Standard model gets right answer for proton, neutron masses

When it comes to weighty matters, quarks and gluons rule the universe, a new study confirms.

One of the largest computational efforts to calculate the masses of protons and neutrons shows that the standard model of particle physics predicts those masses with an uncertainty of less than 4 percent.

Christian Hoelbling, affiliated with the Bergische Universtät Wuppertal in Germany, the Eötvös University in Budapest and the CNRS in Marseille, France, and his colleagues report their findings in the Nov. 21 Science.

Nearly all the mass of ordinary matter consists of atomic nuclei, which are composed of neutrons and protons. These particles are in turn composed of quarks, which are held together by massless particles called gluons.

Gluons are the messenger particles that carry the strong nuclear force and are constantly being exchanged by the quarks, as described by the theory known as quantum chromodynamics, or QCD. These exchanges bind quarks together by changing a quark property known as color charge. This charge is similar to electric charge but comes in three different types, whimsically referred to as red, green and blue. Six different types of quarks interact with eight varieties of gluons to create a panoply of elementary particles.

The new computations confirm a prediction of QCD, the authors say. That prediction is that the masses of particles such as the neutron and proton come from the energy associated with the interactions between quarks and gluons.

Calculating exactly how those interactions generate the masses of protons and neutrons requires several types of approximations. That’s in part because QCD has some peculiar properties: Because the gluon-mediated force between quarks grows stronger as they separate, quarks can never be seen as free agents, but only in pairs. On the other hand, at extremely short distances, which are probed at high energies, quarks and gluons interact very weakly.

In their calculations, Hoelbling and collaborators approximated the continuum of spacetime with a four-dimensional crystal lattice composed of discrete points spaced along columns and rows. The researchers solved the equations of QCD on finer and finer lattices, and then extrapolated the results to the continuum, painstakingly accounting and measuring every approximation and uncertainty along the way.

Although the match between QCD and known particle masses was expected, the finding is still important, team asserts. “It was stated as a conjecture over three decades ago that these fundamental particles — quarks and gluons — would never be observed but rather [only] bound states of them, like the proton and the neutron,” Hoelbling and colleagues explained in an e-mail message. Moreover, the team says, physicists can now more confidently apply QCD to other phenomena in particle physics that still require a QCD explanation.

“Because these accurate calculations agree with laboratory measurements, we now know, rather than just believe, that the source of mass of everyday matter is QCD,” notes Andreas Kronfeld of the Fermi National Accelerator Laboratory in Batavia, Ill., in a commentary accompanying the Science article.

In other words, QCD is QED.