Proton Mass

The mass of the proton, proton mass, is 1.672 621 637(83) x 10 -27 kg, or 938.272013(23) MeV/c2, or 1.007 276 466 77(10) u (that’s unified atomic mass units).

The most accurate measurements of the mass of the proton come from experiments involving Penning traps, which are used to study the properties of stable charged particles. Basically, the particle under study is confined by a combination of magnetic and electric fields in an evacuated chamber, and its velocity reduced by a variety of techniques, such as laser cooling. Once trapped, the mass-to-charge ratio of a proton, deuteron (nucleus of a deuterium atom), singly charged hydrogen molecule, etc can be measured to high precision, and from these the mass of the proton estimated.

It would be nice if the experimentally observed mass of a proton were the same as that derived from theory. But how to work out what the mass of a proton should be, from theory?

The theory is quantum chromodynamics, or QCD for short, and is the strong force counterpart to quantum electrodynamics (QED). As the proton is made up of three quarks – two up and one down – its mass is the mass of those quarks and the mass of binding energy. This is a very difficult calculation to perform, in part because there are so many ways the quarks and gluons in a proton interact, but published results agree with experiment to within a percent or two.

More fundamentally, the proton has mass because of the Higgs boson … at least, it does according to the highly successful Standard Model of particle physics. Only trouble is, the Higgs boson has yet to be detected (the Large Hadron Collider was built with finding the Higgs boson as a key objective!).

Want to know the “official” value? Check out CODATA. And how does the proton mass compare with the mass of the anti-proton? Click here to find out! And how to determine the proton mass from first (theoretical) principles? This article from CNRS explains how.

More to explore, with Universe Today stories: New Estimate for the Mass of the Higgs Boson, Are the Laws of Nature the Same Everywhere in the Universe?, and Forget Neutron Stars, Quark Stars Might be the Densest Bodies in the Universe are three good ones to get you started.

Astronomy Cast episodes The Strong and Weak Nuclear Forces, The Large Hadron Collider and the Search for the Higgs Boson, and Inside the Atom will give you more insight into proton mass; check them out!

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