Electron-neutron mass ratio

$mₑ/mₙ$ = 5.438 673 4416(22) × 10⁻⁴

Value: 5.438 673 4416 × 10⁻⁴
Standard uncertainty: 2.2 × 10⁻¹³
Relative uncertainty: 4.0 × 10⁻¹⁰ (405 parts per trillion)
Status: Measured
Symbol: $mₑ/mₙ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# electron-neutron mass ratio, CODATA 2022 (NIST)
electron_neutron_mass_ratio = 5.4386734416e-4  # standard uncertainty 2.2e-13

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

What it is

The mass of the electron divided by the mass of the neutron. 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	5.438 673 4416(22) × 10⁻⁴	4.0 × 10⁻¹⁰	−0.3 σ of 2018
2018	5.438 673 4424(26) × 10⁻⁴	4.8 × 10⁻¹⁰	−0.1 σ of 2014
2014	5.438 673 4428(27) × 10⁻⁴	5.0 × 10⁻¹⁰	−1.0 σ of 2010
2010	5.438 673 4461(32) × 10⁻⁴	5.9 × 10⁻¹⁰	+0.1 σ of 2006
2006	5.438 673 4459(33) × 10⁻⁴	6.1 × 10⁻¹⁰	−0.6 σ of 2002
2002	5.438 673 4481(38) × 10⁻⁴	7.0 × 10⁻¹⁰	−1.2 σ of 1998
1998	5.438 673 462(12) × 10⁻⁴	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).