Neutron-muon mass ratio

$mₙ/m_μ$ = 8.892 484 08(20)

Value: 8.892 484 08
Standard uncertainty: 2 × 10⁻⁷
Relative uncertainty: 2.2 × 10⁻⁸ (22 parts per billion)
Status: Measured
Symbol: $mₙ/m_μ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# neutron-muon mass ratio, CODATA 2022 (NIST)
neutron_muon_mass_ratio = 8.89248408  # standard uncertainty 2e-7

# SciPy: scipy.constants.physical_constants['neutron-muon mass ratio']

What it is

The mass of the neutron divided by the mass 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	8.892 484 08(20)	2.2 × 10⁻⁸	+0.1 σ of 2018
2018	8.892 484 06(20)	2.2 × 10⁻⁸	−0.1 σ of 2014
2014	8.892 484 08(20)	2.2 × 10⁻⁸	+0.4 σ of 2010
2010	8.892 484 00(22)	2.5 × 10⁻⁸	−0.4 σ of 2006
2006	8.892 484 09(23)	2.6 × 10⁻⁸	+0.3 σ of 2002
2002	8.892 484 02(23)	2.6 × 10⁻⁸	−2.8 σ of 1998
1998	8.892 484 78(27)	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).