Helium-4 mass for electron mass

There is currently a 1.3(0.2) x 10^-10 (6.5-sigma) discrepancy between a recent precision measurement [1] of the mass of ⁴He and the value in CODATA-2018 [2]. Motivated by this discrepancy we are using precision single-ion Penning trap techniques to measure the atomic mass of ⁴He with a precision goal of 1 x 10^-11. A precision atomic mass of ⁴He is needed so that future measurements of the electron-spin flip frequency, f_sf, and cyclotron frequency, f_c, of a single ⁴He⁺ in a strong magnetic field can be used to determine the atomic mass of the electron, m_e, through the relation f_sf/f_c = g_ion(m_ion/2m_e), where m_ion is the mass of ⁴He⁺, and g_ion is the ⁴He⁺ g-factor, which can be calculated to a few parts in 10^-13. Compared to determining m_e from f_sf and f_c in C⁵⁺ [3], which produces the current CODATA value of m_e, ⁴He⁺ has the advantage of smaller nuclear size and QED uncertainties in the g-factor theory. A new “g-factor” m_e is especially pressing due to developments in precision rovibrational spectroscopy and theory for HD⁺. Combined with the atomic mass of the deuteron m_d [4] and the mass ratio m_p/m_d [5], HD⁺ spectroscopy has produced a value for m_e which is competitive, and in 2-sigma tension, with the value obtained from C⁵⁺.

[1] S. Sasidharan, et al, PRL 131, 093201 (2023), [2] E. Tiesinga, et al. RMP 93, 025010 (2021), [3] J. Zatorski, et al. PRA 96, 012502 (2017), [4] S. Rau, et al, Nature, 585, 43 (2020), [5] D. J. Fink and E. G. Myers, PRL 124, 013001(2020); PRL 127, 243001(2021).