Neutron-tau mass ratio

$mₙ/m_τ$ = 0.528 779(36)

Value: 0.528 779
Standard uncertainty: 3.6 × 10⁻⁵
Relative uncertainty: 6.8 × 10⁻⁵ (68 parts per million)
Status: Measured
Symbol: $mₙ/m_τ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# neutron-tau mass ratio, CODATA 2022 (NIST)
neutron_tau_mass_ratio = 5.28779e-1  # standard uncertainty 3.6e-5

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

What it is

The mass of the neutron divided by the mass of the tau. 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	0.528 779(36)	6.8 × 10⁻⁵	+0.0 σ of 2018
2018	0.528 779(36)	6.8 × 10⁻⁵	−0.2 σ of 2014
2014	0.528 790(48)	9.1 × 10⁻⁵	+0.0 σ of 2010
2010	0.528 790(48)	9.1 × 10⁻⁵	+0.6 σ of 2006
2006	0.528 740(86)	1.6 × 10⁻⁴	+0.0 σ of 2002
2002	0.528 740(86)	1.6 × 10⁻⁴	+0.2 σ of 1998
1998	0.528 722(86)	1.6 × 10⁻⁴

“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).