Alpha particle-proton mass ratio

$m_α/mₚ$ = 3.972 599 690 252(70)

Value: 3.972 599 690 252
Standard uncertainty: 7 × 10⁻¹¹
Relative uncertainty: 1.8 × 10⁻¹¹ (18 parts per trillion)
Status: Measured
Symbol: $m_α/mₚ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# alpha particle-proton mass ratio, CODATA 2022 (NIST)
alpha_particle_proton_mass_ratio = 3.972599690252  # standard uncertainty 7e-11

# SciPy: scipy.constants.physical_constants['alpha particle-proton mass ratio']

What it is

The mass of the alpha particle divided by the mass of the proton. 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	3.972 599 690 252(70)	1.8 × 10⁻¹¹	+0.7 σ of 2018
2018	3.972 599 690 09(22)	5.5 × 10⁻¹¹	+2.8 σ of 2014
2014	3.972 599 689 07(36)	9.1 × 10⁻¹¹	−0.7 σ of 2010
2010	3.972 599 689 33(36)	9.1 × 10⁻¹¹	−0.4 σ of 2006
2006	3.972 599 689 51(41)	1.0 × 10⁻¹⁰	+0.8 σ of 2002
2002	3.972 599 689 07(52)	1.3 × 10⁻¹⁰	+4.1 σ of 1998
1998	3.972 599 6846(11)	2.8 × 10⁻¹⁰

“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).