Speaker
Description
One long-standing puzzle in modern physics is the discrepancy between the most accurate proton charge radius measurements from muonic hydrogen spectroscopy and electronic hydrogen spectroscopy [1]. Despite theoretical improvements over the last decade, the mismatch remains [2], potentially hinting at physics beyond the Standard Model [3].
Helium, the next simplest atom after hydrogen, provides another testbed. Muonic helium spectroscopy was performed by the CREMA collaboration [4, 5], while more work has been done in normal helium spectroscopy [6–11]. The nuclear charge radius can be extracted with greater precision from high-accuracy spectroscopic measurements. The $2^3S_1$–$2^3P$ transition in helium is theoretically calculated to 2 MHz accuracy, with predictions of reaching 10 kHz [12]. We can also determine the charge radius difference using difference measurement, enabling direct theory comparison. We aim to measure the $2^3S_1$–$2^3P$ transition frequencies in $^{4}$He and $^{3}$He using ultracold metastable atoms. Ultracold clouds minimize the first-order Doppler shift, a dominant error in previous isotope-shift determinations. We present the design of a sub-kHz-resolution precision absolute laser facility and methods to suppress systematic errors, paving the way for the most precise nuclear charge radius measurement in helium to date.
References
[1] Pohl et al., Nature 466, 213–216 (2010)
[2] Mohr et al., Rev. Mod. Phys. 97, 025002 (2025)
[3] Yu R. Sun & S.-M. Hu, Natl.Sci.Rev. 7, 1818–1827 (2020)
[4] Krauth et al., Nature 589, 527–531 (2021)
[5] Schuhmann et al., Science 388, 854–858 (2025)
[6] van Rooij et al., Science 333, 196–198 (2011)
[7] Cancio Pastor et al., PRL 108, 143001 (2012)
[8] Rengelink et al., Nat. Phys. 14, 1132–1137 (2018)
[9] Zheng et al., PRL 119, 263002 (2017)
[10] van der Werf et al., Science 388, 850–853 (2025)
[11] Clausen and Merkt, PRL 134, 223001 (2025)
[12] Pachucki et al., PRA 95, 062510 (2017)
