Proton-to-electron mass ratio

In physics, the proton-to-electron mass ratio (symbol μ or β) is the rest mass of the proton (a baryon found in atoms) divided by that of the electron (a lepton found in atoms), a dimensionless quantity, namely:

μ = m_p/⁠m_e = 1836.152673426(32).

The number in parentheses is the measurement uncertainty on the last two digits, corresponding to a relative standard uncertainty of 1.7×10⁻¹¹.