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Highly-charged ions
The fourth direction in high precision tests of QED is
research with middle range and even heavy ions in the
highest charge states, i.e. bare ions, hydrogen-like ions
or few-electron systems. Calculations have recently
achieved such a degree of accuracy that these systems
now offer a testing ground of QED in the presence
of strong electric fields. This needs equally accurate
experiments, which involve charge breeding in electron
beam ion traps (EBIT) or electron stripping at relativistic
energies in accelerator facilities, cooling and storing in
ion traps or storage rings. Such experiments will also
benefit from the recent development of high-power
lasers such as PHELIX at GSI or intensive free electron
lasers. Upcoming facilities will provide high-flux photon
beams with energies up to the keV region enabling, for
example, fundamental photon-atom interaction studies
such as precision spectroscopy of highly charged ions.
Further on, experiments on stored and cooled beams of
heavy ions in the highest charge states up to uranium
U 92+ (where the binding energy becomes comparable
to the electron mass), for example the precise measurement
of the ground-state Lamb shift, have already
been conducted in the Experimental Storage Ring ESR
at GSI. The HITRAP facility, a Penning trap setup for
accumulating and storing highly charged ions up to bare
U 92+, will allow for high-accuracy measurements of the
Lamb shift, the ground-state hyperfine splitting, or the
g-factor of the bound electron in an ion of the very same
isotope with up to four electrons. Here, nuclear structure
effects can be eliminated to a large extent and high-
Z QED can be tested without major distortion by the
nuclear size. This opens up an alternative route to an
accurate determination of the fine structure constant.
Also important is the possibility of accurately measuring
the magnetic moment of a heavy ion (at HITRAP in GSI).
This may allow one of the most accurate determinations
of the nuclear magnetic moment, free of nuclear
structure uncertainties, and free of theoretical uncertainties
in the shielding and hyperfine constants. This
is in contrast to standard determinations through the
hyperfine splitting or NMR technique, where the atomic
structure calculations, not considering QED effects,
are not accurate enough to reach a similar precision
for nuclear magnetic moments. Finally, the unexpected
recent observations of non-exponential orbital electron
capture decays of hydrogen-like 140 Pr and 142 Pm ions
(‘GSI oscillations’), which has not yet found a convincing
explanation, deserves to be verified.
Many-electron atoms
In the fifth direction, QED effects are being investigated
for many electron atoms in order to obtain accurate
energy levels, transition rates, isotope shifts and to
test predictions of the Standard Model for the weak
interaction between electrons and nuclei as described
in the previous section. However, the current development
of accurate atomic structure calculations is less
than satisfactory. There is not yet any implementation
of QED theory in multi-electron atoms that accounts
at the same time for correlations and the electron selfenergies.
Although this problem has been investigated
for a long time, so far no systematic approach has been
implemented in the atomic structure codes, so that an
accurate treatment of electron correlations and QED
effects in many electron atoms deserves further investigation.
Finally, we mention speculative ideas, that fundamental
constants such as α are not constant over the
cosmological time, or even on the scale of a couple of
years. These ideas are being verified at many laboratories,
for example at NIST by a comparison of Al + versus
Hg + , the most accurate single-ion optical clocks. As
a result α . / α cannot be larger than 10 -17 /year, which
is so far the most stringent limit on the possible time
variation of α.
QCD and hadronic atoms
Exotic hadronic atoms (see also the chapter on Hadron
Physics) have long been used to study the strong interaction
between the exotic particle and the nucleus at – as
compared to scattering experiments – zero energy, i.e.
at threshold. The lowest energy atomic states experience
a shift and broadening due to the strong interaction
of the exotic hadronic particle and the nucleus, which
are usually measured by precision X-ray spectroscopy.
Activities with pionic atoms at PSI have been successfully
completed, while the DIRAC experiment at CERN is
investigating pionium ππ and πK atoms. Most activity is
concentrated on kaonic atoms and kaonic nuclear bound
states, in Europe at LN Frascati and GSI-Darmstadt, and
in Japan at KEK and in future at J-PARC. Exotic atoms
with more exotic particles like Σ or Ξ are also planned at
J-PARC. Antiprotonic light atoms have been investigated
previously at LEAR, but more studies are planned in the
future at FLAIR.
Pionic atoms
The strong interaction at low energies is described by
non-perturbative QCD. In the case of pionic atoms,
chiral perturbation theory is applicable and the interaction
between the Goldstone bosons, the pions, is
most easy to calculate. This makes systems like ππ and
πK most attractive from the theoretical point of view.
Experimentally, the lifetime of these atoms in matter –
Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010 | 169