Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

# Laser spectroscopy of pionic helium atoms

## Abstract

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.

## Access options

from\$8.99

All prices are NET prices.

## 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.

## References

1. 1.

Ericson, T. & Weise, W. Pions and Nuclei (Clarendon Press, 1988).

2. 2.

Ahmadi, M. et al. Observation of the 1S–2S transition in trapped antihydrogen. Nature 541, 506–510 (2017).

3. 3.

DiSciacca, J. et al. One-particle measurement of the antiproton magnetic moment. Phys. Rev. Lett. 110, 130801 (2013).

4. 4.

Ulmer, S. et al. High-precision comparison of the antiproton-to-proton charge-to-mass ratio. Nature 524, 196–199 (2015).

5. 5.

Korobov, V. I., Hilico, L. & Karr, J. -Ph. 7-order corrections in the hydrogen molecular ions and antiprotonic helium. Phys. Rev. Lett. 112, 103003 (2014).

6. 6.

Hori, M. et al. Buffer-gas cooling of antiproton helium to 1.5 to 1.7 K, and antiproton-to-electron mass ratio. Science 354, 610–614 (2016).

7. 7.

Hori, M. et al. Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio. Nature 475, 484–488 (2011).

8. 8.

Salumbides, E. J., Ubachs, W. & Korobov, V. I. Bounds on the fifth forces at sub-Å length scale. J. Mol. Spectrosc. 300, 65–69 (2014).

9. 9.

Ficek, F. et al. Constraints on exotic spin-dependent interactions between matter and antimatter from antiprotonic helium spectroscopy. Phys. Rev. Lett. 120, 183002 (2018).

10. 10.

Assamagan, K. et al. Upper limit of the muon-neutrino mass and charged-pion mass from momentum analysis of a surface muon beam. Phys. Rev. D 53, 6065–6077 (1996).

11. 11.

Trassinelli, M. et al. Measurement of the charged pion mass using X-ray spectroscopy of exotic atoms. Phys. Lett. B 759, 583–588 (2016).

12. 12.

Daum, M., Frosch, R. & Kettle, P.-R. The charged and neutral pion masses revisited. Phys. Lett. B 796, 11–14 (2019).

13. 13.

Particle Data Group. Review of particle physics. Chin. Phys. C 40, 100001 (2016).

14. 14.

Condo, G. T. On the absorption of negative pions by liquid helium. Phys. Lett. 9, 65–66 (1964).

15. 15.

Russell, J. E. Metastable states of απ e , αK e , and $$\alpha \bar{p}{e}^{-}$$ atoms. Phys. Rev. Lett. 23, 63–64 (1969).

16. 16.

Russell, J. E. Structure of neutral mesonic atoms formed in liquid helium. Phys. Rev. A 1, 721–734 (1970).

17. 17.

Hori, M., Sótér, A. & Korobov, V. I. Proposed method for laser spectroscopy of pionic helium to determine the charged-pion mass. Phys. Rev. A 89, 042515 (2014).

18. 18.

Koch, J. & Scheck, F. Quadrupole interaction in pionic and kaonic atoms. Nucl. Phys. A 340, 221–239 (1980).

19. 19.

Trassinelli, M. & Indelicato, P. Relativistic calculations of pionic and kaonic atoms’ hyperfine structure. Phys. Rev. A 76, 012510 (2007).

20. 20.

Daum, E. et al. Pion absorption at rest in 4He. Nucl. Phys. A 589, 553–584 (1995).

21. 21.

Cernigoi, C. et al. Energy spectra of single neutrons and charged particles emitted following the absorption of stopped negative pions in 4He. Nucl. Phys. A 352, 343–354 (1981).

22. 22.

Yukawa, H. On the interaction of elementary particles. I. Proc. Phys.-Math. Soc. Jpn. 17, 48–57 (1935).

23. 23.

Miller, A. I. Werner Heisenberg and the beginning of nuclear physics. Phys. Today 38, 60–68 (1985).

24. 24.

Lattes, C. M. G., Muirhead, H., Occhialini, G. P. S. & Powell, C. F. Processes involving charged mesons. Nature 159, 694–697 (1947).

25. 25.

Fermi, E. & Teller, E. The capture of negative mesotrons in matter. Phys. Rev. 72, 399–408 (1947).

26. 26.

Gotta, D. Precision spectroscopy of light exotic atoms. Prog. Part. Nucl. Phys. 52, 133–195 (2004).

27. 27.

Fetkovich, J. G. & Pewitt, E. J. Experimental study of the cascade time of negative mesons in a liquid helium bubble chamber. Phys. Rev. Lett. 11, 290–293 (1963).

28. 28.

Block, M. M. et al. Moderation time for nuclear capture of negative pions in liquid 4He. Phys. Rev. Lett. 11, 301–303 (1963).

29. 29.

Block, M. M., Kopelman, J. B. & Sun, C. R. Moderation and cascade time of negative pions and negative kaons in liquid helium. Phys. Rev. 140, B143–B152 (1965).

30. 30.

Zaimidoroga, O. A., Sulyaev, R. M. & Tsupko-Sitnikov, V. M. Measurement of the π meson cascade transition time in gaseous 3He. Sov. Phys. JETP 25, 63–64 (1967).

31. 31.

Nakamura, S. N. et al. Negative-pion trapping by a metastable state in liquid helium. Phys. Rev. A 45, 6202–6208 (1992).

32. 32.

Korobov, V. I., Bekbaev, A. K., Aznabayev, D. T. & Zhaugasheva, S. A. Polarizability of the pionic helium atom. J. Phys. B 48, 245006 (2015).

33. 33.

Obreshkov, B. & Bakalov, D. Collisional shift and broadening of the transition lines of pionic helium. Phys. Rev. A 93, 062505 (2016).

34. 34.

Seidel, M. et al. Production of a 1.3 MW proton beam at PSI. In Proc. IPAC10 1309–1313 (Asian Committee for Future Accelerators, 2010).

35. 35.

Abela, R., Foroughi, F. & Renker, D. Muon beams at PSI. Z. Phys. C 56, S240–S242 (1992).

36. 36.

Heidenreich, G. Carbon and beryllium targets at PSI. Proc. AIP 642, 122–124 (2002).

37. 37.

Ritt, S., Dinapoli, R. & Hartmann, U. Application of the DRS chip for fast waveform digitizing. Nucl. Instrum. Methods Phys. Res. A 623, 486–488 (2010).

38. 38.

Cecil, R. A., Anderson, B. D. & Madey, R. Improved predictions of neutron detection efficiency for hydrocarbon scintillators from 1 MeV to about 300 MeV. Nucl. Instrum. Methods 161, 439–447 (1979).

39. 39.

Cocuzzi, M. D., Schepler, K. L. & Powers, P. E. Narrow-bandwidth, subnanosecond, infrared pulse generation in PPLN pumped by a fiber amplifier-microchip oscillator. IEEE J. Sel. Top. Quantum Electron. 15, 372–376 (2009).

40. 40.

Hori, M. et al. Sub-ppm laser spectroscopy of antiprotonic helium and a CPT-violation limit on the antiprotonic charge and mass. Phys. Rev. Lett. 87, 093401 (2001).

41. 41.

Swann, W. C. & Gilbert, S. L. Pressure-induced shift and broadening of 1560–1630-nm carbon monoxide wavelength-calibration lines. J. Opt. Soc. Am. B 19, 2461–2467 (2002).

42. 42.

White, R. T. et al. Control of frequency chirp in nanosecond-pulsed laser spectroscopy. 3. Spectrotemporal dynamics of an injection-seeded optical parametric oscillator. J. Opt. Soc. Am. B 24, 2601–2609 (2007).

43. 43.

Biesheuvel, J., Karr, J. -Ph., Hilico, L., Eikema, K. S., Ubachs, E. W. & Koelemeij, J. C. J. Probing QED and fundamental constants through laser spectroscopy of vibrational transitions in HD+. Nat. Commun. 7, 10385 (2016).

44. 44.

Alighanbari, S., Hansen, M. G., Korobov, V. I. & Schiller, S. Rotational spectroscopy of cold and trapped molecular ions in the Lamb–Dicke regime. Nat. Physics 14, 555–559 (2018).

45. 45.

Sótér, A. et al. Segmented scintillation detectors with silicon photomultiplier readout for measuring antiproton annihilations. Rev. Sci. Instrum. 85, 023302 (2014).

46. 46.

Todoroki, K. et al. Instrumentation for measurement of in-flight annihilations of 130 keV antiprotons on thin target foils. Nucl. Instrum. Methods Phys. Res. A 835, 110–118 (2016).

47. 47.

Martial, I., Balembois, F., Didierjean, J. & Georges, P. Nd:YAG single-crystal fiber as high peak power amplifier of pulses below one nanosecond. Opt. Express 19, 11667–11679 (2011).

48. 48.

Hori, M. & Dax, A. Chirp-corrected, nanosecond Ti:sapphire laser with 6 MHz linewidth for spectroscopy of antiprotonic helium. Opt. Lett. 34, 1273–1275 (2009).

## Acknowledgements

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.

## Author information

Authors

### Contributions

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.

### Corresponding author

Correspondence to Masaki Hori.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Klaus Jungmann and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Rights and permissions

Reprints and Permissions

Hori, M., Aghai-Khozani, H., Sótér, A. et al. Laser spectroscopy of pionic helium atoms. Nature 581, 37–41 (2020). https://doi.org/10.1038/s41586-020-2240-x

• Accepted:

• Published:

• Issue Date:

• ### Laser Spectroscopy Measurements of Metastable Pionic Helium Atoms at Paul Scherrer Institute

• M. Hori
• , H. Aghai-Khozani
• , A. Sótér
• , A. Dax
•  & D. Barna

Few-Body Systems (2021)

• ### Exotic helium atom lit up

Nature (2020)