Precision measurement of antiprotonic hydrogen and deuterium X-rays

X-rays from antiprotonic hydrogen and deuterium have been measured at low pressures. Using the cyclotron trap, a 105 MeV/c antiproton beam from LEAR was stopped with an efficiency of 86% in 30 mbar hydrogen gas in a volume of only 100 cm³. The X-rays were measured with Si(Li) detectors and a Xe-CH₄ drift chamber. The strong interaction shift and broadening of the Lyman transition and the spin-averaged 2p width in antiprotonic hydrogen was measured with unprecedented accuracy. The triplet component of the ground state in antiprotonic hydrogen was determined for the first time.The authors would like to thank the LEAR staff for their efforts in providing the antiproton beam. The help of P. Gauss from the CERN cryogenic group and of P. Zettwoch from the PS workshop is gratefully acknowledged. We wish to thank K.-P. Wieder for his help with the drawings. This work is part of the Ph.D. thesis of one of us (K.H., University of Karlsruhe, 1990).     

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.     

X-ray transitions from antiprotonic noble gases

The onset of antiprotonic X-ray transitions at high principal quantum numbers and the occurrence of electronic X-rays in antiprotonic argon, krypton, and xenon has been analyzed with the help of Multiconfiguration Dirac-Fock calculations. The shell-by-shell ionisation by Auger electron emission, characterised by appearance and disappearance of X-ray lines, is followed through the antiprotonic cascade by considering transition and binding energies of both the antiproton and the remaining electrons. Electronic lines could be attributed partly to specific states of the antiprotonic atom de-excitation.     

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.     

Sub-ppm laser spectroscopy of antiprotonic helium and a CPT-violation limit on the antiprotonic charge and mass

Six laser-resonant transitions have been detected in metastable antiprotonic helium atoms produced at the CERN Antiproton Decelerator. They include UV transitions from the last metastable states in the v = n-l-1 = 0 and 1 cascades. Zero-density frequencies were obtained from measured pressure shifts with fractional precisions between 1.3 x 10(-7) and 1.6 x 10(-6). By comparing these with QED calculations and the antiproton cyclotron frequency, we deduce that the antiproton and proton charges and masses agree to within 6 x 10(-8) with a confidence level of 90%.     

Hyperfine structure of antiprotonic helium revealed by a laser-microwave-laser resonance method

Using a newly developed laser-microwave-laser resonance method, we observed a pair of microwave transitions between hyperfine levels of the (n,L)=(37,35) state of antiprotonic helium. This experiment confirms the quadruplet hyperfine structure arising from the interaction of the antiproton orbital angular momentum, the electron spin and the antiproton spin as predicted by Bakalov and Korobov. The measured frequencies of nu(+)(HF)=12.895 96+/-0.000 34 GHz and nu(-)(HF)=12.924 67+/-0.000 29 GHz agree with recent theoretical calculations on a level of 6x10(-5).     

Mass and magnetic moment of the antiproton by the exotic atom method

Precision determinations of the mass and magnetic moment of the antiproton were made by the exotic atom method. Antiprotons were stopped in lead or uranium targets. De-excitation X-rays from the antiprotonic atoms were viewed by a high resolution Ge (Li) detector. Six principal transitions of the p?Pb spectrum (16 → 15 to 11 → 10) were analyzed to deduce a value of the antiproton mass. The fine structure splittings of the 11 → 10 and 12 → 11 transitions of p?Pb and p?U were used to determine a value of the antiproton magnetic moment. Our computed values of the energy eigenvalues of the (n, l, j) levels included corrections due to vacuum polarization and higher order radiative terms, electron screening, nuclear finite size and nuclear polarization. In the case of the p?U data, an additional shift due to the dynamic E2 mixing of nuclear rotational levels with antiprotonic orbital levels was included. Noncircular transitions were included in the analysis of the data. The values obtained for the antiproton mass and magnetic moment, 938.179±0.058 MeV and ? 2.791±0.021 nuclear magnetons, respectively, are compared with the corresponding quantities pertaining to the proton, 938.2796 ± 0.0027 MeV and +2.793 nuclear magnetons, respectively (error 1.1 × 10^?6 nuclear magnetons).     

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.     

The antiprotonic helium atom

The Born-Oppenheimer approximation is used to discuss the high rotational and vibrational state of the (He p⁻)e⁻ system in the electronic 1sσ and 2pσ states. Very high angular momentum states, in which the antiproton is well outside the electron orbit, have a Rydberg-like character. states in which the antiproton is within the 1sσ electron orbit have enhanced radiative lifetimes due to the polarization of the 1sσ state by the antiproton. This effect may account for the long-lived component observed in antiproton destruction in He. Preliminary results on the effect of coupling to the 2pσ well, in which the polarization effects enhance the decay rate, are also presented. Some consequences for the suggestion that metastable antiprotonic He atoms may be used to promote antihydrogen formation are discussed.     

High-Precision Calculation of the Energy Levels and Auger Decay Rates of the Metastable States of the Antiprotonic Helium Atoms

We precisely calculated the energy levels and the Auger decay rates of the antiprotonic helium atoms with the coupled rearrangement channel method and the complex-coordinate-rotation method. Calculated transition frequencies were in excellent agreement with observed values. This agreement gives best limit of the antiproton mass with 5×10⁻⁸ uncertainty. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nuclear periphery studied with antiprotonic atoms

Two experimental methods were employed to study the nuclear peripheral density. The first method was based on antiprotonic X ray measurements. The widths and shifts of the last atomic state levels available for the antiproton were deduced from the shape of the X-ray lines. These observables are sensitive to the nuclear matter density at a distance 1.5 fm larger than the charge half-density radius. The other method consisted in studying the antiproton annihilation products and gave information on the neutron-to-proton density ratio at a radius 2.5 fm larger than the charge half-density radius. This article presents the results obtained by the PS209 collaboration in measurements performed with the LEAR facility at CERN. Neutron densities and differences of neutron and proton root-mean-square radii were deduced for isotopes over a wide mass range (A = 40–238).     

Two photon laser spectroscopy of antiprotonic helium atoms at CERN’s AD

The ASACUSA collaboration of CERN has carried out two-photon laser spectroscopy of antiprotonic helium atoms using counter-propagating ultraviolet laser beams. This excited some non-linear transitions of the antiproton at the wavelengths λ = 139.8–197.0 nm, in a way that reduced the thermal Doppler broadening of the observed resonances. The resulting narrow spectral lines allowed the measurement of three transition frequencies with fractional precisions of 2.3–5 parts in 10⁹. By comparing these values with three-body QED calculations, the antiproton-to-electron mass ratio was derived as 1836.1526736(23). We briefly review these results.     

Control of Non-Neutral Plasmas and Generation of Ultra-Slow Antiproton Beam

The Antiproton Decelerator (AD) devoted primarily to atomic physics experiments has been stably operated since 2000. Until now, three proposals have been approved, two of which are on the production and spectroscopy of antihydrogen, and the third one is on atomic collisions and precision spectroscopy of antiprotonic atoms, ASACUSA collaboration. One of the unique features of the ASACUSA collaboration is to develop intense slow and ultra slow antiproton beams of high quality, which will open a new multidisciplinary field involving atomic physics, nuclear physics and elementary particle physics. The ultra slow antiprotons will be prepared by combining the AD (down to 5.3 MeV), the RFQD (Radio Frequency Quadrupole Decelerator) (down to several tens keV), and an electron cooling device which will be called “MUSASHI” (Monoenergetic Ultra Slow Antiproton Source for High-precision Investigations) (down to several eV). MUSASHI produces the eV antiproton beam through an electron cooling of trapped antiprotons and a radial compression followed by an extraction through a transport beam line. The transport beam line is specially designed so that the pressure at the trap region can be maintained more than six orders of magnitude better than the collision region and at the same time the transport efficiency is kept at almost 100%. The ultra slow antiproton beam allows for the first time to study collision dynamics such as antiprotonic atom formation and ionization processes under single collision conditions, and also to study spectroscopic nature of various metastable antiprotonic atoms such as p, He⁺, He⁺⁺, etc. Metastable p are particularly interesting because they allow to make high precision spectroscopy of two body exotic atoms. Production and spectroscopy of antiprotonic atoms consisting of unstable exotic nuclei will also be discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Progress in precision laser spectroscopy of{\displaystyle\overline{p}}-He

The mass of the antiproton (the antiproton-electron mass ratio $m_{\overline{p}}/m_e$ ) can be deduced from laser spectroscopy measurements of antiprotonic helium, compared to 3-body QED calculations. The most precise spectroscopy measurements have so far been limited by Doppler-broadening at finite temperatures. The status of our ongoing precision measurements is presented, within the framework of the Asacusa experiment at CERN.     

Sub-Doppler Two-Photon Laser Spectroscopy of Antiprotonic Helium and the Antiproton-to-Electron Mass Ratio

The ASACUSA collaboration of CERN has recently irradiated antiprotonic helium atoms with two counter-propagating laser beams. This excited some non-linear two-photon transitions of the antiproton at the deep UV wavelengths λ = 139.8–197.0 nm. The counterpropagating geometry of the laser beams reduced the thermal Doppler broadening of the observed resonances. Their narrow spectral lines allowed the measurement of three transition frequencies with fractional precisions of 2.3–5 parts in 10⁹. By comparing the results with three-body QED calculations, the antiproton-to-electron mass ratio was derived as 1836.1526736(23).     

Neutron density distributions deduced from antiprotonic atoms

The differences between neutron and proton density distributions at large nuclear radii in stable nuclei were determined. Two experimental methods were applied: nuclear spectroscopy analysis of the antiproton annihilation residues one mass unit lighter than the target mass and the measurements of strong-interaction effects on antiprotonic x rays. Assuming the validity of two-parameter Fermi neutron and proton distributions at these large radii, the conclusions are that the two experiments are consistent with each other and that for neutron rich nuclei it is mostly the neutron diffuseness which increases and not the half-density radius. The obtained neutron and proton rms radii differences are in agreement with previous results.     

Protonium X-ray spectroscopy

The Lyman and Balmer transitions from antiprotonic hydrogen and deuterium were studied extensively at the Low-Energy-Antiproton Ring LEAR at CERN in order to determine the strong interaction effects. A first series of experiments was performed with semiconductor and gaseous X-ray detectors. In the last years of LEAR operation using a Bragg crystal spectrometer, strong interaction parameters in the 2p states of antiprotonic hydrogen and deuterium were measured directly. The results of the measurements support the meson-exchange models describing the medium and long range part of the nucleon-antinucleon interaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of balmer and lyman X-rays in antiprotonic hydrogen isotopes at pressures below 300 hPa

X-Rays of Balmer and Lyman transitions in antiprotonic hydrogen and of Balmer transitions in antiprotonic deuterium were observed at pressures below 300 hPa using Si(Li) semiconductor detectors. The measurement was performed at the LEAR-facility at a beam momentum of 202 MeV/c. In order to stop antiprotons in a low pressure gaseous target with high efficiency, a novel technique, the cyclotron trap has been used. Absolute yields were determined and compared with cascade calculations. A distinct difference in the cascade of antiprotonic hydrogen and deuterium is found. The parameters of strong interaction in antiprotonic hydrogen are determined to be? _1s=?(620±100) eV,Γ _1s=(1130±170) eV andΓ _2p=(32±10) meV.     

Measurement of strong interaction parameters in antiprotonic hydrogen and deuterium

In the PS207 experiment at CERN, X-rays from antiprotonic hydrogen and deuterium have been measured at low pressure. The strong interaction shift and the broadening of the K _α transition in antiprotonic hydrogen were determined. Evidence was found for the individual hyperfine components of the protonium ground state. This revised version was published online in August 2006 with corrections to the Cover Date.     

High-Precision Spectroscopy of Antiprotonic Helium – First Results from the AD of CERN

New results of the laser and microwave spectroscopy of antiprotonic helium “atomcules” obtained in the first year of operation of the Antiproton Decelerator (AD) facility of CERN are presented. They include the discovery of three new resonant transitions and the determination of the zero-density wavelength of six transitions with an accuracy of 130 ppb in the best case. Auger rates of those states were also determined, and two of them were found to be several orders of magnitude larger than expected from a simple estimate based on the multipolarity Δl, i.e., the jump in angular momentum required for the antiproton to reach the next lower-lying state of ionized He⁺⁺. Furthermore, a first signal of a two-laser microwave triple resonance to measure the hyperfine splitting in antiprotonic helium was observed. This revised version was published online in August 2006 with corrections to the Cover Date.