Laser spectroscopy of antiprotonic helium atoms at CERN’s new ELENA facility

亚稳态反质子氦是一种中性的三体奇异原子，由一个氦核、一个占据1 s态的电子和一个处于大主量子数（n=38）和轨道角动量（l=n-1）的里德堡态的反质子组成。我们建议利用欧洲核子研究中心新的ELENA设施前所未有的高质量光束和最新的激光计量技术，对反质子氦的窄共振进行亚多普勒双光子激光光谱学，最终达到10^-11的分数精度，比过去的实验高两个数量级。这应该可以与Penning陷阱和HD+光谱实验中质子与电子质量比的最佳测定相媲美，并为QED计算提供重要的一致性测试，以及CPT对称性测试。这样就可以得到反质子和氦核之间可能存在的第五种力的上限。在本项目的前三年，我们打算建造和验证一个感应减速器，将由ELENA提供的反质子束的能量从100 keV降低到<30 keV，并使用感应减速器优化反质子氦的激光光谱信号，发现反质子氦的窄双光子共振。

Metastable antiprotonic helium is a neutral three-body exotic atom that consists of a helium nucleus, an electron occupying the 1s state, and an antiproton in a Rydberg state of large principal (n=38) and orbital angular momentum (l=n-1) quantum numbers. We propose to utilize the unprecedented high-quality beam of the new ELENA facility of CERN and the latest laser metrology techniques to carry out sub-Doppler two-photon laser spectroscopy of narrow resonances of antiprotonic helium to ultimately a fractional precision of 10^-11 scale which is two orders of magnitude higher than in past experiments. This should then rival the best determinations of the proton-to-electron mass ratio from Penning trap and HD+ spectroscopy experiments and provide an important consistency test of the QED calculations, as well as a test of CPT symmetry. Upper limits on possible fifth forces between the antiproton and helium nucleus would then be obtained. For the first 3 years of this project, we intend to construct and validate an induction decelerator that reduces the energy of the antiproton beam provided by ELENA from 100 keV to <30 keV, and use the induction decelerator to optimize the laser spectroscopic signal of antiprotonic helium, and discover the narrow two-photon resonances of antiprotonic helium.