Electron to alpha particle mass ratio

$mₑ/m_α$ = 1.370 933 554 733(32) × 10⁻⁴

Value: 1.370 933 554 733 × 10⁻⁴
Standard uncertainty: 3.2 × 10⁻¹⁵
Relative uncertainty: 2.3 × 10⁻¹¹ (23 parts per trillion)
Status: Measured
Symbol: $mₑ/m_α$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# electron to alpha particle mass ratio, CODATA 2022 (NIST)
electron_to_alpha_particle_mass_ratio = 1.370933554733e-4  # standard uncertainty 3.2e-15

# SciPy: scipy.constants.physical_constants['electron to alpha particle mass ratio']

What it is

The mass of the electron divided by the mass of the alpha particle. 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	1.370 933 554 733(32) × 10⁻⁴	2.3 × 10⁻¹¹	−1.2 σ of 2018
2018	1.370 933 554 787(45) × 10⁻⁴	3.3 × 10⁻¹¹	−0.2 σ of 2014
2014	1.370 933 554 798(45) × 10⁻⁴	3.3 × 10⁻¹¹	−1.8 σ of 2010
2010	1.370 933 555 78(55) × 10⁻⁴	4.0 × 10⁻¹⁰	+0.1 σ of 2006
2006	1.370 933 555 70(58) × 10⁻⁴	4.2 × 10⁻¹⁰	−0.1 σ of 2002
2002	1.370 933 555 75(61) × 10⁻⁴	4.4 × 10⁻¹⁰	−1.8 σ of 1998
1998	1.370 933 5611(29) × 10⁻⁴	2.1 × 10⁻⁹

“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).