Sweden ? Two Japanese citizens and an American won the 2008 Nobel Prize in physics on Tuesday for discoveries that help explain the behavior of the smallest particles of matter.

American Yoichiro Nambu, 87, of the University of Chicago, won half of the $1.4 million prize for the discovery of a mechanism called spontaneous broken symmetry.

Makoto Kobayashi and Toshihide Maskawa of Japan shared the other half of the prize for discovering the origin of the broken symmetry that predicted the existence of at least three families of quarks in nature.

In physics, the idea of symmetry refers to a kind of equality or equivalence in a situation. At the subatomic level, for example, you should not be able to tell whether you are watching events unfold directly or in a mirror, or whether a movie of those events is running forward or backward. And particles should behave just like their alter egos, called antiparticles.

If any of these rules is violated, the symmetry is broken.