Neutrons not so neutral after all, study says

Last Updated: Wednesday, September 19, 2007 | 4:30 PM ET

CBC News

Among atomic particles, the neutron seems the most aptly named: Unlike the positively charged proton or the negatively charged electron, neutrons have a charge of zero.

But new experiments conducted in three particle accelerators suggest the neutron is more like an onion when it comes to electromagnetism: with a negatively charged exterior and interior and a positively charged middle sandwiched between them.

While the sum of these charges cancel each other out to produce a net charge of zero, the findings change our understanding of how neutrons interact with the other particles in the atom, said University of Washington physics professor Gerald Miller, writing last week in the online edition of the scientific journal Physical Review Letters.

"When a particle like an electron interacts with a neutron, where it hits will affect how it behaves and where it goes," Miller told CBC News. "These results change what we know of how neutrons interact electrically with anything."

Enrico Fermi had previously suggested neutrons might have a negative exterior and a positive interior, Miller said. But this is the first experimental evidence to suggest the extra layer of complexity.

The findings, obtained from colliding electrons with neutrons and then measuring the angles of outgoing electrons, could help scientists better understand how electromagnetism interacts with the strong nuclear force that keeps protons and neutrons bound together in the nucleus of an atom.

It could also have a profound change on our understanding of neutron stars, the super-dense pulsating stellar objects made up mostly of neutrons that form in the tail end of a star's evolution, Miller said.

If the idea of a neutron having three differently-charged parts seems familiar, it may be because in quantum physics, neutrons are said to be made of three smaller particles called quarks: one "up" quark with a charge of +2/3, and two "down" quarks with a charge of -1/3. When joined together, the three quarks have a charge of zero.

Miller's findings would seem to suggest an actual physical location for these quarks within a neutron, but the truth, he said, is much more complicated.

"We're not talking about location as much as we are talking about density," he said. "What we are saying is the probability of finding a particular charge density is a function of distance. Quarks are constantly moving within a neutron, so this tells us the odds of finding more of them in one area over another."

The connection between quarks and charge location also breaks down when examining the proton, said Miller.

While a proton is made up of one down quark and two up quarks, the same accelerator tests found the proton was positively charged throughout, with the middle region possessing a stronger charge than the interior and exterior.