Helion-electron mass ratio

$mₕ/mₑ$ = 5495.885 279 84(16)

Value: 5495.885 279 84
Standard uncertainty: 1.6 × 10⁻⁷
Relative uncertainty: 2.9 × 10⁻¹¹ (29 parts per trillion)
Status: Measured
Symbol: $mₕ/mₑ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# helion-electron mass ratio, CODATA 2022 (NIST)
helion_electron_mass_ratio = 5.49588527984e3  # standard uncertainty 1.6e-7

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

What it is

The mass of the helion divided by the mass of the electron. 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	5495.885 279 84(16)	2.9 × 10⁻¹¹	−1.0 σ of 2018
2018	5495.885 280 07(24)	4.4 × 10⁻¹¹	+3.1 σ of 2014
2014	5495.885 279 22(27)	4.9 × 10⁻¹¹	+0.8 σ of 2010
2010	5495.885 2754(50)	9.1 × 10⁻¹⁰	−0.2 σ of 2006
2006	5495.885 2765(52)	9.5 × 10⁻¹⁰	+0.7 σ of 2002
2002	5495.885 269(11)	2.0 × 10⁻⁹	+2.6 σ of 1998
1998	5495.885 238(12)	2.2 × 10⁻⁹

“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).