Muon magnetic moment anomaly

$a_μ$ = 1.165 920 62(41) × 10⁻³

Value: 1.165 920 62 × 10⁻³
Standard uncertainty: 4.1 × 10⁻¹⁰
Relative uncertainty: 3.5 × 10⁻⁷ (352 parts per billion)
Status: Measured
Symbol: $a_μ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# muon magnetic moment anomaly, CODATA 2022 (NIST)
muon_magnetic_moment_anomaly = 1.16592062e-3  # standard uncertainty 4.1e-10

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

What it is

The muon's version of the electron anomaly, measured at Brookhaven and then at Fermilab's Muon g−2 experiment. Because the muon is heavier, it is more sensitive to unknown particles, which is why comparisons of this value with theory draw so much attention.

Equations

$a_μ = (|g_μ| − 2)/2$ definition

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	1.165 920 62(41) × 10⁻³	3.5 × 10⁻⁷	−0.4 σ of 2018
2018	1.165 920 89(63) × 10⁻³	5.4 × 10⁻⁷	+0.0 σ of 2014
2014	1.165 920 89(63) × 10⁻³	5.4 × 10⁻⁷	−0.0 σ of 2010
2010	1.165 920 91(63) × 10⁻³	5.4 × 10⁻⁷	+0.4 σ of 2006
2006	1.165 920 69(60) × 10⁻³	5.1 × 10⁻⁷	+1.4 σ of 2002
2002	1.165 919 81(62) × 10⁻³	5.3 × 10⁻⁷	+5.9 σ of 1998
1998	1.165 916 02(64) × 10⁻³	5.5 × 10⁻⁷

“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).