[论文解读] The helion charge radius from laser spectroscopy of muonic helium-3 ions

The CREMA Collaboration, Karsten Schuhmann+38 more|arXiv (Cornell University)|May 19, 2023

Atomic and Molecular Physics被引用 9

一句话总结

激光光谱学在 muonic helium-3 ions 上得到一个极高精度的 helion (3He) rms 电荷半径 r_h = 1.97007(94) fm，通过 Lamb shift、hyperfine 和 fine-structure 测量。

ABSTRACT

Hydrogen-like light muonic ions, in which one negative muon replaces all the electrons, are extremely sensitive probes of nuclear structure, because the large muon mass increases tremendously the wave function overlap with the nucleus. Using pulsed laser spectroscopy we have measured three 2S-2P transitions in the muonic helium-3 ion ($μ^3$He$^+$), an ion formed by a negative muon and bare helium-3 nucleus. This allowed us to extract the Lamb shift $E(2P_{1/2}-2S_{1/2})= 1258.598(48)^{ m exp}(3)^{ m theo}$ meV, the 2P fine structure splitting $E_{ m FS}^{ m exp} = 144.958(114)$ meV, and the 2S-hyperfine splitting (HFS) $E_{ m HFS}^{ m exp} = -166.495(104)^{ m exp}(3)^{ m theo}$ meV in $μ^3$He$^+$. Comparing these measurements to theory we determine the rms charge radius of the helion ($^3$He nucleus) to be $r_h$ = 1.97007(94) fm. This radius represents a benchmark for few nucleon theories and opens the way for precision tests in $^3$He atoms and $^3$He-ions. This radius is in good agreement with the value from elastic electron scattering, but a factor 15 more accurate. Combining our Lamb shift measurement with our earlier one in $μ^4$He$^+$ we obtain $r_h^2-r_α^2 = 1.0636(6)^{ m exp}(30)^{ m theo}$ fm$^2$ to be compared to results from the isotope shift measurements in regular He atoms, which are however affected by long-standing tensions. By comparing $E_{ m HFS}^{ m exp}$ with theory we also obtain the two-photon-exchange contribution (including higher orders) which is another important benchmark for ab-initio few-nucleon theories aiming at understanding the magnetic and current structure of light nuclei.

研究动机与目标

• 测量 muonic helium-3 ions 中的三个 2S-2P 跳跃，以提取关键能量分裂（Lamb shift、2S hyperfine、2P fine）。
• 通过将实验 Lamb shift 与理论进行比较，提取 helion 的 rms 电荷半径。
• 基准测试 light nuclei 中的 two-photon-exchange contributions 与 nuclear structure effects。

提出的方法

• 通过在 5 T 磁化筒内的低压 3He 靶中使 keV μ 停止，形成 muonic He+ 离子。
• 使用带有多次通过的光学腔的脉冲激光光谱学，在大约 850–940 nm 处驱动 2S→2P 跳跃。
• 在与激光发射同振的时间同时检测来自 2P→1S 去激发的 8 keV Kα X 射线，以定位共振。
• 从对饱和和激光能量变化进行校正的线形拟合中提取跃迁频率。
• 使用更新的 2P 能级理论和 2P 重心约定，求解将测量的能量差与 E_LS、E_HFS、E_FS 联系起来的一组方程。

Figure 1: Scheme (not to scale) of the $n=2$ energy levels in $\mu^{3}$ He + and the measured transitions. Due to the negative magnetic moment of the helion, the ordering of the hyperfine levels is reversed.

实验结果

研究问题

• RQ1 通过三次测量的跃迁推断出的 muonic 3He+ 的 Lamb shift E_LS 是多少？
• RQ2 从数据中提取的 2S 超精细分裂 E_HFS 和 2P 精细分裂 E_FS 是多少？
• RQ3 将 E_LS 与现代理论结合时得到的 helion 电荷半径 r_h 是多少？
• RQ4 测得的 E_HFS 如何约束 two-photon-exchange (2PE) 与高阶核结构贡献？
• RQ5 结果与弹性电子散射及核理论预测相比如何？

主要发现

• r_h = 1.97007(94) fm，由 Lamb shift 结合实验与理论输入推导。
• E_LS^exp = 1258.598(48) meV 与 E_HFS^exp = -166.496(104) meV，在与理论结合后，E_FS^exp = 144.958(114) meV。
• 测得的 E_FS^exp 与理论一致：144.979(5) meV，验证了 2P 能级理论。
• 基于同位素位移的量 r_h^2 - r_alpha^2 = 1.0636(6)(30) fm^2，通过组合 muonic 3He+ 与 muonic 4He+ 数据得到，对常规 He 原子中的 Amsterdam 结果存在 3.6 sigma 的张力。
• 此结果将 helion 半径的精度比先前的弹性散射测定提高约 15 倍，并且与该值一致。

Figure 2: Sketch of the experimental setup. CT cyclotron trap, MEC muon extraction channel, Ti:Sa Titanium-sapphire laser, Yb:YAG thin-disk laser, SHG second harmonic generation. The arrows indicate the electric field of the $E\times B$ Wien-filter.