Proton Mass Measured Even More Precisely

We can only calculate the mass of a proton with quantum chromodynamics to a precision of about 1% from first principles, but our latest measurements of its mass published this month is much more precise, about 4.5 parts per trillion:

we obtain a deuteron-to-proton mass ratio, md/mp = 1.999 007 501 272(9). Combined with the atomic mass of the deuteron, we also obtain a new value for the atomic mass of the proton, mp = 1.007 276 466 574(10) u

From Physical Review Letters.

The new proton mass estimate has one-fifth of the uncertainty of the official average value listed by the International Science Council’s Committee on Data (CODATA) and one-third of the Max Planck group’s world leading measurement. All three results agree.

From here.

Some of the relevant CODATA values are as follows:

 

electron mass 

me = 0.510 998 950 00(15) MeV/c^2

= 9.109 383 7015(28)×10^−31 kg

proton mass 

mp = 938.272 088 16(29) MeV/c^2

= 1.672 621 923 69(51)×10^−27 kg

= 1.007 276 466 621(53) u 

= 1836.152 673 43(11) me

neutron mass 

mn = 939.565 420 52(54) MeV/c^2

= 1.008 664 915 95(49) u

deuteron mass 

md =1875.612 942 57(57) MeV/c^2

unified atomic mass unit 

u = (mass12C atom)/12 

= 931.494 102 42(28) MeV/c^2

= 1.660 539 066 60(50)×10^−27 kg