P. Hauser

Balmer α transitions in antiprotonic hydrogen and deuterium

Abstract The strong-interaction shifts ϵ and broadenings Γ of the 2p levels in antiprotonic hydrogen and deuterium have been measured for the first time with a crystal spectrometer. In hydrogen, the 2 3 P 0 hyperfine state could be resolved from the three close-lying states 2 3 P 2 , 2 1 P 1 , and 2 3 P 1 . The hadronic shift was determined to be ϵ 2 3 P 0 =+139±28 meV (attractive). The value found for Γ 2 3 P 0 =120±25 meV is much larger than the spin-averaged 2p-level broadening Γ bal 2p as determined from earlier experiments measuring the intensity balance. The average shift of the three unresolved states is consistent with zero and a mean broadening of Γ 2( 3 P 2 , 1 P 1 , 3 P 1 ) =38±9 meV was measured. In deuterium, the spin-averaged hadronic shift and broadening were found to be ϵ 2p =−243±26 meV (repulsive) and Γ 2p =489±30 meV.

Measurement of the strong interaction parameters in antiprotonic hydrogen and probable evidence for an interference with inner bremsstrahlung

Abstract The spin-averaged values of the 1s strong interaction parameters in antiprotonic hydrogen were measured with good statistics. The results are: shift (ϵ 1s ) = −712.5 ± 20.3 eV and width (Γ 1s ) = 1054± 65 eV . These values are compatible and have approximately the same precision as the world average before this experiment. With some additional constraints, both the triplet and singlet components could be extracted from the same spectrum. The results are: 3 S 1 shift: −785 ± 35 eV , 3 S 1 width: 940 ± 80 eV , 1 S 0 width: 1200 ± 250 eV . In addition, precise values for the K α L α and Ltotal L α yields at low pressure (20 mbar) were obtained: 0.0176 ± 0.0016 and 1.45 ± 0.05 . Furthermore, the 2p mean absorption width of 30.8 ± 3.0 meV was deduced, also compatible with and of similar precision as the world average before this experiment. An indication of interference between the measured Lyman (Ki) x-ray series and the process of inner bremsstrahlung due to charged meson production after annihilation was seen because of the low background conditions of this experiment.

Laser spectroscopy of the Lamb shift in muonic hydrogen

The muonic hydrogen atom in the 2s state provides the possibility of achieving high precision laser spectroscopy experiments from which a high precision value of the proton radius can be deduced. This will ultimately allow an increased precision in the test of QED in bound systems. Important progress has been made in recent years in the ability to stop muons in a low pressure gas target and in the understanding of the 2s-metastability in muonic hydrogen. As a consequence the 2s–2p laser spectroscopy experiment is now feasible and we present here the basic experimental concept considered by our collaboration.

FRICTIONAL COOLING : EXPERIMENTAL RESULTS

The classical methods used in beam cooling are hard to be adapted for a beam of short-lived elementary particles. A novel method, the so-called frictional cooling – that is cooling a beam of low-energy charged particles by moderation in matter and acceleration in an electrostatic field – has been shown to be feasible. In our experiments performed in 1994/1995 a beam of short-lived particles was cooled for the first time ever. Utilizing frictional cooling on a beam of slow negative muons we observed increase in phase space density by about one order of magnitude.

Measurement of the strong interaction parameters in antiprotonic deuterium

Abstract Antiprotonic deuterium K α X-rays were seen for the first time. Their analysis resulted in spin-averaged 1s strong interaction parameters, namely −1050±250 eV (repulsive) for the 1s shift and 1100±750 eV for the 1s width. The large errors are mainly due to low statistics. In addition, values for the K α /L α and L total /L α yields at low pressure (20 mbar) were obtained: 0.005±0.003 and 1.34±0.05. Furthermore, a 2p mean absorption width of 80 ≤ Γ 2p ≤ 350 meV was deduced. The results for the 1s ground state are somewhat surprising, especially the narrow 1s width which is similar to the spin-averaged antiprotonic hydrogen 1s width. However, our 1s width is in agreement with low-energy scattering data [A. Zenoni et al., Phys. Lett. B 461 (1999) 413] (see preceding paper).

A new method to produce a negative muon beam of keV energies

Abstract A method is described by which muons are decelerated from MeV kinetic energies to energies below 50 keV inside a magnetic focusing device (cyclotron trap) and then extracted by an electric field into a magnetic guiding channel. Test measurements were performed which resulted in about 10 4 /s extracted negative muons, in reasonable agreement with Monte-Carlo simulations. Using this method, muon beams in the energy range of 10–50 keV with intensities up to 10 5 μ − /s will be available in the near future.

Kinetic energies of exotic H atoms at formation and cascade

The complex balance of processes occurring at the cascade of exotic H-atoms is usually described by the so-called standard cascade model, but this model neglects variations of the kinetic energy T of the exotic atom during the cascade which are crucial for the analysis of several important experiments. New experimental results on Tµp at H2 pressures between 0.063 and 4 hPa demonstrate the importance of acceleration due to Coulomb de-excitation processes at highly excited µp levels n > 9. The data at the lowest density are sensitive to the initial values of the kinetic energy and n-levels at the moment of atomic capture. From the measured low-energy tail of the Tµp-distribution it can be concluded that a considerable part of the µp(2s) atoms is metastable at pressures of a few hPa.

Absolute X-ray yields of light muonic atoms

The intensities of X-rays from muonic atoms formed in low pressure Ne, O2, N2, He, and H2 gas was measured. For the pressure chosen external electron refilling can be neglected. Absolute yields were extracted and compared to results of cascade calculations. Knowledge of the yields of the circular X-ray transitions allows an in situ efficiency calibration of X-ray detectors down to energies of 1.5 keV.

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 Kα transition in antiprotonic hydrogen were determined. Evidence was found for the individual hyperfine components of the protonium ground state.

Pionic Hydrogen: Status and Outlook

The measurement of the strong interaction shift and width of the ground state in the pionic hydrogen atom determines two different linear combinations of the two isospin separated s-wave scattering lengths of the pion nucleon system. If both quantities are measured with a precision of about 1% a stringent test of chiral perturbation theory and a determination of the pion nucleon coupling constant can be obtained. Past measurements determined the shift with an accuracy better than 1%, and the width with an accuracy of 9%. Additional information from pionic deuterium measurements has been used in order to extract isospin separated scattering lengths with sufficient accuracy. Future measurements plan to directly measure the width of pionic hydrogen with an accuracy on the level on 1%.