for few-nucleon theories, lattice quantum chromodynamics and electron scattering. This agreement also constrains several beyond-standard-model theories proposed to explain the proton-radius puzzle, in line with recent determinations of the proton charge radius, and establishes spectroscopy of light muonic atoms and ions as a precise tool for studies of nuclear properties.
类氢原子系统的能级可以精确计算。从其量子力学解出发，多年来经过不断改进，包括电子自旋、相对论和量子场效应，以及与原子核复杂结构相关的微小能量位移。在由负μ子和原子核组成的类氢系统中，这些由原子核结构引起的能量位移被大大放大，因此这些μ介子离子的光谱可以用于高精度地研究原子核结构。在这里，我们展示了对μ介子氦-4 离子的两个 2S-2P 跃迁的测量，这一测量精确地确定了α粒子的均方根电荷半径为 1.67824(83)飞米。这种来自原子光谱的测定与电子散射的结果非常吻合，但精确了 4.8 倍，为少数核子理论、格点量子色动力学和电子散射提供了基准。这一吻合也限制了几种用于解释质子半径之谜的超越标准模型理论，与最近对质子电荷半径的测定一致，并确立了轻μ子原子和离子的光谱学作为研究核性质的精确工具。
