Electron-muon mass ratio

$mₑ/m_μ$ = 4.836 331 70(11) × 10⁻³

Value: 4.836 331 70 × 10⁻³
Standard uncertainty: 1.1 × 10⁻¹⁰
Relative uncertainty: 2.3 × 10⁻⁸ (23 parts per billion)
Status: Measured
Symbol: $mₑ/m_μ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# electron-muon mass ratio, CODATA 2022 (NIST)
electron_muon_mass_ratio = 4.8363317e-3  # standard uncertainty 1.1e-10

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

What it is

The mass of the electron divided by the mass of the muon. 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	4.836 331 70(11) × 10⁻³	2.3 × 10⁻⁸	+0.1 σ of 2018
2018	4.836 331 69(11) × 10⁻³	2.3 × 10⁻⁸	−0.1 σ of 2014
2014	4.836 331 70(11) × 10⁻³	2.3 × 10⁻⁸	+0.3 σ of 2010
2010	4.836 331 66(12) × 10⁻³	2.5 × 10⁻⁸	−0.4 σ of 2006
2006	4.836 331 71(12) × 10⁻³	2.5 × 10⁻⁸	+0.3 σ of 2002
2002	4.836 331 67(13) × 10⁻³	2.7 × 10⁻⁸	−2.9 σ of 1998
1998	4.836 332 10(15) × 10⁻³	3.1 × 10⁻⁸

“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).