The capture of slow antiprotons in helium, neon and argon

We consider the capture of antiprotons with energies less than roughly 27 eV by helium, neon and argon atoms. We use the adiabatic ‘hidden-crossing’ theory to calculate both the capture cross-sections and the angular distribution of the antiprotons. We find that important differences between helium and the other noble gases can be attributed to the properties of the ‘hidden’ crossings appropriate to each system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Capture and cooling of antiprotons

Approximately one million antiprotons have been captured at a few keV in a half meter long cylindrical Penning trap from a single, fast extracted pulse of antiprotons at the Low Energy Antiproton Ring (LEAR) at CERN. By electron cooling more than 65% were collected in a harmonic Penning trap. Here they can be confined for more than three hours. This is one of the first important steps towards the synthesis of antihydrogen at rest for tests of CPT. This revised version was published online in August 2006 with corrections to the Cover Date.     

Scattering of protons and antiprotons by hydrogen atoms

The two-,four-,five-and fourteen-state approximations of the inpact parameter method have been applied to the excitation of hydrogen atoms by proton(p)and antiproton(p) impact.The effect of both channel and back couplings on the 2s and 2p excitations are investigated.The total cross sections are calculated for incident energies ranging from 1 to 2500keV.it is found that the effect of both channel and back couplings on the antiproton-induced reactions is greater than on that induced by protons.We compare the results with those of other theoretical and experimental works.     

Angular and energy differential electron emission cross sections in collisions between antiprotons and helium atoms

We present theoretical singly- and doubly-differential ionization cross sections in collisions between antiprotons and helium atoms at 50 and 100 keV impact energies. The calculations were performed within the frame work of the continuum distorted wave eikonal initial state approximation and a three-body classical trajectory Monte Carlo method. We find clear evidence for the formation of the anti-cusp in the differential distributions.     

Charge asymmetry in alignment of atoms excited by protons and antiprotons

The multichannel diffraction approximation is used to consider excitation of lithium atom by proton and antiproton impact. The sign-of-charge effect in the alignment of produced 1s²3d excited state and in the linear polarization of the subsequent spontaneous 1s²3d→1s²2p radiation is expected to be considerable.     

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.     

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.     

Protonium formation in collisions of antiprotons with hydrogen atoms

The formation of an antiprotonic hydrogen atom $\{ p\bar p\} $ (protonium) in the ground state and in excited states (nlm) in collisions of 1-to 250-keV antiprotons with hydrogen atoms is considered theoretically. Partial cross sections (both differential and integrated ones) for this reaction are calculated for various values of n, l, and m up to n～100; cross sections summed over l and m are also obtained. Statistical (polarization) tensors of the orbital angular momentum of protonium atoms are evaluated. Corrections due to strong proton-antiproton interaction in the protonium ground (1s) state are analyzed.     

Antihydrogen synthesis by the reaction of antiprotons with excited state positronium atoms

Aspects of the possible reactions of trapped antiprotons with excited state positronium atoms to form antihydrogen are discussed. Conditions are identified whereby the antihydrogen produced may be suitable for capture in a neutral trap. A discussion is given of possible use of antihydrogen to test the quantization of electric charge involving precision comparisons of hydrogen and antihydrogen (Rydberg constants), and proton and antiproton cyclotron frequencies.     

Annihilation of slow antiprotons in flight and in traps

The antiproton-nucleus annihilation in two possible experimental situations (low energy in-flight annihilation and annihilation in a trap) is investigated taking into account the Coulomb interaction. For these two cases, the formulas giving the relation between the antiproton-nucleus scattering lengtha _cs and observables (annihilation cross section and antiprotons time-of-life in a trap) are obtained. These relations are proposed to determinea _cs.     

The scattering of slow atoms by light

A kinetic equation has been obtained describing the motion of atoms in a resonant field of standing light wave. This equation is used to describe the scattering of atoms for conditions close to experimental ones [1]. It is shown that the dependence of atomic scattering has a dip at the resonance frequency of the field.     

ASACUSA: atomic spectroscopy and collisions using slow antiprotons

The ASACUSA experiment [1] was approved last year for the Antiproton Decelerator at CERN. Its aim is to study basic atomic processes of slow antiprotons: stopping power and ionization in low-pressure gases, Coulomb capture of antiprotons and to make laser spectroscopy of antiprotonic transitions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Diffraction of metastable helium atoms by a transmission grating

We report on the diffraction of metastable helium atoms (de Broglie wavelength _dB 1Å) passing a free-standing gold grating with a periodicity of 0.5 m. The observed positions and intensities of the different diffraction peaks are in good agreement with a theoretical model for the grating shape. The overall transmission of the grating for the excited state atoms is about 30%, mainly determined by the grating geometry. Our result indicates that microfabricated transmission gratings can be used as efficient and coherent beam splitters for rare gas atoms in long-lived excited states. This fact offers interesting possibilities in view of atom interferometry with metastable noble gas atoms.     

Elastic scattering of electrons by helium and neon atoms

Summary  Partial and total cross-sections of electrons scattering by helium and neon atoms are calculated at eleven values of incident energy ranging from 0.1 a.u. to 1.1 a.u. The calculations are carried out via model potential (describing the electron target interaction). The iterative Green’s function partial-wave expansion technique was used. The comparison between our results and those obtained by other authors show significant agreement and supports our simple model scattering process.     

Detection of helium implanted into nickel by slow positrons

Slow/monoenergetic positrons were used to detect helium atoms implanted in nickel. Doppler broadening profiles of positron annihilation radiation as a function of incident positron energy were found to be quite sensitive to the helium implantation. The detection of helium implanted with a concentration of several ppm was proved to be possible by this technique.     

Metastable states of antiprotonic helium atoms

We discuss the latest theoretical achievements in calculations of energy transitions in the antiprotonic helium He⁺ p-0304; atoms. New variational calculations of the nonrelativistic energies with precision of ∼10^-10 a.u. and relativistic and QED corrections to the energy levels of mα ⁵ order are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic trapping of metastable helium atoms

Metastable helium atoms were efficiently transferred from a magneto-optical trap (MOT) to a magnetic quadrupole trap, producing samples of up to 3᎒⁷ magnetically trapped atoms at a temperature of about 1 mK. We observe purely exponential decay of the samples with time constants of 9-10 s and derive an upper bound for the rate coefficient of inelastic Penning collisions.