缪原子中核极化效应的从头算理论研究

Nuclear polarization provides insights into structures of nuclei through photo-nuclear reaction experiments. It describes how a nucleus responds to an external electromagnetic probe, through transitions to excited and continuum states. Meanwhile, this nuclear observable also plays an important role in atomic spectroscopy experiments. By applying spectroscopy in muonic hydrogen, the proton electric radius has been recently accurately determined. This new result is different by 7 standard deviation from traditional measurements based on electron-proton scattering and hydrogen spectroscopy. This breakthrough discovery challenges the lepton flavor universality and is called the “proton radius puzzle”. To unravel the mystery, a series of experiments around the world will measure Lamb-shift and hyperfine-structure spectra in different muonic atoms, from which the nuclear electric and magnetic radii will be determined. As the current bottleneck of this field, the nuclear polarization effect to atomic spectra dominates the uncertainty in the determination of nuclear radii from muonic atom experiments. This effect comes from the polarization of the nucleus through two-photon-exchange processes between the nucleus and the muon. Therefore, an accurate calculation of the polarization effects is crucial to this research field. Our project is to combine the state-of-the-art chiral nuclear forces with few-body ab initio methods, and to compute the nuclear polarization effects in muonic atoms. Our theoretical approach will significantly improve the accuracy in predicting nuclear polarization, and will make critical contributions to the determination of nuclear radii in muonic atom experiments. Our work can also be used to test and improve the predictive power and accuracy of chiral nuclear forces.

核极化，是光核反应实验中洞察核结构的探针。它反映了核在电磁外场下向激发态和连续态跃迁的响应过程。这一核物理中的观测量也对原子光谱测量实验有着重要意义。最近，质子的电荷半径在缪氢原子光谱实验中被准确地确定。此结果与传统电子-质子散射和氢原子光谱实验相差7个标准差。这一突破性的发现向轻子味道普适性提出了挑战，被称为“质子半径之谜”。为破解谜题，国际上一系列的实验将要测量不同缪原子中的兰姆位移与超精细结构光谱，由此获得其他原子核的电荷与磁荷半径。作为目前该领域的瓶颈，核极化效应对缪原子光谱的贡献主导着实验中获取核半径的精度。这一效应源自核与缪子交换双光子过程中的极化。为应对该研究领域的迫切需要，本项目将结合先进的手征有效核力与少体从头算理论，精确计算缪原子中的核极化效应。本研究将极大地提高核极化效应的预测精度，为未来的缪原子实验提供关键的理论支持，并可用来检验与提高手征核力的准确度。

核极化效应是μ原子光谱测量实验中精确提取原子核半径的关键理论输入量，在探究“质子半径之谜”这一科学课题中发挥着重要作用。在本项目中，研究团队开展了μ原子中核极化效应的理论计算。通过优化原子理论模型，发展核响应函数电磁多极矩展开计算方法，实现对μ子-原子核双光子交换过程的高精度描述。通过拓展核力模型，在无pi介子有效场理论框架下实现对μ氘原子中核极化效应的精确计算，并对核力模型系统误差进行准确分析。本项目研究取得了一系列成果，也为后续进一步开展相关工作打下了良好的基础。相关工作在项目执行期间在Physical Review C，Journal Physics G与《原子核物理评论》杂志上发表期刊论文共3篇。在国内国际会议做受邀报告共计6次。