Electron-muon magnetic moment ratio

$μₑ/μ_μ$ = 206.766 9881(46)

Value: 206.766 9881
Standard uncertainty: 4.6 × 10⁻⁶
Relative uncertainty: 2.2 × 10⁻⁸ (22 parts per billion)
Status: Measured
Symbol: $μₑ/μ_μ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# electron-muon magnetic moment ratio, CODATA 2022 (NIST)
electron_muon_magnetic_moment_ratio = 2.067669881e2  # standard uncertainty 4.6e-6

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

What it is

The magnetic moment of the electron divided by the magnetic moment 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	206.766 9881(46)	2.2 × 10⁻⁸	−0.0 σ of 2018
2018	206.766 9883(46)	2.2 × 10⁻⁸	+0.1 σ of 2014
2014	206.766 9880(46)	2.2 × 10⁻⁸	−0.3 σ of 2010
2010	206.766 9896(52)	2.5 × 10⁻⁸	+0.4 σ of 2006
2006	206.766 9877(52)	2.5 × 10⁻⁸	−0.3 σ of 2002
2002	206.766 9894(54)	2.6 × 10⁻⁸	+2.8 σ of 1998
1998	206.766 9720(63)	3.0 × 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).