Muon magnetic moment to Bohr magneton ratio

$μ_μ/μ_B$ = −4.841 970 48(11) × 10⁻³

Value: −4.841 970 48 × 10⁻³
Standard uncertainty: 1.1 × 10⁻¹⁰
Relative uncertainty: 2.3 × 10⁻⁸ (23 parts per billion)
Status: Measured
Symbol: $μ_μ/μ_B$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# muon magnetic moment to Bohr magneton ratio, CODATA 2022 (NIST)
muon_magnetic_moment_to_bohr_magneton_ra = -4.84197048e-3  # standard uncertainty 1.1e-10

# SciPy: scipy.constants.physical_constants['muon mag. mom. to Bohr magneton ratio']

What it is

The magnetic moment of the muon, an unstable heavier cousin of the electron (about 207 times its mass) that lives about 2.2 microseconds, expressed in Bohr magnetons.

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	−4.841 970 48(11) × 10⁻³	2.3 × 10⁻⁸	−0.1 σ of 2018
2018	−4.841 970 47(11) × 10⁻³	2.3 × 10⁻⁸	+0.1 σ of 2014
2014	−4.841 970 48(11) × 10⁻³	2.3 × 10⁻⁸	−0.3 σ of 2010
2010	−4.841 970 44(12) × 10⁻³	2.5 × 10⁻⁸	+0.4 σ of 2006
2006	−4.841 970 49(12) × 10⁻³	2.5 × 10⁻⁸	−0.3 σ of 2002
2002	−4.841 970 45(13) × 10⁻³	2.7 × 10⁻⁸	+2.7 σ of 1998
1998	−4.841 970 85(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).