The hadronic width of the ground state of pionic hydrogen has been redetermined by X-ray spectroscopy to be $$\varGamma ^{\pi \mathrm {H}}_{1s}=(856\,\pm \,16_\mathrm{stat}\,\pm \,22_\mathrm{sys})$$  meV. The experiment was performed at the high-intensity low-energy pion beam of the Paul Scherrer Institute by using the cyclotron trap and a high-resolution Bragg spectrometer with spherically bent crystals. Coulomb de-excitation was studied in detail by comparing its influence on the line shape by measuring the three different transitions K $$\alpha $$ , K $$\beta $$ , and K $$\gamma $$ at various hydrogen densities. The pion-nucleon scattering lengths and other physical quantities extracted from pionic-atom data are in good agreement with the results obtained from pion-nucleon and nucleon-nucleon scattering experiments and confirm that a consistent picture is achieved for the low-energy pion-nucleon sector with respect to the expectations of chiral perturbation theory.

After publication of the paper, the authors have noticed an error. Its correction is given in this erratum.

After publication of the paper, the authors have noticed an error. Its correction is given in this erratum.

The 5g - 4f 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. (C) 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

We present a new evaluation of the negatively charged pion mass based on the simultaneous spectroscopy 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 set-up and the methods for deriving the pion mass value from the spatial separation from the 5g - 4 f pi N transition line and the 5g - 4 f mu O 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.

Pion-nucleon scattering lengths are directly related to the ground-state level shift and broadening in pionic hydrogen as well as to the pionic deuterium level shift. The level broadening in deuterium measures the strength of pion threshold-production in proton-proton reactions. However, collisional processes during the atomic de-excitation cascade considerably complicate the analysis of X-ray line shapes in order to extract the hadronic broadening. Therefore, additionally the purely electromagnetic twin system muonic hydrogen was studied. Results of these experiments performed at PSI by using a high-resolution crystal spectrometer are discussed in the context with a new analysis approach for the hadronic broadening.

The hadronic shift in pionic hydrogen has been redetermined to be ε 1 s = 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.

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.

The (3 p — 1 s ) 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.

A set of 14 high-accuracy x-ray transition energies in the 2.4-3.1 keV range is presented, which can be used as x-ray standards. They were measured in two-to four-electron sulfur, chlorine, and argon ions produced in an electron-cyclotron resonance ion source, using a single spherically bent crystal spectrometer. The results include the first measurement of six transitions and improve the accuracy of six other experimental values. These measurements considerably extend the set of high-accuracy x-ray energies reported for highly charged ions. Their relative uncertainties range from 1 to 10 ppm. Theory only reaches such a precision in one-and two-electron ions. Our results thus have two distinct applications. On the one hand, they test predictions in two-electron ions [Artemyev, Shabaev, Yerokhin, Plunien, and Soff, Phys. Rev. A 71, 062104 (2005)], at the precision level of some two-photon QED contributions. We observe an agreement with theory for most of the transitions. On the other hand, the three-and four-electron ion transitions provide new benchmark energies for the calculation of missing theoretical contributions, such as Auger shifts or electronic correlations. Spectra were analyzed with an x-ray tracing simulation that contains all the relevant physics of the spectrometer.

The ground-state level shifts and broadenings of the hydrogen isotopes caused by the strong interaction have been redetermined by using a high-resolution crystal spectrometer. An additional measurement of muonic hydrogen reveals properties of the de-excitation cascade of such electrically neutral exotic atoms, in particular Coulomb de-excitation, the understanding of which is essential for the analysis of the hadronic-atom data.

The strong-interaction shift ε 1s πD and broadening Γ 1s πD in pionic deuterium have been determined in a high statistics study of the πD(3p-1s) X-ray transition using a high-resolution crystal spectrometer. The pionic deuterium shift will provide constraints for the pion-nucleon isospin scattering lengths extracted from measurements of shift and broadening in pionic hydrogen. The hadronic broadening is related to pion absorption and production at threshold. The results are ε 1s πD = (−2356 ± 31) meV (repulsive) and Γ 1s πD meV yielding for the complex πD scattering length a πD = [−(24.99±0.33)+i(6.22 −0.26 +0.12 )] × 10−3 m π −1 . From the imaginary part, the threshold parameter for pion production is obtained to be α = (251 −11 +5 ) μb. This allows, in addition, and by using results from pion absorption in 3He at threshold, the determination of the effective couplings g 0 and g 1 for s-wave pion absorption on isoscalar and isovector NN pairs.

The measurement of the pion-nucleon scattering lengths constitutes a high-precision test of the methods of Chiral Pertubation Theory, which is the low-energy approach of QCD. The pion-nucleon s-wave scattering lengths are related to the strong-interaction shift and width of the s-states of the pionic hydrogen atom. Shift and width are determined from the measured energies and line widths of X-ray transitions to the 1s ground state when compared to the calculated electromagnetic values. A new experiment, set up at the Paul-Scherrer-Institut, has completed a first series of measurements.

An unusual but effective way to determine at threshold the dpi<-->NN transition strength alpha is to exploit the hadronic ground-state broadening Gamma(1s) in pionic deuterium, accessible by x-ray spectroscopy. The broadening is dominated by the true absorption channel dpi(-)-->nn, which is related to s-wave pion production pp-->dpi(+) by charge symmetry and detailed balance. Using the exotic atom circumvents the problem of Coulomb corrections to the cross section as necessary in the production experiments. Our dedicated measurement finds Gamma(1s)=(1171(-49)(+23)) meV yielding alpha=(252(-11)(+5)) microb.

The line shape of the (3p − 1s) X-ray transition in muonic hydrogen was measured for the first time with a high-resolution crystal spectrometer. The assumption of a statistical population of the hyperfine levels was directly confirmed by experiment, and a measured value for the hyperfine splitting is reported. An X-ray line broadening due to Doppler effect could be clearly identified and attributed to different Coulomb de-excitation transitions which precede the measured radiative transition. The results allow a decisive test of advanced cascade model calculations and establish an alternative and “model free” method to extract the strong-interaction parameters from pionic hydrogen data.

The international conference "Exotic atoms and related topics 2005" (EXA05) was the second such meeting organized by the Stefan Meyer Institute for Subatomic Physics of the Austrian Academy of Sciences in Vienna. The first meeting in 2002 was an international workshop that lasted three days; 33 talks were presented and about 80 scientists attended. The next conference lasted four days, with 53 speakers and more than 120 participants. The large increase in both speakers and participants shows how dynamic the fields being discussed are. Both meetings presented those fields of physics that can be investigated using exotic atoms, namely, fundamental symmetries and interactions. Examples include the testing of CPT symmetry with atoms containing antiparticles, and the investigation of low-energy QCD with hadronic atoms in low-lying states. For antiprotonic atoms, the spectroscopy of antiprotonic helium is defining the limits of the equality of proton-antiproton charge and mass with increasing accuracy. The formation and spectroscopy of antihydrogen, potentially the best candidate for precision tests of CPT symmetry in atomic systems, is being undertaken by several experiments at the Antiproton Decelerator at CERN, although formation studies are still in their early stages. The future of research into low-energy antiproton physics seems promising, due to the partial approval of the FLAIR facility at the future FAIR facility in GSI. Currently, the fastest developing field is the kaon-nucleon interaction, where the existence of a series of deeply bound states has been predicted. The first experimental evidence for this was shown at the conference and further experiments in progress were described. If confirmed, these so-called "K-clusters" will open new possibilities for studying dense cold matter in the laboratory, since calculations predict nuclear densities in these systems to be as high as those in neutron stars.

The experimental and spectrum analysis procedures that led to about 15 new, high-precision, relative x-ray line energy measurements are presented. The measured lines may be used as x-ray reference lines in the 2.4-3.1 keV range. Applications also include tests of the atomic theory, and in particular of quantum electrodynamics and of relativistic many-body theory calculations. The lines originate from 2- to 4-electron ions of sulfur (Z = 16), chlorine (Z = 17) and argon (Z = 18). The precision reached for their energy ranges from a few parts per million (ppm) to about 50 ppm. This places the new measurements among the most precise performed in mid-Z highly charged ions (Z is the nuclear charge number). The elements of the experimental setup are described: the ion source (an electron cyclotron resonance ion trap), the spectrometer (a single, spherically bent crystal spectrometer), as well as the spectrum acquisition camera (low-noise, high-efficiency CCD). The spectrum analysis procedure, which is based on a full simulation of the spectrometer response function, is also presented.