Muon g factor

$g_μ$ = −2.002 331 841 23(82)

Value: −2.002 331 841 23
Standard uncertainty: 8.2 × 10⁻¹⁰
Relative uncertainty: 4.1 × 10⁻¹⁰ (410 parts per trillion)
Status: Measured
Symbol: $g_μ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# muon g factor, CODATA 2022 (NIST)
muon_g_factor = -2.00233184123  # standard uncertainty 8.2e-10

# SciPy: scipy.constants.physical_constants['muon g factor']

What it is

The g-factor of the muon, an unstable heavier cousin of the electron (about 207 times its mass) that lives about 2.2 microseconds: the pure number that relates its magnetic moment to its spin angular momentum. A negative value means the moment 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	−2.002 331 841 23(82)	4.1 × 10⁻¹⁰	+0.4 σ of 2018
2018	−2.002 331 8418(13)	6.5 × 10⁻¹⁰	+0.0 σ of 2014
2014	−2.002 331 8418(13)	6.5 × 10⁻¹⁰	+0.0 σ of 2010
2010	−2.002 331 8418(13)	6.5 × 10⁻¹⁰	−0.3 σ of 2006
2006	−2.002 331 8414(12)	6.0 × 10⁻¹⁰	−1.5 σ of 2002
2002	−2.002 331 8396(12)	6.0 × 10⁻¹⁰	−5.8 σ of 1998
1998	−2.002 331 8320(13)	6.5 × 10⁻¹⁰

“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).