Standard model gets right answer for proton, neutron masses

Science News ^ | November 20th, 2008 | Ron Cowen

[neverdem]

Correct calculation strengthens theory of quark-gluon interactions in nuclear particles

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...

[SunkenCiv]

some sidebars to this 2008 topic you posted. :')

• Physics: Proton radius smaller than believed, European scientists say
• The proton -- smaller than thought