In physics, the proton-to-electron mass ratio, 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 = mp/me = 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.