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A New Basis for Nuclear Structure

Evidence being sought that the structure of the bound proton and neutron has changed in a nucleus would herald a new paradigm for the structure of atomic nuclei.

Evidence being sought that the structure of the bound proton and neutron has changed in a nucleus would herald a new paradigm for the structure of atomic nuclei.

By Anthony Thomas

The idea that the internal structure of protons might change under certain circumstances is being put to the test, and could help to explain some inconsistencies in theoretical physics.

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At the start of the 20th century Rutherford discovered that the atom was largely empty space, with most of its mass concentrated in a tiny nucleus, just one hundred thousandth of the size of the atom itself. The only particles known at that time were the proton and the electron and there was no way for the laws of physics to bind them into such a small volume.

It took another 20 years, until the discovery of the neutron, to resolve this mystery. Since that discovery, the overwhelming majority of studies of nuclear structure have adopted the hypothesis that the protons and neutrons inside a nucleus are immutable objects whose internal structure never changes. These immutable objects interact through non-relativistic two- and three-body forces and the challenge is primarily to accurately solve the many-body problem.

In the 1970s this pattern was in a sense repeated, with the discovery of the quarks. The proton and neutron, far from being elementary particles, are themselves mainly empty space containing apparently point-like quarks bound by force carriers known as gluons. The proton consists of two "up" quarks and one down and vice-versa for the neutron.

In the early 80s an experiment at CERN by the European Muon Collaboration (EMC) discovered the remarkable result that the distribution of momentum on the quarks in a nucleus were significantly...

The full text of this article can be purchased from Informit.