Muon magnetic moment

$μ_μ$ = −4.490 448 30(10) × 10⁻²⁶ J T⁻¹

Value: −4.490 448 30 × 10⁻²⁶ J T⁻¹
Standard uncertainty: 1 × 10⁻³³ J T⁻¹
Relative uncertainty: 2.2 × 10⁻⁸ (22 parts per billion)
Status: Measured
Symbol: $μ_μ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# muon magnetic moment, CODATA 2022 (NIST)
muon_magnetic_moment = -4.4904483e-26  # J T^-1, standard uncertainty 1e-33

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

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: how strongly it behaves like a tiny bar magnet. A negative value means it points opposite to the spin.

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	−4.490 448 30(10) × 10⁻²⁶	2.2 × 10⁻⁸	+0.0 σ of 2018
2018	−4.490 448 30(10) × 10⁻²⁶	2.2 × 10⁻⁸	−0.4 σ of 2014
2014	−4.490 448 26(10) × 10⁻²⁶	2.2 × 10⁻⁸	−1.3 σ of 2010
2010	−4.490 448 07(15) × 10⁻²⁶	3.3 × 10⁻⁸	−1.3 σ of 2006
2006	−4.490 447 86(16) × 10⁻²⁶	3.6 × 10⁻⁸	+0.3 σ of 2002
2002	−4.490 447 99(40) × 10⁻²⁶	8.9 × 10⁻⁸	+0.6 σ of 1998
1998	−4.490 448 13(22) × 10⁻²⁶	4.9 × 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).