The proton radius extracted from the measurements of the 2S-2P energy splitting in muonic hydrogen
(μp) has attracted great attention because of a 7σ discrepancy with the values extracted from
electron scattering and hydrogen (H) spectroscopy. Hundreds of publications have been devoted to the
so called “proton radius puzzle” ranging from studies of physics beyond the standard model, to reanalysis
of electron scattering data, refinements of bound-state QED calculations, new theories describing
the proton structure, and proposals for new scattering and H spectroscopy experiments.
As next step, I plan two new (i.e., never before attempted) measurements: the ground-state hyperfine
splitting (1S-HFS) in both μp and μ3He+ with 1 ppm relative accuracy by means of pulsed laser
spectroscopy. From these measurements the nuclear-structure contributions (two-photon-exchange)
can be extracted with a relative accuracy of 100 ppm which in turn can be used to extract the corresponding
Zemach radii (with a relative accuracy of 0.1%) and polarizability contributions. The Zemach radii
can provide magnetic radii when form-factor data or models are assumed.
These radii are benchmarks for lattice QCD and few-nucleon theories. With the polarizability contribution
they impact our models of the proton and of the 3He nucleus. Moreover, the μp measurement
can be used to solve the discrepancy between the magnetic radii values as extracted from polarized and
unpolarized electron scattering and to further test bound-state QED predictions of the 1S-HFS in H.
These two experiments require a muon beam line, a target with an optical cavity, detector, and laser
systems. As weak M1 transitions must be probed, large laser-pulse energies are needed, thus cutting-edge
laser technologies (mainly thin-disk laser and parametric down-conversion) need to be developed.
Laser schemes of potentially high industrial impact that I have just patented will be implemented and
Fields of science
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