Pionic Hydrogen: Status and Outlook,
D.F. Anagnostopoulos et al.,
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%.
Recent theoretical and experimental studies of the exotic atoms
with Z=1 are reviewed.
An interplay between the atomic internal and external degrees of
freedom is essential for a good description of the atomic cascade.
The perspective of ab initio cascade calculations is outlined.
The atomic cascades in $\mu^-p$, $\pi^-p$, and $K^-p$ have been studied
in detail using a new Monte Carlo kinetics code.
For the first time, a nuclear absorption in the atomic cascade was
treated in a way fully consistent with the elastic scattering and
Stark mixing processes.
This allowed us to get rid of tuning parameters commonly used in earlier
calculations. An interplay between the atomic internal and external
degrees of freedom is shown to play an important role in the atomic cascade.