Two Japanese scientists and a Tokyo-born American shared the 2008 Nobel Prize for physics for discoveries in sub-atomic particles, the prize committee said.

The Nobel committee lauded Yoichiro Nambu, a Tokyo-born American citizen, and Makoto Kobayashi and Toshihide Maskawa of Japan for separate work that helped explain why the universe is made up mostly of matter and not anti-matter via processes known as broken symmetries.

They helped figure out the existence and behaviour of the very tiniest particles known as quarks.

Nambu, a professor at the University of Chicago, was recognised for his discovery of the mechanism of spontaneous broken symmetry. It helps underlie the Standard Model of physics, which unites three of the four fundamental forces of nature: strong, weak and electromagnetic, leaving out gravity.

Nambu also influenced the development of quantum chromodynamics, a theory that describes some of the interactions between protons and neutrons, which make up atoms, and the quarks that make up the protons and neutrons.

Nambu shared half of the prestigious 10 million Swedish crown ($1.4 million) prize with Kobayashi of Japan's High Energy Accelerator Research Organisation and Maskawa of Kyoto University.

Kobayashi and Maskawa proposed the six types of quarks -- up, down, strange, charm, bottom, and top. All were later discovered in high-energy particle physics experiments.

"The fact that our world does not behave perfectly symmetrically is due to deviations from symmetry at the microscopic level," the committee said. This broken symmetry allowed particles of matter to outnumber particles of anti-matter.

This is lucky for all living things -- because if the universe were symmetrical, anti-matter would be constantly meeting matter and exploding in a burst of energy. Kobayashi said the news came as a shock. "It is my great honour and I can't believe this," he said. But Maskawa said he was not surprised.

"There is a pattern to how the Nobel prize is awarded. I did not think I would get the award up until last year, but I predicted it pretty much this year," he was quoted as saying by Kyodo news agency.

"I am very happy that Professor Yoichiro Nambu was awarded. I myself am not that happy. It's a noisy celebration for society."    

Physicists are now searching for the spontaneous broken symmetry, the Higgs mechanism, which threw the universe into its current imbalance at the time of the Big Bang 13.7 billion years ago.

This Higgs mechanism gave the particles their masses and there should be a Higgs particle, theory predicts. Scientists at the world's most powerful particle accelerator, the Large Hadron Collider at the European Organisation for Nuclear Research or CERN in Switzerland, will be looking for this particle when they re-start the collider in spring of 2009.

The prize, awarded by the Nobel Committee for Physics at the Royal Swedish Academy of Sciences, was the second of this year's crop of Nobel prizes. The prizes are handed out annually for achievements in science, peace, literature and economics. The prizes bearing the name of Alfred Nobel were first awarded in 1901 in accordance with the 1895 will of the Swedish dynamite millionaire.- Reuters