Electron-helion mass ratio

$mₑ/mₕ$ = 1.819 543 074 649(53) × 10⁻⁴

Value: 1.819 543 074 649 × 10⁻⁴
Standard uncertainty: 5.3 × 10⁻¹⁵
Relative uncertainty: 2.9 × 10⁻¹¹ (29 parts per trillion)
Status: Measured
Symbol: $mₑ/mₕ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# electron-helion mass ratio, CODATA 2022 (NIST)
electron_helion_mass_ratio = 1.819543074649e-4  # standard uncertainty 5.3e-15

# SciPy: scipy.constants.physical_constants['electron-helion mass ratio']

What it is

The mass of the electron divided by the mass of the helion. Ratios like this are often measured more precisely than either quantity on its own, which is why CODATA lists them.

How the value changed, 1998–2022

Each point is one CODATA adjustment, plotted as its difference from the 2022 value; the bars show one standard uncertainty (an arrowhead means the bar runs off the chart).

Relative uncertainty

CODATA	Value (uncertainty)	Relative uncertainty	Change from previous
2022	1.819 543 074 649(53) × 10⁻⁴	2.9 × 10⁻¹¹	+1.0 σ of 2018
2018	1.819 543 074 573(79) × 10⁻⁴	4.3 × 10⁻¹¹	−3.2 σ of 2014
2014	1.819 543 074 854(88) × 10⁻⁴	4.8 × 10⁻¹¹	−0.7 σ of 2010
2010	1.819 543 0761(17) × 10⁻⁴	9.3 × 10⁻¹⁰

“Change from previous” is the shift in the value divided by the standard uncertainty of the previous adjustment. Earlier values are converted into the current unit where NIST changed it (for example J s to J Hz⁻¹).