In effect this is a comparison between the helion particle (the nucleus of helium-3) and the alpha particle (nucleus of helium-4). Such measurements contribute to searches for yet unknown effects beyond our current understanding of physics, known as the Standard Model.
In the experiment, helium-3 is cooled to near absolute zero temperature (the quantum regime), and suspended in a focused ultraviolet laser beam. The ultracold atoms are then excited with an infrared ultrastable laser connected to an atomic clock to obtain more than 12 digits of accuracy. In this manner the tiny influence of the nucleus on the atomic absorption spectrum can be measured with high precision. Combined with previous measurements on helium-4 [2] and theoretical calculations, the charge radius difference can be determined from this measurement with unprecedented accuracy.
Surprisingly, the measured charge radius difference between the helion and alpha particle did not agree with a competing determinion by the CREMA collaboration based on exotic ‘muonic’ helium ions [3], but deviated by 3.6 sigma. While the papers of both experiments were posted on the arXiv and were under review with Science, this puzzling situation encouraged the development of more advanced calculations for helium-3 by two theory groups. With these new calculations a re-interpretation of the measurements now shows that both experiments do agree within 1.2 sigma. This is a remarkable success given the very different type of atoms the experiments used and the wide range of physics involved. The VU and CREMA paper will be published together in Science on May 22.
References
[1] Yuri van der Werf, Kees Steinebach, Raphael Jannin, Hendrick L. Bethlem, Kjeld S.E. Eikema, Alpha and helion particle charge radius difference determined from quantum-degenerate helium, Science 388, 850-853 (2025).
[2] Robert J. Rengelink, Yuri van der Werf, Remi P.M.J.W. Notermans, Rafael Jannin, Kjeld S.E. Eikema, Maarten D. Hoogerland and Wim Vassen, Precision spectroscopy of helium in a magic wavelength optical dipole trap, Nature Physics 14, 1132-1137 (2018)
[3] Karsten Schumann et al., The helion charge radius from laser spectroscopy of muonic helium-3 ions, Science 388, 854-858 (2025).
See also our previous publication about spectroscopy in helium-3 and the Pauli-blocking effect:
[4] Raphael Jannin, Yuri van der Werf, Kees Steinebach, Hendrick L. Bethlem and Kjeld S.E. Eikema, Pauli blocking of stimulated emission in a degenerate Fermi gas, Nature Communications 13, 6479 2022.