Stockholm - US researcher Yoichiro Nambu and his Japanese colleagues Makoto Kobayashi and Toshihide Maskawa have won this year's Nobel Prize for Physics, the Royal Swedish Academy of Sciences announced in Stockholm on Tuesday.

Their discoveries were linked to describing the smallest building blocks in nature and nature's order, the academy said.

The academy cited Nambu of the Enrico Fermi Institute at the University of Chicago "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics."

Kobayashi and Maskawa were cited "for the discovery of the origin of broken symmetry which predicts the existence of at least three families of quarks in nature."

Tokyo-born Nambu won half the award, worth 10 million kronor (1.5 million dollars), while Kobayashi and Maskawa share the other half.

Nambu - born 1921 - formulated his mathematical description theory in 1960 of a model that "unifies the smallest building blocks of all matter and three of nature's four forces in one single theory," the academy said.

Kobayashi and Maskawa helped explain "broken symmetry within the framework of the standard model," the academy said of the model that is a "synthesis of all insights into the innermost parts of matter" based on quantum physics and the theory of relativity.

Broken symmetry is also linked to the origin of the cosmos, dating back to the Big Bang some 14 billion years ago. "If equal amounts of matter and antimatter were created, they ought to have annihilated each other," the academy said, noting that the cosmos appeared to have survived because of "broken symmetry."

Further research, including that at the new particle accelerator, the Large Hadron Collider at the European Organization for Nuclear Research (CERN) in Geneva, may contribute to further understanding.

Kobayashi's initial reaction was a "great surprise," he told reporters via a loudspeaker telephone at the academy's headquarters in Stockholm, adding "I don't know what to say."

Kobayashi, born 1944, is a retired professor with the High Energy Accelerator Research Organization (KEK) said he had not expected the award.

Maskawa, born 1940, is also professor emeritus, and worked at Yukawa Institute for Theoretical Physics (YITP) at Kyoto University.

The physics prize was the second of the 2008 Nobel Awards to be announced.

On Monday, the Nobel Prize for Medicine was awarded to German Harald zur Hausen for discovering the human papilloma virus which causes cervical cancer, while French scientists Francoise Barre- Sinoussi and Luc Montagnier, were awarded for their discovery of the human immunodeficiency virus (HIV) that causes AIDS.

On Wednesday, the chemistry prize is due to be announced. Later this week, winners of the literature and peace prizes are to be named.

The prizes were endowed by Swedish industrialist Alfred Nobel, the inventor of dynamite.

The economic sciences prize - a prize not endowed by Nobel and awarded since 1968 - is slated to be announced on MOnday.

The award ceremonies are held December 10, the anniversary of Nobel's 1896 death in San Remo, Italy. (dpa)