The particle zoo may house such exotica as K mesons and pentaquarks, but the stuff of our everyday lives is made up of just three beasts: the proton, the electron, and the neutron. The mass of the proton is of interest not just because of the primal nature of the particle but also because the proton mass is used as input in precise determinations of the neutron mass and other quantities. Now a team led by Sven Sturm of the Max Planck Institute for Nuclear Physics has measured the proton mass with a precision of 32 parts per trillion, a threefold improvement on the precision of the currently accepted value. Their result, mp = 1.007276466583 amu (atomic mass unit), is lower than the accepted value by about 300 ppt.
As with other groups who have precisely measured the proton mass, Sturm and colleagues made use of a Penning trap,...
