STOCKHOLM (AP) — 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 10 million kronor (US$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.

The academy said the trio "presented theoretical insights that give us a deeper understanding of what happens far inside the tiniest building blocks of matter.

In physics, the idea of symmetry refers 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.

One big broken symmetry arose immediately after the big bang, when just a tiny bit more matter than antimatter was created. Because these two kinds of particles annihilate each other when they meet, that excess of matter was responsible for seeding the universe.

Nambu introduced his description of spontaneous symmetry violation into particle physics in 1960.

The Nobel citation said Nambu's theories now permeate the Standard Model of physics, which is the basic theory of how the universe operates. For example, they help explain why different particles have different masses.

In 1972, Kobayashi and Maskawa explained why an experiment eight years before had found that some subatomic particles called kaons failed to follow the rules of symmetry. Their explanation predicted the existence of three unknown subatomic particles called quarks. In fact, scientists discovered those predicted particles between 1974 and 1994.

Kobayashi and Maskawa also predicted that symmetry would be broken in the behavior of other particles, called B-mesons. As early as 2001, scientists confirmed that prediction, too.

Nambu said he was awakened by the academy, which called to tell him about the prize.

"I was surprised and honored. I didn't expect it. I've been told for many years that I was on the list (to get the award)," he said. "I had almost given up."

The Japanese-born Nambu moved to the United States in 1952 and is a professor at the Enrico Fermi Institute at the University of Chicago, where he has worked for 40 years. He became a U.S. citizen in 1970.

Kobayashi, 64, works for the High Energy Accelerator Research Organization, or KEK, in Tsukuba, Japan. Maskawa, 68, is a physics professor at Kyoto Sangyo University in Japan's ancient capital of Kyoto, who also teaches at Nagoya University in his hometown in central Japan.

"I wasn't expecting the prize," Kobayashi said. "I've been only pursuing my interest."

"It's an honor to receive the prize for my work from long time ago," Kobayashi said at a news conference in Japan. "I wrote that paper more than 30 years ago."

Kobayashi seemed to be astonished by the big crowd of reporters and said "Looks like it's a big deal."

His news conference was interrupted several times by a phone call from Prime Minster Taro Aso, who congratulated the 64-year-old professor.

In a separate news conference at his university, Maskawa said, "As a scientist, I'm not thrilled by the prize."

"I was happier when our findings were acknowledged around 2002. The Nobel prize is a rather mundane thing."

The last Japanese citizen to win the physics prize was Masatoshi Koshiba of the University of Tokyo in 2002. He shared half of the prize with Raymond Davis Jr. of the U.S. for work in detecting cosmic neutrinos. American Riccardo Giacconi received the other half of the prize for his work that led to the discovery of cosmic X-ray sources.

The 2008 prize is "recognizing one of the most basic and fundamental aspects of existence," said Phil Schewe, a physicist and spokesman for the American Institute of Physics in College Park, Md. "Nature works in strange ways, and these three physicists helped to explain that strangeness in an ingenious way."

• 2007: France's Albert Fert and Germany's Peter Gruenberg for work on the discovery of giant magnetoresistance.

• 2006: Americans John C. Mather and George F. Smoot for work examining the infancy of the universe, aiding the understanding of galaxies and stars and increasing support for the Big Bang theory of the beginning of the universe.

• 2005: Americans John L. Hall and Roy J. Glauber and German Theodor W. Haensch, for research explaining the behavior of light particles and determining the frequency of light with great precision.

• 2004: Americans David J. Gross, H. David Politzer and Frank Wilczeck, for their work in the discovery and exploration of strong force and quarks.

• 2003: Alexei A. Abrikosov, United States and Russia, Anthony J. Leggett, United States and Britain, and Vitaly L. Ginzburg, Russia, for their work concerning superconductivity and superfluidity in the field of quantum physics.

• 2002: Raymond Davis, Jr., United States, and Masatoshi Koshiba, Japan, for their research into cosmic neutrinos; and Riccardo Giacconi, United States, for pioneering contributions to astrophysics that led to the discovery of cosmic X-ray sources.

• 2001: Eric A. Cornell and Carl E. Wieman, United States, and U.S.-based researcher Wolfgang Ketterle of Germany for creating a new state of matter, an ultra-cold gas known as Bose-Einstein condensate.

• 2000: Zhores I. Alferov, Russia, U.S.-based researcher Herbert Kroemer of Germany, and Jack Kilby, United States, for work that helped create modern information technology.

• 1999: Gerardus 't Hooft and Martinus J.G. Veltman, Netherlands, for their theoretical work on the structure and motion of subatomic particles.

• 1998: Robert B. Laughlin, United States, Horst L. Stoermer, Germany, and Daniel C. Tsui, United States, for discovering a new form of quantum fluid that gives more profound insights into the general inner structure and dynamics of matter.

• 1997: Steven Chu and William D. Phillips, United States, and Claude Cohen-Tannoudji, France, for their work in cooling and trapping atoms with laser light.

• 1996: David M. Lee, Douglas D. Osheroff and Robert C. Richardson, United States, for their discovery of superfluidity in helium-3.

• 1995: Martin L. Perl and Frederick Reines, United States, for pioneering experimental contributions to lepton physics.

• 1994: Bertram N. Brockhouse, Canada, and Clifford G. Shull, United States, for developing methods of neutron scattering techniques for studies of condensed matter.

• 1993: Russell A. Hulse and Joseph H. Taylor, Jr., United States, for finding a twin star: a binary pulsar that helped prove Einstein's theory of relativity.

• 1992: Georges Charpak, France, for developing particle detectors and the multiwire proportional chamber.

• 1991: Pierres-Gilles de Gennes, France, for developing systems for analyzing complex matter such as liquid crystals and polymers.

• 1990: Jerome I. Friedman and Henry W. Kendall, United States, and Richard E. Taylor, Canada, for investigating the scattering of electrons and refining models of quarks.

• 1989: Norman F. Ramsey and Hans G. Dehmelt, United States, and Wolfgang Paul, West Germany, for inventing methods used in atomic clocks and ion trap techniques.

• 1988: Leon M. Lederman, Melvin Schwartz and Jack Steinberger, United States, for developing the neutrino beam and discovering new types of neutrinos.

• 1987: J. Georg Bednorz, West Germany, and K. Alexander Muller, Switzerland, for work revealing superconductivity in ceramics.

• 1986: Ernst Ruska and Gerd Binnig, West Germany, and Heinrich Rohrer, Switzerland, for designing the electron and scanning tunneling microscopes.

• 1985: Klaus von Klitzing, West Germany, for discovering the quantized Hall effect.

• 1984: Carlo Rubbia, Italy, and Simon van der Meer, Netherlands, for contributions to the discovery of field particles involved in weak interaction.

• 1983: Subramanyan Chandrasekhar and William A. Fowler, United States, for theories explaining the chemical and physical process between stars and the universe.

• 1982: Kenneth G. Wilson, United States, for developing the theory of phase transitions.

• 1981: Nicolaas Bloembergen and Arthur L. Schawlow, United States, and Kai M. Siegbahn, Sweden, for contributing to the development of laser and electron spectroscopy.

• 1980: James Cronin and Val Fitch, United States, for discovering new aspects of neutral K-mesons.