Method for laser spectroscopy of metastable pionic helium atoms
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Method for laser spectroscopy of metastable pionic helium atoms M. Hori1 · A. S´ot´er1 · H. Aghai-Khozani1 · D. Barna2 · A. Dax3 · R. S. Hayano4 · Y. Murakami4 · H. Yamada4
© Springer International Publishing Switzerland 2015
Abstract The PiHe collaboration is currently attempting to carry out laser spectroscopy of metastable pionic helium atoms using the high-intensity π − beam of the ring cyclotron facility of the Paul Scherrer Institute. These atoms are heretofore hypothetical three-body Coulomb systems each composed of a helium nucleus, a π − occupying a Rydberg state, and an electron occupying the 1s ground state. We briefly review the proposed method by which we intend to detect the laser spectroscopic signal. This complements our experiments on metastable antiprotonic helium atoms at CERN. Keywords Pionic atoms · Exotic atoms · Laser spectroscopy
1 Introduction The Pionic Helium (PiHe) collaboration of Paul Scherrer Institut is currently attempting to carry out laser spectroscopy [1] of metastable pionic helium (π He+ ≡ π − + He2+ + e− ) atoms. These are heretofore hypothetical three-body atoms [2, 3], each composed of a helium nucleus, an electron occupying the 1s ground state, and a π − occupying a circular
Proceedings of the International Conference on Exotic Atoms and Related Topics (EXA 2014), Vienna, Austria, 15–19 September 2014. M. Hori
[email protected] 1
Max-Planck-Institut f¨ur Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany
2
CERN, 1211, Geneva 23, Switzerland
3
Paul Scherrer Institut, 5232 Villigen, Switzerland
4
Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
M. Hori et al.
Rydberg state with principal and orbital angular momentum quantum numbers of n ∼ + 1 ∼ 16. The atoms are assumed to be formed by the capture process π − +He → π He+ +e− , that occurs when a beam of π − is allowed to come to rest in a liquid helium target. The atom is the pionic analog of the metastable antiprotonic helium atom pHe+ studied for many years by the ASACUSA collaboration at CERN. In this paper, we review our proposed method to carry out laser spectroscopy as detailed in Ref. [1], and briefly describe the status of our experiment. The Rydberg pionic orbitals have very little overlap with the nucleus, and are expected to retain nanosecond-scale lifetimes against the combined effects of π − absorption in the helium nucleus and π − → μ− + ν μ decay. The electromagnetic cascade processes that normally lead to immediate nuclear absorption are expected to be suppressed in this atom. The existence of π He+ has been indirectly inferred from several experiments carried out in the 1960’s [4–6] and 1990’s [7] that observed that some π − retain a lifetime of ∼ 0.2–7 ns in liquid helium bubble chambers, liquid 4 He targets, and high-pressure 3 He gas targets. Quantitative understanding of these experimental results is difficult; the lifetimes of some π He+ states calculated recently [1] using the complex-coordinate rotation method appear to differ by
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