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 = 1836.15267343(11).

The number in parentheses is the measurement uncertainty on the last two digits, corresponding to a relative standard uncertainty of 6.0×10⁻¹¹.