Halpern, Paul, Wesson, Paul. "4 Darkness Apparent: The Hidden Stuff of the Cosmos." Brave New Universe: Illuminating the Darkest Secrets of the Cosmos. Washington, DC: The National Academies Press, 2006.
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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos
draw a chuckle by using that name. Fittingly, they are believed to exist because they cleansed the strong nuclear interaction of one of its original puzzling properties. To understand what property was removed, let’s revisit the topic of symmetry breaking in the early universe.
One type of symmetry in nature is called CP (charge-parity) invariance. That means that, if the charges of an interacting system are reversed (from positive to negative, or vice versa) and the system is reflected in a mirror, it would look exactly the same as the original. For a single rotating particle, the latter process—known as parity reversal—involves switching the direction of rotation (from clockwise to counterclockwise, or the converse). Thus, in short, by flipping pluses to minuses and clockwise spins to counterclockwise spins, the system under consideration should return to its initial state.
The highly successful theory of the electroweak interaction includes a term that explicitly violates CP invariance. The term entered the theory because of experiments showing that CP violation is an inherent feature of the weak interaction. For certain processes involving the weak interaction, changing the signs of all the electrically charged particles and then reflecting them in the mirror leads to a system as different from the original as right-handed gloves from left-handed gloves. This disparity can be seen, for example, in the decay of certain elementary particles called kaons.
Theorists hoping to extend the standard model to describe the strong interaction found the need to include a similar term. Yet they ran up against a brick wall. Strong processes all respect CP symmetry. Like Isaac Asimov’s perfectly programmed robots, under no circumstance will they break this rule.
A good example of this property involves the neutron. Strong-force models predict that this particle should possess a physical feature called an electric dipole moment. According to electromagnetic theory, the actions of electric and magnetic fields can be grouped into “moments”: dipole, quadrupole, and so on. The electric dipole