Nuclear structure from laser spectroscopy of light muonic atoms and ions
Randolf Pohl for the CREMA collaboration
Muonic atoms and ions are hydrogen-like systems that are formed when negative
muons are stopped in ordinary matter, thereby replacing all of the atom's
electrons by a single muon. The muon's Bohr radius is 200 times smaller than
the corresponding electronic Bohr radius in ordinary H-like ions, due to the
200 times larger mass of the muon, compared to the electron. This results in a
tremendously increased sensitivity (200^3) of the muonic atom's S-states to
the finite charge and magnetic radius of the nucleus.
We have recently determined the proton charge radius by laser spectroscopy of
the 2S-2P transition ("Lamb shift") in muonic hydrogen [1,2]. Our value of
Rp=0.84087(39) fm is ten times more accurate, but 7 sigma discrepant from the
world average, which is based on elastic electron-proton scattering and
precision spectroscopy of regular (electronic) hydrogen. This so-called
"proton radius puzzle" has sparked tremendous interest both in atomic and
nuclear physics. Possile explanations range from experimental errors to
unexpected behaviour of the proton and to physics beyond the Standard Model .
To shed new light on this discrepancy, we have measured the Lamb shift in
muonic deuterium and extracted a value of the charge radius of the deuteron
. This year,. we have also measured several 2S-2P transitions in muonic
helium-3 and -4. This will improve the accuracy of the charge radii of all
helium isotopes by a factor of ten . In future, spectroscopy of of muonic
lithium, beryllium and boron ions may be used for significantly improved
charge radius value of the lightest isotopes .
(*) Charge Radius Experiment using Muonic Atoms
 R. Pohl et al. (CREMA coll.), Nature 466, 213 (2010).
 A. Antognini et al. (CREMA coll.), Science 339, 417 (2013).
 R. Pohl et al., Ann. Rev. Nucl. Part. Sci 63, 175 (2013)
 CREMA coll., in preparation.
 A. Antognini et al. (CREMA coll.), Can. J. Phys. 89, 47 (2011).
 Drake, Byer, PRA 32, 713 (1985).