Published online 7 October 2008 | Nature | doi:10.1038/news.2008.1155


Nobel Prize in Physics for symmetry breakdown

Japanese-born theorists rewarded for work on fundamental symmetries in particle physics.

BelleThe Belle detector in Japan helped to confirm the symmetry breaking effects predicted by theoretical physicists.KEK

A trio of Japanese-born physicists has been awarded the 2008 Nobel Prize in Physics for their work on understanding how the fundamental symmetries of nature are broken.

Makoto Kobayashi of Japan's High Energy Accelerator Research Organization (KEK) in Tsukuba and Toshihide Maskawa of the Yukawa Institute for Theoretical Physics (YITP) at Kyoto University were awarded a quarter of the prize each for discovering the origin of the 'broken symmetry' that contributed to a preponderance of matter over antimatter in the Universe.

The other half went to Yoichiro Nambu of the University of Chicago in Illinois, who showed that the concept of spontaneous broken symmetry can explain the variety of particles and forces seen in the quantum world1.

"I am very glad that I have received the prize," Kobayashi said by telephone to a gathering of reporters in Stockholm, where the prize announcement was made.

A broken mirror

Symmetry breaking describes how physical systems can suddenly show a preference for one direction over another. For example, a pencil balancing on its tip appears symmetric from the top. But when it falls, and points in a particular direction, that symmetry is broken.

The same concept applies to many systems, but it was Nambu who extended the theory to fundamental particles, explains John Ellis, a theoretical physicist at CERN, Europe's particle-physics lab in Geneva, Switzerland. Ellis is one of thousands of scientists hoping to use CERN's Large Hadron Collider (LHC) to find an infamous product of symmetry breaking — the Higgs boson, which is believed to endow other particles with mass.

Kobayashi and Maskawa, meanwhile, showed how violation of a particular symmetry could create more matter than antimatter in the Universe — a long-standing mystery in particle physics. In a 1973 paper2, the pair calculated that interactions between quarks via the weak force - one of the four fundamental forces of nature - naturally gives rise to a charge-parity (CP) violation, where particles don't behave as precise mirror-image likenesses of their antimatter counterparts.

"They wrote down this huge expression whose physical interpretation is the violation of symmetry between matter and antimatter," says Ken Peach, a physicist at the University of Oxford, UK. The equations also predicted a third family of quarks, an idea that seemed "far-fetched" at the time, according to Ellis - until the quarks were found.

Powerful violation

Kobayashi and Maskawa's work has since been verified by two high-energy experiments. The Japanese Belle experiment at KEK and the BaBar experiment at the Stanford Linear Accelerator Center in California both measured the decay of particles that included bottom quarks, and their findings bore out the original predictions. "All [the findings] are consistent," says Peach.

However, the CP-violation discovered by the pair is still not enough to explain the total dominance of matter over antimatter in the present-day Universe. Peach says many physicists believe another, more powerful symmetry violation may yet be found — perhaps also in measurements at the LHC.

This is the first time that three Japanese-born physicists have been jointly awarded the prize, according to Tatsuya Nakada, a physicist at CERN. The fact that a Japanese experiment had a major role further shows that the country is now a world-class player in high-energy physics, he says. "I think the success of Belle showed that high-energy physics in Japan can compete," Nakada says. "This means a lot." 

  • References

    1. Nambu, Y. A 'Superconductor' Model of Elementary Particles and its Consequences. In Broken Symmetries, Selected Papers by Y. Nambu, (eds Eguchi, T. and Nishijima, K.) (World Scientific, 1995).
    2. Kobayashi, M. and Maskawa, K. Progr. Theor. Phys. 49, 652 (1973). | Article | ChemPort |


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    The same concept applies to many systems, but it was Nambu who extended the theory to fundamental particles, explains John Ellis, a theoretical physicist at CERN, Europe's particle-physics lab in Geneva, Switzerland Fabric Linen

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