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Standard model gets right answer; calculation of nucleon mass supports quark-gluon theory.

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 within 4 percent. Christian Hoelbling of the Bergische Universtat Wuppertal in Germany and colleagues report their findings in the Nov. 21 Science.

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

Gluons 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 comes in three different forms, 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, part of particle physics' standard model, that the masses of particles such as neutrons and protons come from the energy associated with interactions between quarks and gluons.

In their calculations, Hoelbling and colleagues approximated the continuum of spacetime with a four-dimensional 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.

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"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.

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Title Annotation:Matter & Energy
Author:Cowen, Ron
Publication:Science News
Article Type:Brief article
Geographic Code:1USA
Date:Dec 20, 2008
Words:296
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