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Laser spectroscopy of pionic helium atoms


Charged pions1 are the lightest and longest-lived mesons. Mesonic atoms are formed when an orbital electron in an atom is replaced by a negatively charged meson. Laser spectroscopy of these atoms should permit the mass and other properties of the meson to be determined with high precision and could place upper limits on exotic forces involving mesons (as has been done in other experiments on antiprotons2,3,4,5,6,7,8,9). Determining the mass of the π meson in particular could help to place direct experimental constraints on the mass of the muon antineutrino10,11,12,13. However, laser excitations of mesonic atoms have not been previously achieved because of the small number of atoms that can be synthesized and their typically short (less than one picosecond) lifetimes against absorption of the mesons into the nuclei1. Metastable pionic helium (π4He+) is a hypothetical14,15,16 three-body atom composed of a helium-4 nucleus, an electron and a π occupying a Rydberg state of large principal (n ≈ 16) and orbital angular momentum (l ≈ n − 1) quantum numbers. The π4He+ atom is predicted to have an anomalously long nanosecond-scale lifetime, which could allow laser spectroscopy to be carried out17. Its atomic structure is unique owing to the absence of hyperfine interactions18,19 between the spin-0 π and the 4He nucleus. Here we synthesize π4He+ in a superfluid-helium target and excite the transition (nl) = (17, 16)  (17, 15) of the π-occupied π4He+ orbital at a near-infrared resonance frequency of 183,760 gigahertz. The laser initiates electromagnetic cascade processes that end with the nucleus absorbing the π and undergoing fission20,21. The detection of emerging neutron, proton and deuteron fragments signals the laser-induced resonance in the atom, thereby confirming the presence of π4He+. This work enables the use of the experimental techniques of quantum optics to study a meson.

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Fig. 1: Energy level diagram of π4He+.
Fig. 2: Experimental layout.
Fig. 3: Detection of π arrivals and nuclear absorptions in the helium target, and time spectra of the emerging fission fragments measured with and without laser irradiation.
Fig. 4: Laser resonance signals of the transition (nl) = (17, 16) → (17, 15) in metastable π4He+ atoms.

Data availability

As the datasets generated during and/or analysed during the current study are in total 20 TBytes in size consisting primarily of waveform data, they are available from the corresponding author on reasonable request.


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This work was supported by the Max-Planck Gesellschaft and the European Research Council (ERC-Stg). We thank the staff members of the PSI cyclotron and beamline operations groups, the PSI laboratory for particle physics (LTP), the PSI water, cryogenic and electrical engineering groups, the CERN cryogenics laboratory and its magnet and scintillator workshops, D. Bakalov, T. W. Hänsch, R. S. Hayano, V. I. Korobov, J. Mouleyre, A. Newborough, B. Obreshkov, N. Picque, O. Pirotte, R. Pohl, A. Sakai, T. Udem and T. Yamazaki. The scintillator array and cryogenic target were constructed at CERN within the framework of the ASACUSA collaboration of the Antiproton Decelerator facility. The DRS4 ASIC was procured from PSI LTP. S. Ritt provided technical advice for the design of our electronics. The E × B Wien filter was developed by the PSI beamline operations group. The quadrupole doublet magnet was donated by the CERN magnet group. Y. Murakami, K. Todoroki and H. Yamada provided vital assistance during some phases of the experiments.

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Authors and Affiliations



M.H. conceived the experimental method and developed the DPSS Nd:YAG and OPG-OPA laser systems, 140-channel photomultiplier array with active breeders and high-voltage bias generators, scintillators, data acquisition electronics of the particle detectors, the concentric pair of cylindrical target chambers with 500-μm-thick aluminium walls, cryogenic and beam delivery optics, and field-programmable gate array software. A.S. designed the cryogenic target, including its thermal radiation shields, the compact housing and supports for the particle detectors and electronics, and the scanning scintillation counter used to measure the diameter of the π beam, and carried out Monte Carlo simulations of the experiment. M.H., the CERN cryogenics group and A.S. designed the Joule–Thomson cryocooler and cryogenic pumping line used to cool the target to superfluid temperature. H.A.-K. designed the mechanical support of the cryogenic target including linear translation stages. D.B. wrote the data acquisition software. All authors contributed to the assembly of the experiment and data taking. D.B., A.S. and M.H. independently wrote analysis programs. M.H. wrote the manuscript and all authors discussed the results and contributed to the editing.

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Correspondence to Masaki Hori.

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Hori, M., Aghai-Khozani, H., Sótér, A. et al. Laser spectroscopy of pionic helium atoms. Nature 581, 37–41 (2020).

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