D. F. Anagnostopoulos

On the characterisation of a Bragg spectrometer with X-rays from an ECR source

Narrow X-ray lines from helium-like argon emitted from a dedicated ECR source have been used to determine the response function of a Bragg crystal spectrometer equipped with large area spherically bent silicon (111) or quartz (10¯ crystals. The measured spectra are compared with simulated ones created by a ray-tracing code based on the expected theoretical crystal’s rocking curve and the geometry of the experimental set-up.

Hadronic shift in pionic hydrogen

The hadronic shift in pionic hydrogen has been redetermined to be ε1s = 7.086 ± 0.007(stat) ± 0.006(sys) eV by X-ray spectroscopy of ground-state transitions applying various energy calibration schemes. The experiment was performed at the high-intensity low-energy pion beam of the Paul Scherrer Institut by using the cyclotron trap and an ultimate-resolution Bragg spectrometer with bent crystals.

Highly charged ions in exotic atoms research at PSI

During their de-excitation, exotic atoms formed in low pressure gases reach a state of high or even complete ionization. X-rays emitted from higher n-states of electron-free atoms have well defined energies with the error originating only from the error in the mass values of the constituent particles. They served as a basis for a new determination of the pion mass as well as for a high precision measurement of the pionic hydrogen ground state shift. The response function of the Bragg spectrometer has been determined with X-rays from completely ionized pionic carbon and with a dedicated electron cyclotron resonance ion trap (ECRIT). A further extension of the ECRIT method implemented in the experiment allows a direct calibration of exotic atom transitions as well as a precise determination of the energy of fluorescence lines.

Precision measurements in pionic hydrogen

Abstract The strong interaction in the pion nucleon system leads to a shift and a broadening of the 1s-ground state in pionic hydrogen. These two quantities are being measured in an experiment at the Paul Scherrer Institute with much improved precision and allow an experimental test of recent calculations in the framework of Chiral Perturbation Theory. The experimental techniques using high resolution crystal spectroscopy are described as well as recent results.

Measurement of the charged pion mass using X-ray spectroscopy of exotic atoms

Abstract The 5 g − 4 f transitions in pionic nitrogen and muonic oxygen were measured simultaneously by using a gaseous nitrogen–oxygen mixture at 1.4 bar. Due to the precise knowledge of the muon mass the muonic line provides the energy calibration for the pionic transition. A value of (139.57077 ± 0.00018) MeV/c 2 (± 1.3 ppm) is derived for the mass of the negatively charged pion, which is 4.2 ppm larger than the present world average.

Conclusions from recent pionic--atom experiments

The most recent pionic—hydrogen experiment marks the completion of a whole series of measurements, the main goal of which was to provide conclusive data on pion—nucleon interaction at threshold for comparison with calculations from Chiral perturbation theory. The precision achieved for hadronic shift and broadening of 0.2% and 2%, respectively, became possible by comprehensive studies of cascade effects in hydrogen and other light exotic atoms including results from the last years of LEAR operation. In order to obtain optimum conditions for the Bragg crystal spectrometer, the cyclotron trap II has been used to provide a suitable X—ray source. To characterize the bent crystal spectrometer, the cyclotron trap has been modified to operate as an electron—cyclotron resonance source, which produces with high intensity narrow X‐ray transitions in the few keV range originating from highly charged ions.

Measurement of the charged pion mass using a low-density target of light atoms

We present a new evaluation of the negatively charged pion mass based on the simultaneous spec-troscopy of pionic nitrogen and muonic oxygen transitions using a gaseous target composed by a N 2 /O 2 mixture at 1.4 bar. We present the experimental setup and the methods for deriving the pion mass value from the spatial separation from the 5g − 4 f πN transition line and the 5g − 4 f µO transition line used as reference. Moreover, we discuss the importance to use dilute targets in order to minimize the influence of additional spectral lines from the presence of remaining electrons during the radiative emission. The occurrence of possible satellite lines is investigated via hypothesis testing methods using the Bayes factor.

Precision measurement of the (3p–1s) X-ray transition in muonic hydrogen

The (3p — 1s) X-ray transition to the muonic hydrogen ground state was measured with a highresolution crystal spectrometer. The assumption of a statistical population of the hyperfine levels of the muonic hydrogen ground state was directly confirmed by the experiment and measured values for the hyperfine splitting can be reported. The measurement supplements studies on line broadening effects induced by Coulomb de-excitation hindering the direct extraction of the pion-nucleon scattering lengths from pionic hydrogen and deuterium X-ray lines.

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%.

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.