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arXiv:2607.07125

Density effects in precision laser spectroscopy of exotic helium atoms

Hubert J. Jóźwiak, Dimitar Bakalov, Michał Przybytek, Michail Stoilov, Piotr Wcisło

physics.atom-phphysics.chem-phquant-phSubmitted Jul 8, 2026• Announced Jul 9, 2026• 0 citations
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Abstract

Exotic helium atoms act as unique atomic traps for heavy, negatively charged particles, protecting them from nuclear annihilation and nuclear capture on timescales long enough to enable high-precision laser spectroscopy. Such measurements serve as stringent tests of three-body quantum electrodynamics and offer a direct route to determining fundamental particle masses. Motivated by upcoming spectroscopic efforts targeting pionic ($π^{-\,4}\mathrm{He}^+$) and kaonic ($K^{-\,4}\mathrm{He}^+$) helium, we present a rigorous theoretical evaluation of the collisional and density effects governing these systems. Using an ab initio potential energy surface and coupled-channel quantum scattering calculations, we study the collisional stability of the candidate metastable states against inelastic quenching in a cryogenic helium buffer gas. Furthermore, we provide theoretical reference values for the pressure broadening and pressure shift coefficients of the targeted transitions. These results establish an essential benchmark for future experiments, paving the way for refined determinations of the pion and kaon masses.

Comments
15 pages, 8 figures

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Exotic helium atoms act as unique atomic traps for heavy, negatively charged particles, protecting them from nuclear annihilation and nuclear capture on timescales long enough to enable high-precision laser spectroscopy. Such measurements serve as stringent tests of three-body quantum electrodynamics and offer a direct route to determining fundamental particle masses. Motivated by upcoming spectroscopic efforts targeting pionic (π− 4He+π^{-\,4}\mathrm{He}^+π−4He+) and kaonic (K− 4He+K^{-\,4}\mathrm{He}^+K−4He+) helium, we present a rigorous theoretical evaluation of the collisional and density effects governing these systems. Using an ab initio potential energy surface and coupled-channel quantum scattering calculations, we study the collisional stability of the candidate metastable states against inelastic quenching in a cryogenic helium buffer gas. Furthermore, we provide theoretical reference values for the pressure broadening and pressure shift coefficients of the targeted transitions. These results establish an essential benchmark for future experiments, paving the way for refined determinations of the pion and kaon masses.
A laser spectroscopic method was developed to directly observe the lifetimes of individual states in metastable antiprotonic helium atoms (p‾4He+).(\overline{p}{}^{4}{\mathrm{He}}^{+}).(p​4He+). Collisional effects with surrounding helium atoms were studied in cryogenic gas targets at pressures between 0.1 and 9.0 bars and at temperatures between 5.5 and 7.0 K. The metastable state (n,l)=(39,35)(n,\mathcal{l})=(39,35)(n,l)=(39,35) showed a lifetime of 1.5 $\ensuremath{\mu}\mathrm{s}$, regardless of density, whereas the lifetime of the energetically lower-lying state (37,34)(37,34)(37,34) was found to shorten from 1.3 to 0.1 $\ensuremath{\mu}$s with increasing density. The lifetimes of the states (38,34)(38,34)(38,34) and (36,33)(36,33)(36,33) were determined to be 10 ns and 5 ns, respectively, showing the effects of the internal Auger process. When extrapolated to zero density, the measured lifetimes agreed with the results of calculations for the radiative and Auger widths. Using the fact that the state (37,34)(37,34)(37,34) becomes short lived at high density, the resonance $(38,35)\ensuremath{\rightarrow}(37,34)$ at 529.62 nm was detected.
Some cases in which the interpretation of experiments in nuclear, particle, and low temperature physics requires understanding of molecular effects have been studied.Neutrino-mass determination from β decay of T2 requires knowledge of the energy loss due to molecular excitations of the daughter HeT+ molecule. The theoretical final-state distribution has been studied, in particular the role of nonadiabatic effects. Possible origins of unphysical experimental results leading to a negative mass squared are discussed, as are various methods for experimental verification of the final-state distribution.Pionic hydrogen (pπ-) is used in experiments that determine the mass difference mπ- - mp0, and other fundamental properties of pions. Formation of three-body resonances (ppπ-)* in the cascade of pionic hydrogen atoms has been studied. The decay of these state provides an efficient accelerationmechanism for the pionic atoms, with impact on the experimental analysis.Cold antihydrogen will be used at CERN to study the CPT theorem and the weak equivalence principle. We study the collisional interaction of antihydrogen with ultracold hydrogen. It has been found that cooling dominates over annihilation at antihydrogen temperatures down to ~ 0.1 K.In Bose-Einstein condensation of atomic hydrogen the shape of the 1s-2s line is used to probe the temperature and density. Long-range interactions between excited 2s atoms have been studied in order to improve the understanding of line shapes above and below the condensation temperature.
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A series of magic wavelengths for the 23S1 → 23PJ (J = 0, 1, 2) transitions in helium is determined with high precision using the relativistic configuration interaction combined with the radiative potential method. The calculation explicitly incorporates relativistic, finite nuclear mass, and quantum electrodynamic corrections through the inclusion of mass shift and radiative potential operators into the Dirac-Coulomb-Breit Hamiltonian. The resulting magic wavelengths lie entirely within the visible spectrum (400-700 nm) and converge to seven significant digits, making them directly accessible for experimental verification. These accurate predictions serve as benchmarks for testing atomic structure theory and support the development of state-insensitive optical trapping schemes in precision spectroscopy.
The current status of the physics of exotic atoms is reviewed. The problem of the anomalous width of the kaonic helium 2p level is analyzed, experimental CPT invariance tests for the antihydrogen are discussed, and results of investigations on the metastability of the antiproton (as well as of ??, ??, K?, ??, ...) in helium are considered. A comparison is made of the well-known Condo model and the ionic model. In the latter, the ion (?ee) ? a so-called 'ioncule' ? forms after is stopped in its motion in helium. One of the ioncule's electrons is weakly bound, resides at one of the Efimov levels, and orbits at ~ 20 a.u. in the dipole field of the neutral 'atomcule' (?e). The lifetime of the ioncule is long (> 10? 5 s) and the antiproton's vibrational ? rotational transition frequencies ? are practically equal to the atomcule value (?? /? ~ 10? 5 ?10? 6 ) ? in agreement with laser-spectroscopy data. While the ionic model results on the quenching cross sections of metastable states by impurity atoms and molecules in helium agree well with experiment, the atomcule-based Condo model underrates the cross sections by 3 ? 4 orders of magnitude. The ionic model is also consistent with other experiments. Direct experimental tests of this model are discussed. A new interpretation of E Zavattini's group's laser-spectroscopy results on the 2s ? 2p-splitting in muonic helium is presented.