The \overline{\mbox{\sf P}}{\sf ANDA} detector at FAIR

Selected problems of collisional processes in exotic atoms are discussed. The emphasis is on the collisional effects in antiprotonic helium including formation of antiprotonic atoms, collisional quenching of hot and thermalized metastable antiprotonic helium, shift and broadening of E1 and M1 spectral lines.     

Capture of slow antiprotons by helium atoms

A consistent quantum mechanical calculation of partial cross-sections leading to different final states of antiprotonic helium atom was performed. For the four-body scattering wave function, corresponding to the initial state, as well as for the antiprotonic helium wave function, appearing in the final state, adiabatic approximations were used. Further, symmetric and non-symmetric effective charge (SEC, NEC) approximations were introduced for the two-electron wave functions in the field of the two fixed charges of the He nucleus and the antiproton. Calculations were carried out for a wide range of antiprotonic helium states and incident energies of the antiproton below the first ionization threshold of the He atom. The origin of the rich low-energy structure of certain cross-sections is discussed in detail.     

Observation of cold, long-lived antiprotonic helium ions

Cold, two-body antiprotonic helium ions p 4He2+ and p 3He2+ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ni=28-32 and li=ni-1 have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature T approximately 10 K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p 3He2+ against annihilation induced by collisions were shorter than those of p 4He2+, and decreased for larger-ni states.     

Microwave spectroscopy measurements of the hyperfine structure in antiprotonic 3He

We shall present here the first experimental results for microwave spectroscopy of the hyperfine structure of antiprotonic He-3 and a comparison to numerical simulations of the measurement. Due to the helium nuclear spin, antiprotonic He-3 has a more complex hyperfine structure than antiprotonic He-4 which has already been studied before. Thus a comparison between theoretical calculations and the experimental results will provide a more stringent test of the three-body quantum electrodynamics (QED) theory. The comparison of measured data to simulations allows to investigate the collisional processes between the helium atoms of the target medium and the antiprotonic helium atomcules. The collision rates can not be calculated exactly, but estimated by comparison of numeric simulations with the experimental results. Two out of four super-super-hyperfine (SSHF) transition lines of the (n, L)?=?(36, 34) state were observed. The measured frequencies of the individual transitions are 11.12559(14)?GHz and 11.15839(18)?GHz, less than 1?MHz higher than the current theoretical values, but still within their estimated errors. The frequency difference between the two lines also agrees with theoretical calculations.     

Possible formation of antihydrogen atoms from metastable antiprotonic helium atoms and positrons/positroniums

The possible formation of antihydrogen atoms via the collision of metastable antiprotonic helium atoms with positrons and positroniums is discussed based on the known behavior of positrons in helium media.     

Antiprotonic helium

Recent measurements of the 2p and 3d atomic levels and lifetimes in antiprotonic helium seem to indicate an inconsistency, when interpreted in terms of an optical potential based on parameters extracted from heavier antiprotonic atoms. Here it is shown that this inconsistency is removed, when the data is described in terms of a multiple scattering formalism that treats correctly the number of nucleons and their correlations in the rescattering terms.     

Kaonic helium atoms

Recent progress on the x-ray spectroscopy of kaonic helium atoms as well as on the precision laser spectroscopy of antiprotonic helium atoms are presented. Some historical background connecting these two exotic helium atoms is also discussed.     

The {\bar p}-He atom – an antiproton trap

The experiments on metastable antiprotonic helium atoms are reviewed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fine and hyperfine structure of antiprotonic helium

We consider the fine and hyperfine splitting of the energy levels of antiprotonic helium pHe⁺ in highly excited rotational states in connection with the interpretation of the recently discovered delayed annihilation of antiprotons in helium, based on the Condo model.     

Quenching of metastable states of antiprotonic helium by foreign gas admixtures

The interaction of antiprotonic helium atoms with admixtures of foreign gases was investigated using laser spectroscopy techniques. This revised version was published online in August 2006 with corrections to the Cover Date.     

Antiprotonic helium spectroscopy. Toward better than 10 digit precision

The recent progress in precision spectroscopy of the antiprotonic helium is outlined. It allows to claim that an accuracy achieved in the theoretical predictions for the ro-vibrational transition frequencies has relative uncertainty at a level of 10^?10.     

Auger decay rates of antiprotonic helium

Auger decay rates of the metastable antiprotonic helium ^3,4He e are calculated. The variational method and solution of coupled differential equations are combined to determine the initial metastable state wave function. Besides metastable states, the calculation reveals specific short-lived states of the antiprotonic helium with an essentially different structure of the wave function. An effect of mixture of the wave functions is taken into account to calculate the decay rate for a few metastable states, which are close in energy to the short-lived ones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Search for electric dipole moments at storage rings

Twenty years ago, we published a paper entitled “Discovery of antiproton trapping by long-lived metastable states in liquid helium”. In retrospect, this was the discovery of antiprotonic helium atoms, the study of which is actively being done at CERN’s antiproton decelerator. A brief overview of this interesting exotic atom is given, together with some historical background.     

Metastable antiprotonic helium atomcules

The present status and future scope of the metastabe antiprotonic helium atomcules ¯pHe⁺ are described. Recently developed laser resonance spectroscopy has not only established the atomcule model but also triggered high-precision calculations of these atomcules.     

Resonances in Three-Body Systems Studied by a Full Angular-Momentum, Smooth-Exterior Complex-Scaling Finite-Element Method

 The development of a tool for calculating resonances and bound states in three-body systems described by a single-potential energy surface is reported. The method has been applied to the antiprotonic helium, doubly excited states in helium, the ¹¹Li nuclear halo, the NeICl van der Waals molecule, and the recently found FHD reaction complex. Received November 26, 2001; accepted for publication November 28, 2001     

Two-photon spectroscopy of antiprotonic helium

The precision of laser spectroscopy of antiprotonic helium (a helium atom with one of its electrons replaced by an antiproton) has improved by almost 4 orders of magnitude over its 20 years of history. Experimental transition frequencies can be compared to 3-body QED calculations to derive the antiproton-electron mass ratio. In the latest measurements of the Asacusa experiment at CERN, two-photon transitions of antiprotonic helium were excited using two counter-propagating laser beams. This method reduces the Doppler-broadening caused by the thermal motion of the atoms, and allowed us to measure the transition frequencies with a fractional precision of 2.5–5 parts in 10⁹. From these frequencies, we derived an antiproton-electron mass ratio of 1836.1526736(23). Our precision approaches that of the experimental value of the proton-electron mass ratio, and agrees with the latter within errors. Assuming CPT symmetry (i.e. \(m_{p}=m_{\overline {p}}\) ), we further derived the electron’s atomic mass as m _e = 0.0005485799091(7)u from the more accurately known atomic mass of the proton.     

Anomalous bumpy structures in the capture cross sections of antiprotons by helium

We investigate the state-specified capture process of antiprotons by helium. Freezing one of the two electrons, we reduce this four-body rearrangement problem into a three-body problem. The capture cross sections are calculated by solving the Chew-Goldberger-type integral equation. Differing from the capture of antiprotons by hydrogen atoms, the bumpy structures are revealed in the total angular momentum dependent capture cross sections. Further analysis shows that the bumps arise from the partial channel closing due to the removal of the energy degeneracy in the antiprotonic helium.     

Global-Vector Representation of the Angular Motion of Few-Particle Systems

The angular motion of a few-body system is described by introducing angles that depend on the positions of the particles. The most adequate angles are determined variationally. The numerical examples presented for the molecule, Li atom, antiprotonic helium atom, triton and alpha particle convincingly show that this method is an excellent alternative of the partial-wave decomposition of the wave function. Received November 4, 1997; accepted in final form January 19, 1998     

Measurement of the hyperfine structure of antiprotonic helium

This paper discusses the level splitting of the antiprotonic helium atomcule, an exotic atom consisting of a helium nucleus, an electron, and an antiproton which occupies highly excited states with angular momentum of L ≈ 30–35. The observation of a splitting in a laser transition between two levels is presented, and an experiment in preparation at the forthcoming AD facility at CERN is described which aims at directly measuring the level splitting by using a 2-laser microwave triple resonance technique. This experiment has the potential to determine the magnetic moment of the antiproton with higher precision than currently known. This revised version was published online in August 2006 with corrections to the Cover Date.     

Metastable States in Antiprotonic Helium Atoms: An Island of Stability in a Sea of Continuum

 In this contribution we consider a phenomenon of metastable states in antiprotonic helium atoms, precise spectroscopy of these states and a present-day study of the electromagnetic properties of antiprotons. Received October 16, 2001; accepted for publication November 13, 2001