Recent results of laser spectroscopy experiments of pionic helium atoms at PSI

Masaki Hori,Anna Sótér,Andreas Dax,H Aghai-Khozani,Daniel Barna

doi:10.21468/scipostphysproc.5.026

Abstract

A review of a recent experiment carried out at PSI involving laser spectroscopy of metastable pionic helium (\pi{^4He}^+\equiv\pi^{-}+{^4He}^{2+}+e^-π4He+≡π−+4He2++e−) atoms is presented. An infrared transition (n,\ell)=(17,16)(n,ℓ)=(17,16)\rightarrow→(17,15)(17,15) at a resonance frequency of \nu\approx 183760ν≈183760 GHz was detected.

Highlights

Metastable pionic helium is a neutral exotic atom [1,2,3,4,5,6,7,8] that contains a helium nucleus with an electron in the ground state, and a negatively-charged pion (π−) occupying a state having high principal and orbital angular momentum quantum numbers of around n ∼ + 1 ∼ 16. These states have nanosecond-scale lifetimes against the competing cascade processes of π− nuclear absorption and π− → μ− + νμ decay. This longevity arises because the π− orbitals have very small overlap with the nucleus and so the rates of electromagnetic cascade processes involving the rapid deexcitation of the π−, such as Auger and radiative decays, are significantly reduced
By comparing the atomic frequencies measured by laser spectroscopy with the results of quantum electrodynamics (QED) calculations, the π− mass [10,11,12] can, in principle, be determined with a high precision
An earlier version of the electronics based on the DRS4 application-specific integrated circuit (ASIC) was used in an experiment to determine upper limits on the annihilation cross sections of antiprotons of kinetic energy E ≈ 125 keV on thin target foils [51, 55, 56], the results of which were compared with the cross sections measured at higher energies E = 5.3 MeV [57, 58]

Summary

Introduction

Metastable pionic helium is a neutral exotic atom [1,2,3,4,5,6,7,8] that contains a helium nucleus with an electron in the ground state, and a negatively-charged pion (π−) occupying a state having high principal and orbital angular momentum quantum numbers of around n ∼ + 1 ∼ 16. In the recent PSI experiment, laser pulses excited a transition from a pionic state of the neutral atom that had a nanosecond-scale lifetime, to a state with a picosecond-scale lifetime against Auger decay [5] (Figure 26.1).

Results

Conclusion