Anomalies in pionic atoms

Large anomalies have recently been observed in the level shifts and widths of pionic3d states in very heavy atoms. It is shown that the shifts can be well accounted for by including the previously neglected energy dependence of the multiple scattering optical potential without introducing any new free parameter. It is essential to couple this energy dependence to minimal coupling. The widths are better described but are still anomalously small. The present effect has consequences for ^± scattering.Presented at the symposium Mesons and Light Nuclei, Liblice, Czechoslovakia, June 1981.     

Finite-range effects in pionic atoms

Pionic atom data are refitted in terms of conventional π^? nuclear optical potentials, modified to allow for finite range in the underlying p-wave πN interaction. We find that level shifts and widths in light and medium-weight nuclei cannot distinguish between the various ranges tested. In heavy nuclei, and for reasonable choices of neutron radii, the data tend to exclude ranges of order 0.5 fm and larger.     

Total and transport elastic cross-sections for excited muonic and pionic hydrogen atoms

The results of an experiment on muon catalyzed dd-fussion in HD gas are presented. The experiment was performed at the muon beam of PSI using a high-pressure ionization chamber filled with pure HD-gas of low D₂ concentration on the level 1%, at temperatures 50, 150 and 300 K. The non-resonant character of ddμ-molecule formation on HD molecules was confirmed by measuring the ratio of yields of the two ddμ-fusion channels, R=Y(³He,n)/Y(³H,p), which proved to be close to unity. The ddμ formation rate was found to vary from λ_ddμ-HD=0.05· 10⁶ s^-1 at T=50 K to λ_ddμ-HD=0.12· 10⁶ s^-1 at T=300 K, in agreement with the theoretical prediction. A prominent peak at t<60 ns was observed in the time spectrum of fusion neutrons indicating a resonant contribution of ddμ formation from epithermal dμ atoms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quadrupole interactions in pionic and kaonic atoms

We discuss the quadrupole hyperfine structure in mesonic atoms for nuclei with spin I 1. The optical potential is expanded in terms of the non-spherical density contributions. Examples are given for the hyperfine splitting of peripheral and deeper-lying states in pionic and kaonic atoms.     

Chiral symmetry probed with pionic atoms

Study of pionic atom has been revisited by a recent discovery of deeply bound pionic states in the context of possible sensitivity to the partial chiral restoration in the nuclear medium. Precisely measured binding energies and widths of the deeply bound π^? Is states in ^115,119,123Sn in ^116,120,124Sn(d,³He) reactions at recoil-free conditions are used to deduce the medium-modified isovector scattering amplitude as b₁ = 0.115 ± 0.007m _π ^?1. The observed enhancement of b ₁ over the free πN value infers 34% reduction of $\left\langle {\bar qq} \right\rangle $ at the normal nuclear density, ρ = 0.17 fm^?3.     

Chiral dynamics of deeply bound pionic atoms

We present and discuss a systematic calculation, based on two-loop chiral perturbation theory, of the pion-nuclear s-wave optical potential. A proper treatment of the explicit energy dependence of the off-shell pion self-energy together with (electromagnetic) gauge invariance of the Klein-Gordon equation turns out to be crucial. Accurate data for the binding energies and widths of the 1s and 2p levels in pionic 205Pb and 207Pb are well reproduced without need for a notorious "missing repulsion" in the pion-nuclear s-wave optical potential. The connection with the in-medium change of the pion decay constant is clarified.     

Dynamic nuclear effects in pionic and kaonic atoms

Corrections have been calculated to absorption widths in pionic and kaonic atoms due to the admixture of excited nuclear states. Effects as large as 12% and 27% are indicated for pionic and kaonic atoms, respectively.     

E2 nuclear resonance effect in pionic atoms

The quadrupole nuclear resonance effect in pionic atoms is considered, and the results of calculations for pionic atoms with distorted even-even nuclei are presented.     

Kinetic energy distributions in pionic hydrogen

A framework for analyzing the Doppler broadening of the X-ray lines in pionic hydrogen is proposed. It is shown that the kinetic energy distributions at the instant of the (n = 2-4) radiative transitions are related to each other. In order to establish the connection, the collisional processes for pionic hydrogen scattering from hydrogen atoms and molecules have been calculated in different models. The proposed method can be used to determine reliably the strong interaction width of the ground state of pionic hydrogen from the X-ray line profiles measured recently at the Paul-Scherrer-Institut.Received: 6 August 2004, Published online: 28 September 2004PACS: 34.50.-s Scattering of atoms and molecules - 36.10.Gv Mesonic atoms and molecules, hyperonic atoms and molecules     

Low-energy X-ray standards from hydrogenlike pionic atoms

We demonstrate the first step of a complete program, which consists in establishing an x-ray energy standard scale with the use of few-body atoms, in the few keV range. Light pionic and muonic atoms as well as one and two-electron ions from electron-cyclotron ion sources are used. The transition energies are calculable from quantum-electrodynamics, meaning that only a very limited subset need be measured and compared with theory, while providing a large number of standard lines. Here we show that circular transitions in pionic neon atoms, completely stripped from their electrons, reveal spectral lines which are narrow, symmetric, and well reproducible. We use these lines for the energy determination of transition energies in complex electronic systems, like the Kalpha(1,2) transitions in metallic Ti, which may serve as secondary standard.     

Binding effects in the optical potential of pionic atoms

We calculate the contribution from nucleon binding to the coefficient Re B₀ in the optical potential of pionic atoms. The relevant Feynman graphs are deduced for nuclear matter. They depend on off-shell values for pion-nucleon scattering lengths a_πN and for the nucleon-binding potential V_NA. Off-shell effects are found to be very important: A pole model for a_πN, extrapolated off-shell, increases binding effects by a factor of three over the use of on-shell values. Two simple models are proposed for the off-shell continuation of V_NA. One leads to attractive, the other to repulsive contributions to Re B₀.     

Structure and formation of deeply bound pionic atoms

We discuss some recent developments of the theoretical studies for the structure and formation of the deeply bound pionic atoms, which were found experimentally using (d,³He) reactions.     

The pionic hydrogen experiment at PSI

A new high-precision determination of the strong-interaction shift (∈_1s) and broadening (Γ_1s) of the ground state in pionic hydrogen (πH) has been started at PSI. This constitutes a direct measurement of the πN scattering lengths and is an important test of the methods of chiral perturbation theory.The experiment will be conducted in two steps. In the first step we aim for a precision of 0.2% for ∈_1s and 3% for Γ_1s. A new method of energy calibration is used: by measuring πH simultaneously with pionic oxygen (πO), systematic errors are suppressed. The goal for the second step is an accuracy of 1% for Γ_1s.     

E2 nuclear resonance effects in pionic and kaonic atoms

The attenuation due to the E2 nuclear resonance effect has been measured in hadronic atoms using pions with ¹¹¹Cd and ¹¹²Cd, and for kaons with ¹²²Sn. Energies of the relevant X-ray and γ-ray transitions and of the X-ray cascade intensities have also been measured so as to give a self-consistent set of information. The results are found to be in very good agreement with theoretical calculations.