ConstantID

Electron-tau mass ratio

Atomic & nuclearMeasured

me/mτ = 2.875 85(19) × 10−4

Value
2.875 85 × 10−4
Standard uncertainty
1.9 × 10−8
Relative uncertainty
6.6 × 10−5 (66 parts per million)
Status
Measured
Symbol
me/mτ
Source
CODATA 2022 via NIST · NIST entry

Use in a calculation

# electron-tau mass ratio, CODATA 2022 (NIST)
electron_tau_mass_ratio = 2.87585e-4  # standard uncertainty 1.9e-8

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

What it is

The mass of the electron 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).

electron-tau mass ratio: CODATA values 1998–2022 relative to the 2022 value, in parts per million, with one-standard-uncertainty bars−150−100−500+50+100+150parts per million1998200220062010201420182022

Relative uncertainty

electron-tau mass ratio: relative standard uncertainty in each CODATA adjustment, logarithmic scale10⁻⁵10⁻⁴10⁻³1998200220062010201420182022
CODATAValue (uncertainty)Relative uncertaintyChange from previous
20222.875 85(19) × 10−46.6 × 10−5 +0.0 σ of 2018
20182.875 85(19) × 10−46.6 × 10−5 −0.3 σ of 2014
20142.875 92(26) × 10−49.0 × 10−5 +0.0 σ of 2010
20102.875 92(26) × 10−49.0 × 10−5 +0.6 σ of 2006
20062.875 64(47) × 10−41.6 × 10−4 +0.0 σ of 2002
20022.875 64(47) × 10−41.6 × 10−4 +0.2 σ of 1998
19982.875 55(47) × 10−41.6 × 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).

Related constants

Atomic & nuclear −e/me Electron charge to mass quotient −1.758 820 008 38(55) × 1011 C kg−1 Atomic & nuclear μe/μd Electron-deuteron magnetic moment ratio −2143.923 4921(56) Atomic & nuclear me/md Electron-deuteron mass ratio 2.724 437 107 629(47) × 10−4 Atomic & nuclear ge Electron g-factor −2.002 319 304 360 92(36) Atomic & nuclear γe Electron gyromagnetic ratio 1.760 859 627 84(55) × 1011 s−1 T−1 Atomic & nuclear γe/2π Electron gyromagnetic ratio in MHz/T 28 024.951 3861(87) MHz T−1 Atomic & nuclear me/mh Electron-helion mass ratio 1.819 543 074 649(53) × 10−4 Atomic & nuclear μe Electron magnetic moment −9.284 764 6917(29) × 10−24 J T−1