The exotic helium atom, consisting of a helium nucleus
He2+, an electron ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$, and a massive, negatively charged particle ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$, such as an antiproton (p\ifmmode\bar\else\textasciimacron\fi{}) or a kaon (${\mathit{K}}^{\mathrm{\ensuremath{-}}}$), is studied theoretically. This exotic atom is more like a polar molecule with two nuclei
He2+ and ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$, if ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ is in a low-lying orbital and ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ is in a highly excited orbital. Such states are generally formed in the capture of ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ by helium. The Born-Oppenheimer (BO) separation of the ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ motion and the
He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ motion is a good approximation for these states, and affords a transparent, unified perspective of exotic helium with different ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ and different isotopes of
He2+. A propensity rule that favors small transition energies (except for the smallest one) is found for radiative emission by low vibrational and high rotational levels. This rule resembles the selection rule for infrared emission by the usual diatomic polar molecules, and contradicts the conventional expectation that radiative transitions in exotic atoms favor larger transition energies. The radiative lifetimes \ensuremath{\tau} of highly excited states formed immediately after the capture of ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ are of the order of \ensuremath{\mu}sec for
4He2+-p\ifmmode\bar\else\textasciimacron\fi{}-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ and
3He2+-p\ifmmode\bar\else\textasciimacron\fi{}-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$, and those for
4He2+-${\mathit{K}}^{\mathrm{\ensuremath{-}}}$-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ are about half the values for the antiprotonic helium.The effect of the electronic charge polarization by ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ on \ensuremath{\tau} is explained quantitatively in a simple picture in terms of the molecular dipole moment. Low vibrational, high rotational states are described as circular or near-circular states in the atomic picture. They are stable against Auger decay, which the nonadiabatic perturbation on the BO states induces. An infinite series of bound \ensuremath{\Sigma} electronic states of exotic helium negative ions (
He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$)-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ exists for any
He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ distances R beyond 0.819 a.u. in the BO approximation. An infinite \ensuremath{\Pi} series exists for R>3.811 a.u., and an infinite \ensuremath{\Delta} series for R>9.532 a.u.
Antiproton, the antiparticle of proton, is a unique projectile in the study of atomic collision physics, which can be treated theoretically either as a `negative proton' or a `heavy electron'. Atomic capture of an antiproton will result in formation of a highly excited exotic atom. Antiprotonic helium atom has been studied intensively by means of precision laser spectroscopy, which has led to a stringent determination of antiproton mass and charge to a level of ppb. Comparison of these values with those of proton gives one of the best tests of CPT invariance, the most fundamental symmetry in physics. However, the dynamic processes of antiproton capture remain unclarified, except for some indirect information given by those optical and X-ray observations. With an aim to produce an antiproton beam at atomic-physics energies for `pure' collision experiments, we have so far developed techniques to decelerate, cool and confine antiprotons in vacuo, using a sequential combination of the Antiproton Decelerator (AD) at CERN, a Radio-Frequency Quadrupole Decelerator (RFQD), and an electromagnetic trap. Our recent success in stable extraction of monoenergetic ultra-slow antiprotons, about 3×105 in number available every 5 minutes, has opened up the possibility to study ionization and atomic capture processes between an antiproton thus provided as a beam and an atom prepared in the form of a supersonic gas-jet target, at an unprecedented low energy from 10 eV to 1 keV under the single-collision condition. Our design and strategy of the cross-beam experiments are discussed.
About 3% of antiprotons () stopped in helium are long-lived with microsecond lifetimes, against picoseconds in all other materials. This unusual longevity has been ascribed to the trapping of on metastable bound states in He+ helium atom-molecules thus named atomcules. Apart from their unique dual structure investigated by laser spectroscopy – a near-circular quasi-classical Rydberg atom with l ~ n – 1 ~ 37 or a special diatomic molecule with a negatively charged nucleus in high rotational state with J = l – the chemical physics aspects of their interaction with other atoms or molecules constitute an interesting topic for molecular physics. While atomcules may resist to million collisions in helium, molecular contaminants such as H2 are likely to destroy them in a single one, down to very low temperatures. In the Born-Oppenheimer framework, we interpret the molecular interaction obtained by ab initio quantum chemical calculations in terms of classical reactive channels, with activation barriers accounting for the experiments carried out in He and H2. From classical trajectory Monte Carlo simulations, we show that the thermalization stage strongly quenches initial populations, thus reduced to a recovered 3 % trapping fraction. This work illustrates the pertinence of chemical physics concepts to the study of exotic processes involving antimatter. New insights into the physico-chemistry of cold interstellar radicals are anticipated.
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+) and kaonic (
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.
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.
The international conference "Exotic atoms and related topics 2005" (EXA05) was the second such meeting organized by the Stefan Meyer Institute for Subatomic Physics of the Austrian Academy of Sciences in Vienna. The first meeting in 2002 was an international workshop that lasted three days; 33 talks were presented and about 80 scientists attended. The next conference lasted four days, with 53 speakers and more than 120 participants. The large increase in both speakers and participants shows how dynamic the fields being discussed are. Both meetings presented those fields of physics that can be investigated using exotic atoms, namely, fundamental symmetries and interactions. Examples include the testing of CPT symmetry with atoms containing antiparticles, and the investigation of low-energy QCD with hadronic atoms in low-lying states. For antiprotonic atoms, the spectroscopy of antiprotonic helium is defining the limits of the equality of proton-antiproton charge and mass with increasing accuracy. The formation and spectroscopy of antihydrogen, potentially the best candidate for precision tests of CPT symmetry in atomic systems, is being undertaken by several experiments at the Antiproton Decelerator at CERN, although formation studies are still in their early stages. The future of research into low-energy antiproton physics seems promising, due to the partial approval of the FLAIR facility at the future FAIR facility in GSI. Currently, the fastest developing field is the kaon-nucleon interaction, where the existence of a series of deeply bound states has been predicted. The first experimental evidence for this was shown at the conference and further experiments in progress were described. If confirmed, these so-called "K-clusters" will open new possibilities for studying dense cold matter in the laboratory, since calculations predict nuclear densities in these systems to be as high as those in neutron stars.