Helion-proton mass ratio

$mₕ/mₚ$ = 2.993 152 671 552(70)

Value: 2.993 152 671 552
Standard uncertainty: 7 × 10⁻¹¹
Relative uncertainty: 2.3 × 10⁻¹¹ (23 parts per trillion)
Status: Measured
Symbol: $mₕ/mₚ$
Source: CODATA 2022 via NIST · NIST entry

Use in a calculation

# helion-proton mass ratio, CODATA 2022 (NIST)
helion_proton_mass_ratio = 2.993152671552  # standard uncertainty 7e-11

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

What it is

The mass of the helion 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	2.993 152 671 552(70)	2.3 × 10⁻¹¹	−0.9 σ of 2018
2018	2.993 152 671 67(13)	4.3 × 10⁻¹¹	+4.2 σ of 2014
2014	2.993 152 670 46(29)	9.7 × 10⁻¹¹	−0.1 σ of 2010
2010	2.993 152 6707(25)	8.4 × 10⁻¹⁰	−0.2 σ of 2006
2006	2.993 152 6713(26)	8.7 × 10⁻¹⁰	+0.7 σ of 2002
2002	2.993 152 6671(58)	1.9 × 10⁻⁹	+9.2 σ of 1998
1998	2.993 152 658 50(93)	3.1 × 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).