Muon-neutron mass ratio

$m_μ/mₙ$ = 0.112 454 5168(25)

Value: 0.112 454 5168
Standard uncertainty: 2.5 × 10⁻⁹
Relative uncertainty: 2.2 × 10⁻⁸ (22 parts per billion)
Status: Measured
Symbol: $m_μ/mₙ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# muon-neutron mass ratio, CODATA 2022 (NIST)
muon_neutron_mass_ratio = 1.124545168e-1  # standard uncertainty 2.5e-9

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

What it is

The mass of the muon divided by the mass of the neutron. 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	0.112 454 5168(25)	2.2 × 10⁻⁸	−0.1 σ of 2018
2018	0.112 454 5170(25)	2.2 × 10⁻⁸	+0.1 σ of 2014
2014	0.112 454 5167(25)	2.2 × 10⁻⁸	−0.4 σ of 2010
2010	0.112 454 5177(28)	2.5 × 10⁻⁸	+0.3 σ of 2006
2006	0.112 454 5167(29)	2.6 × 10⁻⁸	−0.3 σ of 2002
2002	0.112 454 5175(29)	2.6 × 10⁻⁸	+2.8 σ of 1998
1998	0.112 454 5079(34)	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).