Nuclear size puzzles

The size of the proton should not depend on the way it is measured. For 8 years now there is a large discrepancy between measurements of the proton charge radius in three branches of physics. Electron scattering and laser spectroscopy in atomic hydrogen agree, while laser spectroscopy in muonic hydrogen, where the electron is replaced by a 200 times heavier muon, disagrees by 6σ. Such a difference between muonic and electronic systems cannot be explained within the Standard Model and hints at violation of lepton universality. This so-called ‘proton size puzzle’, in 2016 extended to the deuteron, still stands today despite strong efforts to solve it. Recently laser spectroscopy in muonic He+ was performed, which will soon provide very accurate sizes of the α-particle and the helion (the nuclei of the 4He resp. 3He atom). The question is whether this nuclear size puzzle also extends to helium. I propose to confront the upcoming muonic helium results with nuclear size measurements in ‘normal’ electronic 4He and 3He atoms, laser cooled to a temperature ~0.2 μK. A Bose-Einstein condensate of 4He and a degenerate Fermi gas of 3He atoms in the metastable triplet state, tightly confined at the anti-nodes of an optical lattice (a one-dimensional standing wave) at a ‘magic’ wavelength of 319.8 nm will be used to perform high-precision laser spectroscopy. The inherently narrow transition (8 Hz) to the singlet metastable state will be measured with <10 Hz accuracy (5 parts in 10^14) for each isotope. In combination with highly accurate Quantum Electrodynamics calculations of level energies the difference of the squared nuclear charge radii of the helion and the α-particle will be deduced with an accuracy more than a factor of ten higher than anticipated from muonic He+ spectroscopy. It will be very clear whether also a helium-size puzzle exists.