Muonic atoms are atomic bound states of a negative muon and a nucleus.
The muon, which is the 200 times heavier cousin of the electron,
orbits the nucleus with a 200 times smaller Bohr radius. This
enhances the sensitivity of the atomic energy levels to the nucleus
finite size tremendously.
By performing laser spectroscopy of the 2S-2P transitions in muonic
hydrogen we have determined the proton root mean square charge radius
20 times more precisely than previously obtained. However, this value
disagrees by 4 standard deviations from the value extracted from
``regular `` hydrogen spectroscopy and also by 6 standard deviations
from electron-proton scattering data.
The variance of the various proton radius values has led to a very
lively discussion in various fields of physics: particle and nuclear
physics (proton structure, new physics, scattering analysis), in
atomic physics (hydrogen energy level theory, fundamental constants)
and fundamental theories (bound-state problems, QED, effective field
theories). The origin of this discrepancy is not yet known and the
various (im)possibilities will be presented here.
Here we present also preliminary results of muonic deuterium and
helium-ion (He-3 and He-4) spectroscopy, which beside helping to
disentangle the origin of the observed ``proton radius puzzle'' also
provide values of the corresponding nuclear charge radii with high
accuracy. This knowledge open the way for enhanced bound-state QED
test in regular atoms and provide benchmark for few-nucleon theories.