应用光核反应计算μ原子中的核极化效应

Application of Photonuclear Reaction to Evaluating Nuclear Polarizability in Muonic Atoms

近年来,瑞士保罗谢勒研究所的CREMA合作组通过测量μ氢原子兰姆位移显著提升了质子半径的测量精确度。然而这一新实验结果与已知质子半径标准值(CODATA)相差5.6个标准差,被称为质子半径之谜,受到众多物理学家的关注。受此启发,CREMA合作组在不同的μ原子中展开了一系列兰姆位移光谱的测量实验。他们计划从这些μ原子的测量中得到轻核(包括^(2,3)H,^(3,4)He)的电荷半径。除了对光谱测量精度的要求外,轻核半径的实验精度当前仍被一项理论输入量限制:核极化效应对μ原子光谱的修正。核极化效应体现了μ子与原子核进行双光子交换中对核的虚激发,进而对μ原子能谱产生高阶修正。因此,这一效应与光核反应以及康普顿散射直接相关。核极化效应对兰姆位移的修正可通过计算光核吸收截面以及虚光子康普顿振幅的求和规则而得到。本工作运用第一性原理的核结构计算方法,研究了μ原子中的核极化效应。通过结合现代核力模型与超球简谐基展开多体方法,计算了一系列与核极化相关的光核反应及康普顿散射求和规则。这一理论研究为从μ原子光谱测量中对核半径的精确提取提供了关键性的理论输入。

Recent measurements by CREMA Collaboration in Paul Scherrer Institute(Switzerland) determined the proton radius in Lamb shift spectroscopy of muonic hydrogen with a significantly improved precision.However,they discovered that this determination differs from the well-accepted CODATA value by 5.6 standard deviation.This discovery is named the "proton radius puzzle",and attracted interests of many physicists.Inspired by this work,the CREMA Collaboration extended their experiments in muonic hydrogen to a series of light muonic atoms/ions,including μ^(2,3)H and μ^(3,4) He+.They planned to extract the radii of light nuclei(i.e.,^(2,3)H,^(3,4)He) from Lamb shift measurements in muonic atoms.Besides the spectroscopy precision,the accuracy of nuclear radii is limited by one theoretical input,i.e.,nuclear polarizability.Nuclear polarizability originates from virtual excitation of the nucleus during the two-photon exchange process.This effect can make higher-order corrections to the muonic atom spectrum.Polarizability is strongly connected with the photonuclear reaction and the virtual Compton scattering.Therefore,its correction to the Lamb shift can be obtained by evaluating the sum rules of photoabsorption cross sections and forward virtual Compton amplitudes.Using ab initio methods,we calculated the nuclear polarizability effects in muonic atoms.By utilizing modern nuclear force models and the hyperspherical harmonic manybody approaches,we calculated a series of photonuclear sum rules,which are correlated with the nuclear polarizability.This theoretical work provides key input to the high-precision determination of nuclear radii in muonic atom spectroscopy.