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Breaking their way. Makoto Kobayashi (left), Toshihide Maskawa (center), and Yoichiro Nambu share the prize for work on two different aspects of "broken symmetry."

2008 Physics Nobel Prize Honors American and Japanese Particle Theorists

Staff Writer

This year's Nobel Prize in physics honors three particle theorist of Japanese origin, one for pioneering the use of a key conceptual tool and the other two for making, in essence, an inspired educated guess that expanded the family of fundamental subatomic particles.

Yoichiro Nambu, 87, of the University of Chicago in Illinois receives half the $1.4 million prize for, in the early 1960s, applying to particle physics the concept of spontaneous symmetry breaking--the sort of thing that happens when all the ions in a magnet suddenly align the same way--instead of all pointing in random directions. That concept now lies at the heart of the so-called standard model of fundamental particles. Makoto Kobayashi, 64, now at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, and countryman Toshihide Maskawa, 68, of Kyoto University in Japan share the other half of the prize for predicting in 1972 that an asymmetry between matter and antimatter that had been observed 8 years earlier could be explained if there were at least six types of particles called quarks. Only three quarks were known at the time, but the others have since been found and the theory confirmed to high precision.

Experts say the awards are richly deserved. "A lot of us thought that [Nambu] deserved it a long time ago," says Jonathan Ellis, a theorist at the European particle physics laboratory, CERN, near Geneva, Switzerland. "It's high time." Michael Gronau, a theorist at the Technion-Israel Institute of Technology in Haifa, says that "the [particle] physics of the last 10 years has been dominated by Kobayashi and Maskawa."

Nambu's peers say he could have won the prize for several different things, but he receives the Nobel specifically for applying the concept of spontaneous symmetry breaking to interactions between the protons and neutrons found in atomic nuclei (also known as nucleons) and particles called pions. Earlier, Japanese theorist Hideki Yukawa had theorized that the strong nuclear force that binds protons together is carried by pions, which zip back and forth between nucleons. Nambu showed that the nature of pions, and indeed their very existence, could be explained as a consequence of spontaneous symmetry breaking. "I think this is the work that I value the most," Nambu told Science.

Spontaneous symmetry breaking occurs whenever the forces within a system are in someway symmetric, but the lowest energy "ground state" that the system nestles into does not share that symmetry. The classic example is one of a pencil balanced on its tip, Ellis explains. Gravity will pull the pencil down, but it doesn't tug the pencil in any particular horizontal direction. "Which direction will it fall?" Ellis says. "A priori you have no way of predicting." In order to reach its ground state, the pencil has to spontaneously break the symmetry of the forces and tip one way.

Nambu assumed a much more subtle symmetry between protons and neutrons involving the way the particles spin and the fact that, to the strong force, a proton is essentially indistinguishable from a neutron. He then showed that if the symmetry were spontaneously broken, the pions had to emerge from the conceptual process with exactly the mass and other properties they were already known to have. That was a big victory for the time, and it laid the groundwork for many further advances, as spontaneous symmetry breaking of other sorts shows up throughout the standard model. In fact, physicists believe that the spontaneous breaking of a different symmetry explains how the particles in the theory obtain their mass and that the long-sought Higgs boson is a necessary byproduct of that bit of physics. "It's a very general phenomenon in many areas, including, of course, the Higgs boson that everyone is looking for," says Jean Iliopoulos of the École Normale Supérieure in Paris.

Kobayashi and Maskawa set out to explain the mysterious asymmetry between matter and antimatter known as charge-parity (CP) violation. That subtle effect had been seen in 1964 in the decays of particles called K mesons and their antimatter foils, which themselves were found to consist of smaller particles called quarks. At the time, physicists knew of three kinds of quarks: Up and down quarks joined in trios to form protons and neutrons, and the strange quark that could bind with an anti-up or an anti-down anti-quark to form K mesons. Kobayashi and Maskawa found that they could explain CP violation if there existed at least three other quarks. The presence of those extra quarks creates just enough mathematical wiggle room so that in meson decay, affects akin to the interference between waves, can create small difference between how matter and antimatter behave. "It was very bold," Gronau says. "Nobody took the model seriously when it came out."

Yet it didn't take long for the other quarks to appear in the bits of matter blasted out of ever-more powerful particle collisions. The charm quark surfaced in 1974, the heavier bottom quark emerged in 1977, and the superheavy top quark showed up in 1995. "We proposed this theory over 30 years ago, and step by step the evidence accumulated," Kobayashi said at a press conference today. "It was very fulfilling as a researcher to watch that progress." Most importantly, starting in 1999, researchers at the Stanford Linear Accelerator Center (SLAC) in Menlo Park, California, and at KEK studied CP violation in B mesons, which contain bottom quarks and are the only other particles to exhibit CP violation. The results from those experiments confirm the exact predictions of the Kobayashi and Maskawa "mechanism" to within a few percent.

That's a great intellectual victory for physicists but also, ironically, something of a defeat. Researchers know that the standard model simply does not contain enough CP violation to explain a bigger mystery: why the universe evolved to contain so much matter and so little antimatter. Many had been hoping that the SLAC and KEK experiments would show that Kobayashi and Maskawa were not exactly right and provide hints to a deeper theory. "Proving [the theory] was a tremendous achievement," says Yosef Nir, a theorist at the Weizmann Institute of Science in Rehovot, Israel. "Disproving it would have been a revolution."

Ellis notes that both achievements help set the conceptual table for the world's new most powerful atom smasher, the Large Hadron Collider (LHC) at CERN. Although currently down for repairs, next spring the LHC will start to chase the Higgs and take an even closer look at matter-antimatter asymmetry.

With reporting by Dennis Normile.

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